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1.1 root 1: \input texinfo
2: @setfilename cpp.info
3: @settitle The C Preprocessor
4:
5: @ignore
6: @ifinfo
7: @format
8: START-INFO-DIR-ENTRY
9: * Cpp: (cpp). The C preprocessor.
10: END-INFO-DIR-ENTRY
11: @end format
12: @end ifinfo
13: @end ignore
14:
15: @c @smallbook
16: @c @cropmarks
17: @c @finalout
18: @setchapternewpage odd
19: @ifinfo
20: This file documents the GNU C Preprocessor.
21:
22: Copyright 1987, 1989, 1991, 1992, 1993 Free Software Foundation, Inc.
23:
24: Permission is granted to make and distribute verbatim copies of
25: this manual provided the copyright notice and this permission notice
26: are preserved on all copies.
27:
28: @ignore
29: Permission is granted to process this file through Tex and print the
30: results, provided the printed document carries copying permission
31: notice identical to this one except for the removal of this paragraph
32: (this paragraph not being relevant to the printed manual).
33:
34: @end ignore
35: Permission is granted to copy and distribute modified versions of this
36: manual under the conditions for verbatim copying, provided also that
37: the entire resulting derived work is distributed under the terms of a
38: permission notice identical to this one.
39:
40: Permission is granted to copy and distribute translations of this manual
41: into another language, under the above conditions for modified versions.
42: @end ifinfo
43:
44: @titlepage
45: @c @finalout
46: @title The C Preprocessor
47: @subtitle Last revised July 1992
48: @subtitle for GCC version 2
49: @author Richard M. Stallman
50: @page
51: @vskip 2pc
52: This booklet is eventually intended to form the first chapter of a GNU
53: C Language manual.
54:
55: @vskip 0pt plus 1filll
56: Copyright @copyright{} 1987, 1989, 1991, 1992 Free Software Foundation, Inc.
57:
58: Permission is granted to make and distribute verbatim copies of
59: this manual provided the copyright notice and this permission notice
60: are preserved on all copies.
61:
62: Permission is granted to copy and distribute modified versions of this
63: manual under the conditions for verbatim copying, provided also that
64: the entire resulting derived work is distributed under the terms of a
65: permission notice identical to this one.
66:
67: Permission is granted to copy and distribute translations of this manual
68: into another language, under the above conditions for modified versions.
69: @end titlepage
70: @page
71:
72: @node Top, Global Actions,, (DIR)
73: @chapter The C Preprocessor
74:
75: The C preprocessor is a @dfn{macro processor} that is used automatically by
76: the C compiler to transform your program before actual compilation. It is
77: called a macro processor because it allows you to define @dfn{macros},
78: which are brief abbreviations for longer constructs.
79:
80: The C preprocessor provides four separate facilities that you can use as
81: you see fit:
82:
83: @itemize @bullet
84: @item
85: Inclusion of header files. These are files of declarations that can be
86: substituted into your program.
87:
88: @item
89: Macro expansion. You can define @dfn{macros}, which are abbreviations
90: for arbitrary fragments of C code, and then the C preprocessor will
91: replace the macros with their definitions throughout the program.
92:
93: @item
94: Conditional compilation. Using special preprocessor commands, you
95: can include or exclude parts of the program according to various
96: conditions.
97:
98: @item
99: Line control. If you use a program to combine or rearrange source files into
100: an intermediate file which is then compiled, you can use line control
101: to inform the compiler of where each source line originally came from.
102: @end itemize
103:
104: C preprocessors vary in some details. This manual discusses the GNU C
105: preprocessor, the C Compatible Compiler Preprocessor. The GNU C
106: preprocessor provides a superset of the features of ANSI Standard C.
107:
108: ANSI Standard C requires the rejection of many harmless constructs commonly
109: used by today's C programs. Such incompatibility would be inconvenient for
110: users, so the GNU C preprocessor is configured to accept these constructs
111: by default. Strictly speaking, to get ANSI Standard C, you must use the
112: options @samp{-trigraphs}, @samp{-undef} and @samp{-pedantic}, but in
113: practice the consequences of having strict ANSI Standard C make it
114: undesirable to do this. @xref{Invocation}.
115:
116: @menu
117: * Global Actions:: Actions made uniformly on all input files.
118: * Commands:: General syntax of preprocessor commands.
119: * Header Files:: How and why to use header files.
120: * Macros:: How and why to use macros.
121: * Conditionals:: How and why to use conditionals.
122: * Combining Sources:: Use of line control when you combine source files.
123: * Other Commands:: Miscellaneous preprocessor commands.
124: * Output:: Format of output from the C preprocessor.
125: * Invocation:: How to invoke the preprocessor; command options.
126: * Concept Index:: Index of concepts and terms.
127: * Index:: Index of commands, predefined macros and options.
128: @end menu
129:
130: @node Global Actions, Commands, Top, Top
131: @section Transformations Made Globally
132:
133: Most C preprocessor features are inactive unless you give specific commands
134: to request their use. (Preprocessor commands are lines starting with
135: @samp{#}; @pxref{Commands}). But there are three transformations that the
136: preprocessor always makes on all the input it receives, even in the absence
137: of commands.
138:
139: @itemize @bullet
140: @item
141: All C comments are replaced with single spaces.
142:
143: @item
144: Backslash-Newline sequences are deleted, no matter where. This
145: feature allows you to break long lines for cosmetic purposes without
146: changing their meaning.
147:
148: @item
149: Predefined macro names are replaced with their expansions
150: (@pxref{Predefined}).
151: @end itemize
152:
153: The first two transformations are done @emph{before} nearly all other parsing
154: and before preprocessor commands are recognized. Thus, for example, you
155: can split a line cosmetically with Backslash-Newline anywhere (except
156: when trigraphs are in use; see below).
157:
158: @example
159: /*
160: */ # /*
161: */ defi\
162: ne FO\
163: O 10\
164: 20
165: @end example
166:
167: @noindent
168: is equivalent into @samp{#define FOO 1020}. You can split even an escape
169: sequence with Backslash-Newline. For example, you can split @code{"foo\bar"}
170: between the @samp{\} and the @samp{b} to get
171:
172: @example
173: "foo\\
174: bar"
175: @end example
176:
177: @noindent
178: This behavior is unclean: in all other contexts, a Backslash can be
179: inserted in a string constant as an ordinary character by writing a double
180: Backslash, and this creates an exception. But the ANSI C standard requires
181: it. (Strict ANSI C does not allow Newlines in string constants, so they
182: do not consider this a problem.)
183:
184: But there are a few exceptions to all three transformations.
185:
186: @itemize @bullet
187: @item
188: C comments and predefined macro names are not recognized inside a
189: @samp{#include} command in which the file name is delimited with
190: @samp{<} and @samp{>}.
191:
192: @item
193: C comments and predefined macro names are never recognized within a
194: character or string constant. (Strictly speaking, this is the rule,
195: not an exception, but it is worth noting here anyway.)
196:
197: @item
198: Backslash-Newline may not safely be used within an ANSI ``trigraph''.
199: Trigraphs are converted before Backslash-Newline is deleted. If you
200: write what looks like a trigraph with a Backslash-Newline inside, the
201: Backslash-Newline is deleted as usual, but it is then too late to
202: recognize the trigraph.
203:
204: This exception is relevant only if you use the @samp{-trigraphs}
205: option to enable trigraph processing. @xref{Invocation}.
206: @end itemize
207:
208: @node Commands, Header Files, Global Actions, Top
209: @section Preprocessor Commands
210:
211: @cindex preprocessor commands
212: @cindex commands
213: Most preprocessor features are active only if you use preprocessor commands
214: to request their use.
215:
216: Preprocessor commands are lines in your program that start with @samp{#}.
217: The @samp{#} is followed by an identifier that is the @dfn{command name}.
218: For example, @samp{#define} is the command that defines a macro.
219: Whitespace is also allowed before and after the @samp{#}.
220:
221: The set of valid command names is fixed. Programs cannot define new
222: preprocessor commands.
223:
224: Some command names require arguments; these make up the rest of the command
225: line and must be separated from the command name by whitespace. For example,
226: @samp{#define} must be followed by a macro name and the intended expansion
227: of the macro.
228:
229: A preprocessor command cannot be more than one line in normal circumstances.
230: It may be split cosmetically with Backslash-Newline, but that has no effect
231: on its meaning. Comments containing Newlines can also divide the command into
232: multiple lines, but the comments are changed to Spaces before the command
233: is interpreted. The only way a significant Newline can occur in a preprocessor
234: command is within a string constant or character constant. Note that
235: most C compilers that might be applied to the output from the preprocessor
236: do not accept string or character constants containing Newlines.
237:
238: The @samp{#} and the command name cannot come from a macro expansion. For
239: example, if @samp{foo} is defined as a macro expanding to @samp{define},
240: that does not make @samp{#foo} a valid preprocessor command.
241:
242: @node Header Files, Macros, Commands, Top
243: @section Header Files
244:
245: @cindex header file
246: A header file is a file containing C declarations and macro definitions
247: (@pxref{Macros}) to be shared between several source files. You request
248: the use of a header file in your program with the C preprocessor command
249: @samp{#include}.
250:
251: @menu
252: * Header Uses:: What header files are used for.
253: * Include Syntax:: How to write @samp{#include} commands.
254: * Include Operation:: What @samp{#include} does.
255: * Once-Only:: Preventing multiple inclusion of one header file.
256: * Inheritance:: Including one header file in another header file.
257: @end menu
258:
259: @node Header Uses, Include Syntax, Header Files, Header Files
260: @subsection Uses of Header Files
261:
262: Header files serve two kinds of purposes.
263:
264: @itemize @bullet
265: @item
266: @findex system header files
267: System header files declare the interfaces to parts of the operating
268: system. You include them in your program to supply the definitions and
269: declarations you need to invoke system calls and libraries.
270:
271: @item
272: Your own header files contain declarations for interfaces between the
273: source files of your program. Each time you have a group of related
274: declarations and macro definitions all or most of which are needed in
275: several different source files, it is a good idea to create a header
276: file for them.
277: @end itemize
278:
279: Including a header file produces the same results in C compilation as
280: copying the header file into each source file that needs it. But such
281: copying would be time-consuming and error-prone. With a header file, the
282: related declarations appear in only one place. If they need to be changed,
283: they can be changed in one place, and programs that include the header file
284: will automatically use the new version when next recompiled. The header
285: file eliminates the labor of finding and changing all the copies as well as
286: the risk that a failure to find one copy will result in inconsistencies
287: within a program.
288:
289: The usual convention is to give header files names that end with @file{.h}.
290:
291: @node Include Syntax, Include Operation, Header Uses, Header Files
292: @subsection The @samp{#include} Command
293:
294: @findex #include
295: Both user and system header files are included using the preprocessor
296: command @samp{#include}. It has three variants:
297:
298: @table @code
299: @item #include <@var{file}>
300: This variant is used for system header files. It searches for a file
301: named @var{file} in a list of directories specified by you, then in a
302: standard list of system directories. You specify directories to
303: search for header files with the command option @samp{-I}
304: (@pxref{Invocation}). The option @samp{-nostdinc} inhibits searching
305: the standard system directories; in this case only the directories
306: you specify are searched.
307:
308: The parsing of this form of @samp{#include} is slightly special
309: because comments are not recognized within the @samp{<@dots{}>}.
310: Thus, in @samp{#include <x/*y>} the @samp{/*} does not start a comment
311: and the command specifies inclusion of a system header file named
312: @file{x/*y}. Of course, a header file with such a name is unlikely to
313: exist on Unix, where shell wildcard features would make it hard to
314: manipulate.@refill
315:
316: The argument @var{file} may not contain a @samp{>} character. It may,
317: however, contain a @samp{<} character.
318:
319: @item #include "@var{file}"
320: This variant is used for header files of your own program. It
321: searches for a file named @var{file} first in the current directory,
322: then in the same directories used for system header files. The
323: current directory is the directory of the current input file. It is
324: tried first because it is presumed to be the location of the files
325: that the current input file refers to. (If the @samp{-I-} option is
326: used, the special treatment of the current directory is inhibited.)
327:
328: The argument @var{file} may not contain @samp{"} characters. If
329: backslashes occur within @var{file}, they are considered ordinary text
330: characters, not escape characters. None of the character escape
331: sequences appropriate to string constants in C are processed. Thus,
332: @samp{#include "x\n\\y"} specifies a filename containing three
333: backslashes. It is not clear why this behavior is ever useful, but
334: the ANSI standard specifies it.
335:
336: @item #include @var{anything else}
337: This variant is called a @dfn{computed #include}. Any @samp{#include}
338: command whose argument does not fit the above two forms is a computed
339: include. The text @var{anything else} is checked for macro calls,
340: which are expanded (@pxref{Macros}). When this is done, the result
341: must fit one of the above two variants---in particular, the expanded
342: text must in the end be surrounded by either quotes or angle braces.
343:
344: This feature allows you to define a macro which controls the file name
345: to be used at a later point in the program. One application of this
346: is to allow a site-configuration file for your program to specify the
347: names of the system include files to be used. This can help in
348: porting the program to various operating systems in which the
349: necessary system header files are found in different places.
350: @end table
351:
352: @node Include Operation, Once-Only, Include Syntax, Header Files
353: @subsection How @samp{#include} Works
354:
355: The @samp{#include} command works by directing the C preprocessor to scan
356: the specified file as input before continuing with the rest of the current
357: file. The output from the preprocessor contains the output already
358: generated, followed by the output resulting from the included file,
359: followed by the output that comes from the text after the @samp{#include}
360: command. For example, given two files as follows:
361:
362: @example
363: /* File program.c */
364: int x;
365: #include "header.h"
366:
367: main ()
368: @{
369: printf (test ());
370: @}
371:
372:
373: /* File header.h */
374: char *test ();
375: @end example
376:
377: @noindent
378: the output generated by the C preprocessor for @file{program.c} as input
379: would be
380:
381: @example
382: int x;
383: char *test ();
384:
385: main ()
386: @{
387: printf (test ());
388: @}
389: @end example
390:
391: Included files are not limited to declarations and macro definitions; those
392: are merely the typical uses. Any fragment of a C program can be included
393: from another file. The include file could even contain the beginning of a
394: statement that is concluded in the containing file, or the end of a
395: statement that was started in the including file. However, a comment or a
396: string or character constant may not start in the included file and finish
397: in the including file. An unterminated comment, string constant or
398: character constant in an included file is considered to end (with an error
399: message) at the end of the file.
400:
401: The line following the @samp{#include} command is always treated as a
402: separate line by the C preprocessor even if the included file lacks a final
403: newline.
404:
405: @node Once-Only, Inheritance, Include Operation, Header Files
406: @subsection Once-Only Include Files
407: @cindex repeated inclusion
408:
409: Very often, one header file includes another. It can easily result that a
410: certain header file is included more than once. This may lead to errors,
411: if the header file defines structure types or typedefs, and is certainly
412: wasteful. Therefore, we often wish to prevent multiple inclusion of a
413: header file.
414:
415: The standard way to do this is to enclose the entire real contents of the
416: file in a conditional, like this:
417:
418: @example
419: #ifndef __FILE_FOO_SEEN__
420: #define __FILE_FOO_SEEN__
421:
422: @var{the entire file}
423:
424: #endif /* __FILE_FOO_SEEN__ */
425: @end example
426:
427: The macro @code{__FILE_FOO_SEEN__} indicates that the file has been
428: included once already; its name should begin with @samp{__} to avoid
429: conflicts with user programs, and it should contain the name of the file
430: and some additional text, to avoid conflicts with other header files.
431:
432: The GNU C preprocessor is programmed to notice when a header file uses
433: this particular construct and handle it efficiently. If a header file
434: is contained entirely in a @samp{#ifndef} conditional, then it records
435: that fact. If a subsequent @samp{#include} specifies the same file,
436: and the macro in the @samp{#ifndef} is already defined, then the file
437: is entirely skipped, without even reading it.
438:
439: @findex #pragma once
440: There is also an explicit command to tell the preprocessor that it need
441: not include a file more than once. This is called @samp{#pragma once},
442: and was used @emph{in addition to} the @samp{#ifndef} conditional around
443: the contents of the header file. @samp{#pragma once} is now obsolete
444: and should not be used at all.
445:
446: In the Objective C language, there is a variant of @samp{#include}
447: called @samp{#import} which includes a file, but does so at most once.
448: If you use @samp{#import} @emph{instead of} @samp{#include}, then you
449: don't need the conditionals inside the header file to prevent multiple
450: execution of the contents.
451:
452: @samp{#import} is obsolete because it is not a well-designed feature.
453: It requires the users of a header file---the applications
454: programmers---to know that a certain header file should only be included
455: once. It is much better for the header file's implementor to write the
456: file so that users don't need to know this. Using @samp{#ifndef}
457: accomplishes this goal.
458:
459: @node Inheritance,, Once-Only, Header Files
460: @section Inheritance and Header Files
461: @cindex inheritance
462: @cindex overriding a header file
463:
464: @dfn{Inheritance} is what happens when one object or file derives some
465: of its contents by virtual copying from another object or file. In
466: the case of C header files, inheritance means that one header file
467: includes another header file and then replaces or adds something.
468:
469: If the inheriting header file and the base header file have different
470: names, then inheritance is straightforward: simply write @samp{#include
471: "@var{base}"} in the inheriting file.
472:
473: Sometimes it is necessary to give the inheriting file the same name as
474: the base file. This is less straightforward.
475:
476: For example, suppose an application program uses the system header file
477: @file{sys/signal.h}, but the version of @file{/usr/include/sys/signal.h}
478: on a particular system doesn't do what the application program expects.
479: It might be convenient to define a ``local'' version, perhaps under the
480: name @file{/usr/local/include/sys/signal.h}, to override or add to the
481: one supplied by the system.
482:
483: You can do this by using the option @samp{-I.} for compilation, and
484: writing a file @file{sys/signal.h} that does what the application
485: program expects. But making this file include the standard
486: @file{sys/signal.h} is not so easy---writing @samp{#include
487: <sys/signal.h>} in that file doesn't work, because it includes your own
488: version of the file, not the standard system version. Used in that file
489: itself, this leads to an infinite recursion and a fatal error in
490: compilation.
491:
492: @samp{#include </usr/include/sys/signal.h>} would find the proper file,
493: but that is not clean, since it makes an assumption about where the
494: system header file is found. This is bad for maintenance, since it
495: means that any change in where the system's header files are kept
496: requires a change somewhere else.
497:
498: @findex #include_next
499: The clean way to solve this problem is to use
500: @samp{#include_next}, which means, ``Include the @emph{next} file with
501: this name.'' This command works like @samp{#include} except in
502: searching for the specified file: it starts searching the list of header
503: file directories @emph{after} the directory in which the current file
504: was found.
505:
506: Suppose you specify @samp{-I /usr/local/include}, and the list of
507: directories to search also includes @file{/usr/include}; and suppose that
508: both directories contain a file named @file{sys/signal.h}. Ordinary
509: @samp{#include <sys/signal.h>} finds the file under
510: @file{/usr/local/include}. If that file contains @samp{#include_next
511: <sys/signal.h>}, it starts searching after that directory, and finds the
512: file in @file{/usr/include}.
513:
514: @node Macros, Conditionals, Header Files, Top
515: @section Macros
516:
517: A macro is a sort of abbreviation which you can define once and then
518: use later. There are many complicated features associated with macros
519: in the C preprocessor.
520:
521: @menu
522: * Simple Macros:: Macros that always expand the same way.
523: * Argument Macros:: Macros that accept arguments that are substituted
524: into the macro expansion.
525: * Predefined:: Predefined macros that are always available.
526: * Stringification:: Macro arguments converted into string constants.
527: * Concatenation:: Building tokens from parts taken from macro arguments.
528: * Undefining:: Cancelling a macro's definition.
529: * Redefining:: Changing a macro's definition.
530: * Macro Pitfalls:: Macros can confuse the unwary. Here we explain
531: several common problems and strange features.
532: @end menu
533:
534: @node Simple Macros, Argument Macros, Macros, Macros
535: @subsection Simple Macros
536:
537: A @dfn{simple macro} is a kind of abbreviation. It is a name which
538: stands for a fragment of code. Some people refer to these as
539: @dfn{manifest constants}.
540:
541: Before you can use a macro, you must @dfn{define} it explicitly with the
542: @samp{#define} command. @samp{#define} is followed by the name of the
543: macro and then the code it should be an abbreviation for. For example,
544:
545: @example
546: #define BUFFER_SIZE 1020
547: @end example
548:
549: @noindent
550: defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the text
551: @samp{1020}. Therefore, if somewhere after this @samp{#define} command
552: there comes a C statement of the form
553:
554: @example
555: foo = (char *) xmalloc (BUFFER_SIZE);
556: @end example
557:
558: @noindent
559: then the C preprocessor will recognize and @dfn{expand} the macro
560: @samp{BUFFER_SIZE}, resulting in
561:
562: @example
563: foo = (char *) xmalloc (1020);
564: @end example
565:
566: @noindent
567: the definition must be a single line; however, it may not end in the
568: middle of a multi-line string constant or character constant.
569:
570: The use of all upper case for macro names is a standard convention.
571: Programs are easier to read when it is possible to tell at a glance which
572: names are macros.
573:
574: Normally, a macro definition must be a single line, like all C preprocessor
575: commands. (You can split a long macro definition cosmetically with
576: Backslash-Newline.) There is one exception: Newlines can be included in
577: the macro definition if within a string or character constant. By the same
578: token, it is not possible for a macro definition to contain an unbalanced
579: quote character; the definition automatically extends to include the
580: matching quote character that ends the string or character constant.
581: Comments within a macro definition may contain Newlines, which make no
582: difference since the comments are entirely replaced with Spaces regardless
583: of their contents.
584:
585: Aside from the above, there is no restriction on what can go in a macro
586: body. Parentheses need not balance. The body need not resemble valid C
587: code. (Of course, you might get error messages from the C compiler when
588: you use the macro.)
589:
590: The C preprocessor scans your program sequentially, so macro definitions
591: take effect at the place you write them. Therefore, the following input to
592: the C preprocessor
593:
594: @example
595: foo = X;
596: #define X 4
597: bar = X;
598: @end example
599:
600: @noindent
601: produces as output
602:
603: @example
604: foo = X;
605:
606: bar = 4;
607: @end example
608:
609: After the preprocessor expands a macro name, the macro's definition body is
610: appended to the front of the remaining input, and the check for macro calls
611: continues. Therefore, the macro body can contain calls to other macros.
612: For example, after
613:
614: @example
615: #define BUFSIZE 1020
616: #define TABLESIZE BUFSIZE
617: @end example
618:
619: @noindent
620: the name @samp{TABLESIZE} when used in the program would go through two
621: stages of expansion, resulting ultimately in @samp{1020}.
622:
623: This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
624: The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
625: specify---in this case, @samp{BUFSIZE}---and does not check to see whether
626: it too is the name of a macro. It's only when you @emph{use} @samp{TABLESIZE}
627: that the result of its expansion is checked for more macro names.
628: @xref{Cascaded Macros}.
629:
630: @node Argument Macros, Predefined, Simple Macros, Macros
631: @subsection Macros with Arguments
632:
633: A simple macro always stands for exactly the same text, each time it is
634: used. Macros can be more flexible when they accept @dfn{arguments}.
635: Arguments are fragments of code that you supply each time the macro is
636: used. These fragments are included in the expansion of the macro according
637: to the directions in the macro definition.
638:
639: To define a macro that uses arguments, you write a @samp{#define} command
640: with a list of @dfn{argument names} in parentheses after the name of the
641: macro. The argument names may be any valid C identifiers, separated by
642: commas and optionally whitespace. The open-parenthesis must follow the
643: macro name immediately, with no space in between.
644:
645: For example, here is a macro that computes the minimum of two numeric
646: values, as it is defined in many C programs:
647:
648: @example
649: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
650: @end example
651:
652: @noindent
653: (This is not the best way to define a ``minimum'' macro in GNU C.
654: @xref{Side Effects}, for more information.)
655:
656: To use a macro that expects arguments, you write the name of the macro
657: followed by a list of @dfn{actual arguments} in parentheses, separated by
658: commas. The number of actual arguments you give must match the number of
659: arguments the macro expects. Examples of use of the macro @samp{min}
660: include @samp{min (1, 2)} and @samp{min (x + 28, *p)}.
661:
662: The expansion text of the macro depends on the arguments you use.
663: Each of the argument names of the macro is replaced, throughout the
664: macro definition, with the corresponding actual argument. Using the
665: same macro @samp{min} defined above, @samp{min (1, 2)} expands into
666:
667: @example
668: ((1) < (2) ? (1) : (2))
669: @end example
670:
671: @noindent
672: where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.
673:
674: Likewise, @samp{min (x + 28, *p)} expands into
675:
676: @example
677: ((x + 28) < (*p) ? (x + 28) : (*p))
678: @end example
679:
680: Parentheses in the actual arguments must balance; a comma within
681: parentheses does not end an argument. However, there is no requirement
682: for brackets or braces to balance, and they do not prevent a comma from
683: separating arguments. Thus,
684:
685: @example
686: macro (array[x = y, x + 1])
687: @end example
688:
689: @noindent
690: passes two arguments to @code{macro}: @samp{array[x = y} and @samp{x +
691: 1]}. If you want to supply @samp{array[x = y, x + 1]} as an argument,
692: you must write it as @samp{array[(x = y, x + 1)]}, which is equivalent C
693: code.
694:
695: After the actual arguments are substituted into the macro body, the entire
696: result is appended to the front of the remaining input, and the check for
697: macro calls continues. Therefore, the actual arguments can contain calls
698: to other macros, either with or without arguments, or even to the same
699: macro. The macro body can also contain calls to other macros. For
700: example, @samp{min (min (a, b), c)} expands into this text:
701:
702: @example
703: ((((a) < (b) ? (a) : (b))) < (c)
704: ? (((a) < (b) ? (a) : (b)))
705: : (c))
706: @end example
707:
708: @noindent
709: (Line breaks shown here for clarity would not actually be generated.)
710:
711: If a macro @code{foo} takes one argument, and you want to supply an
712: empty argument, you must write at least some whitespace between the
713: parentheses, like this: @samp{foo ( )}. Just @samp{foo ()} is providing
714: no arguments, which is an error if @code{foo} expects an argument. But
715: @samp{foo0 ()} is the correct way to call a macro defined to take zero
716: arguments, like this:
717:
718: @example
719: #define foo0() @dots{}
720: @end example
721:
722: If you use the macro name followed by something other than an
723: open-parenthesis (after ignoring any spaces, tabs and comments that
724: follow), it is not a call to the macro, and the preprocessor does not
725: change what you have written. Therefore, it is possible for the same name
726: to be a variable or function in your program as well as a macro, and you
727: can choose in each instance whether to refer to the macro (if an actual
728: argument list follows) or the variable or function (if an argument list
729: does not follow).
730:
731: Such dual use of one name could be confusing and should be avoided
732: except when the two meanings are effectively synonymous: that is, when the
733: name is both a macro and a function and the two have similar effects. You
734: can think of the name simply as a function; use of the name for purposes
735: other than calling it (such as, to take the address) will refer to the
736: function, while calls will expand the macro and generate better but
737: equivalent code. For example, you can use a function named @samp{min} in
738: the same source file that defines the macro. If you write @samp{&min} with
739: no argument list, you refer to the function. If you write @samp{min (x,
740: bb)}, with an argument list, the macro is expanded. If you write
741: @samp{(min) (a, bb)}, where the name @samp{min} is not followed by an
742: open-parenthesis, the macro is not expanded, so you wind up with a call to
743: the function @samp{min}.
744:
745: You may not define the same name as both a simple macro and a macro with
746: arguments.
747:
748: In the definition of a macro with arguments, the list of argument names
749: must follow the macro name immediately with no space in between. If there
750: is a space after the macro name, the macro is defined as taking no
751: arguments, and all the rest of the line is taken to be the expansion. The
752: reason for this is that it is often useful to define a macro that takes no
753: arguments and whose definition begins with an identifier in parentheses.
754: This rule about spaces makes it possible for you to do either this:
755:
756: @example
757: #define FOO(x) - 1 / (x)
758: @end example
759:
760: @noindent
761: (which defines @samp{FOO} to take an argument and expand into minus the
762: reciprocal of that argument) or this:
763:
764: @example
765: #define BAR (x) - 1 / (x)
766: @end example
767:
768: @noindent
769: (which defines @samp{BAR} to take no argument and always expand into
770: @samp{(x) - 1 / (x)}).
771:
772: Note that the @emph{uses} of a macro with arguments can have spaces before
773: the left parenthesis; it's the @emph{definition} where it matters whether
774: there is a space.
775:
776: @node Predefined, Stringification, Argument Macros, Macros
777: @subsection Predefined Macros
778:
779: @cindex predefined macros
780: Several simple macros are predefined. You can use them without giving
781: definitions for them. They fall into two classes: standard macros and
782: system-specific macros.
783:
784: @menu
785: * Standard Predefined:: Standard predefined macros.
786: * Nonstandard Predefined:: Nonstandard predefined macros.
787: @end menu
788:
789: @node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
790: @subsubsection Standard Predefined Macros
791:
792: The standard predefined macros are available with the same meanings
793: regardless of the machine or operating system on which you are using GNU C.
794: Their names all start and end with double underscores. Those preceding
795: @code{__GNUC__} in this table are standardized by ANSI C; the rest are
796: GNU C extensions.
797:
798: @table @code
799: @item __FILE__
800: @findex __FILE__
801: This macro expands to the name of the current input file, in the form of
802: a C string constant. The precise name returned is the one that was
803: specified in @samp{#include} or as the input file name argument.
804:
805: @item __LINE__
806: @findex __LINE__
807: This macro expands to the current input line number, in the form of a
808: decimal integer constant. While we call it a predefined macro, it's
809: a pretty strange macro, since its ``definition'' changes with each
810: new line of source code.
811:
812: This and @samp{__FILE__} are useful in generating an error message to
813: report an inconsistency detected by the program; the message can state
814: the source line at which the inconsistency was detected. For example,
815:
816: @smallexample
817: fprintf (stderr, "Internal error: "
818: "negative string length "
819: "%d at %s, line %d.",
820: length, __FILE__, __LINE__);
821: @end smallexample
822:
823: A @samp{#include} command changes the expansions of @samp{__FILE__}
824: and @samp{__LINE__} to correspond to the included file. At the end of
825: that file, when processing resumes on the input file that contained
826: the @samp{#include} command, the expansions of @samp{__FILE__} and
827: @samp{__LINE__} revert to the values they had before the
828: @samp{#include} (but @samp{__LINE__} is then incremented by one as
829: processing moves to the line after the @samp{#include}).
830:
831: The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
832: if a @samp{#line} command is used. @xref{Combining Sources}.
833:
834: @item __INCLUDE_LEVEL__
835: @findex __INCLUDE_LEVEL_
836: This macro expands to a decimal integer constant that represents the
837: depth of nesting in include files. The value of this macro is
838: incremented on every @samp{#include} command and decremented at every
839: end of file.
840:
841: @item __DATE__
842: @findex __DATE__
843: This macro expands to a string constant that describes the date on
844: which the preprocessor is being run. The string constant contains
845: eleven characters and looks like @samp{"Jan 29 1987"} or @w{@samp{"Apr
846: 1 1905"}}.
847:
848: @item __TIME__
849: @findex __TIME__
850: This macro expands to a string constant that describes the time at
851: which the preprocessor is being run. The string constant contains
852: eight characters and looks like @samp{"23:59:01"}.
853:
854: @item __STDC__
855: @findex __STDC__
856: This macro expands to the constant 1, to signify that this is ANSI
857: Standard C. (Whether that is actually true depends on what C compiler
858: will operate on the output from the preprocessor.)
859:
860: @item __GNUC__
861: This macro is defined if and only if this is GNU C. This macro is
862: defined only when the entire GNU C compiler is in use; if you invoke
863: the preprocessor directly, @samp{__GNUC__} is undefined.
864:
865: @item __GNUG__
866: The GNU C compiler defines this when the compilation language is
867: C++; use @samp{__GNUG__} to distinguish between GNU C and GNU
868: C++.
869:
870: @item __cplusplus
871: The draft ANSI standard for C++ used to require predefining this
872: variable. Though it is no longer required, GNU C++ continues to define
873: it, as do other popular C++ compilers. You can use @samp{__cplusplus}
874: to test whether a header is compiled by a C compiler or a C++ compiler.
875:
876: @item __STRICT_ANSI__
877: This macro is defined if and only if the @samp{-ansi} switch was
878: specified when GNU C was invoked. Its definition is the null string.
879: This macro exists primarily to direct certain GNU header files not to
880: define certain traditional Unix constructs which are incompatible with
881: ANSI C.
882:
883: @item __BASE_FILE__
884: @findex __BASE_FILE__
885: This macro expands to the name of the main input file, in the form
886: of a C string constant. This is the source file that was specified
887: as an argument when the C compiler was invoked.
888:
889: @item __VERSION__
890: This macro expands to a string which describes the version number of
891: GNU C. The string is normally a sequence of decimal numbers separated
892: by periods, such as @samp{"1.18"}. The only reasonable use of this
893: macro is to incorporate it into a string constant.
894:
895: @item __OPTIMIZE__
896: This macro is defined in optimizing compilations. It causes certain
897: GNU header files to define alternative macro definitions for some
898: system library functions. It is unwise to refer to or test the
899: definition of this macro unless you make very sure that programs will
900: execute with the same effect regardless.
901:
902: @item __CHAR_UNSIGNED__
903: This macro is defined if and only if the data type @code{char} is
904: unsigned on the target machine. It exists to cause the standard
905: header file @file{limit.h} to work correctly. It is bad practice
906: to refer to this macro yourself; instead, refer to the standard
907: macros defined in @file{limit.h}. The preprocessor uses
908: this macro to determine whether or not to sign-extend large character
909: constants written in octal; see @ref{#if Command,,The @samp{#if} Command}.
910: @end table
911:
912: @node Nonstandard Predefined,, Standard Predefined, Predefined
913: @subsubsection Nonstandard Predefined Macros
914:
915: The C preprocessor normally has several predefined macros that vary between
916: machines because their purpose is to indicate what type of system and
917: machine is in use. This manual, being for all systems and machines, cannot
918: tell you exactly what their names are; instead, we offer a list of some
919: typical ones. You can use @samp{cpp -dM} to see the values of
920: predefined macros; @pxref{Invocation}.
921:
922: Some nonstandard predefined macros describe the operating system in use,
923: with more or less specificity. For example,
924:
925: @table @code
926: @item unix
927: @findex unix
928: @samp{unix} is normally predefined on all Unix systems.
929:
930: @item BSD
931: @findex BSD
932: @samp{BSD} is predefined on recent versions of Berkeley Unix
933: (perhaps only in version 4.3).
934: @end table
935:
936: Other nonstandard predefined macros describe the kind of CPU, with more or
937: less specificity. For example,
938:
939: @table @code
940: @item vax
941: @findex vax
942: @samp{vax} is predefined on Vax computers.
943:
944: @item mc68000
945: @findex mc68000
946: @samp{mc68000} is predefined on most computers whose CPU is a Motorola
947: 68000, 68010 or 68020.
948:
949: @item m68k
950: @findex m68k
951: @samp{m68k} is also predefined on most computers whose CPU is a 68000,
952: 68010 or 68020; however, some makers use @samp{mc68000} and some use
953: @samp{m68k}. Some predefine both names. What happens in GNU C
954: depends on the system you are using it on.
955:
956: @item M68020
957: @findex M68020
958: @samp{M68020} has been observed to be predefined on some systems that
959: use 68020 CPUs---in addition to @samp{mc68000} and @samp{m68k}, which
960: are less specific.
961:
962: @item _AM29K
963: @findex _AM29K
964: @itemx _AM29000
965: @findex _AM29000
966: Both @samp{_AM29K} and @samp{_AM29000} are predefined for the AMD 29000
967: CPU family.
968:
969: @item ns32000
970: @findex ns32000
971: @samp{ns32000} is predefined on computers which use the National
972: Semiconductor 32000 series CPU.
973: @end table
974:
975: Yet other nonstandard predefined macros describe the manufacturer of
976: the system. For example,
977:
978: @table @code
979: @item sun
980: @findex sun
981: @samp{sun} is predefined on all models of Sun computers.
982:
983: @item pyr
984: @findex pyr
985: @samp{pyr} is predefined on all models of Pyramid computers.
986:
987: @item sequent
988: @findex sequent
989: @samp{sequent} is predefined on all models of Sequent computers.
990: @end table
991:
992: These predefined symbols are not only nonstandard, they are contrary to the
993: ANSI standard because their names do not start with underscores.
994: Therefore, the option @samp{-ansi} inhibits the definition of these
995: symbols.
996:
997: This tends to make @samp{-ansi} useless, since many programs depend on the
998: customary nonstandard predefined symbols. Even system header files check
999: them and will generate incorrect declarations if they do not find the names
1000: that are expected. You might think that the header files supplied for the
1001: Uglix computer would not need to test what machine they are running on,
1002: because they can simply assume it is the Uglix; but often they do, and they
1003: do so using the customary names. As a result, very few C programs will
1004: compile with @samp{-ansi}. We intend to avoid such problems on the GNU
1005: system.
1006:
1007: What, then, should you do in an ANSI C program to test the type of machine
1008: it will run on?
1009:
1010: GNU C offers a parallel series of symbols for this purpose, whose names
1011: are made from the customary ones by adding @samp{__} at the beginning
1012: and end. Thus, the symbol @code{__vax__} would be available on a Vax,
1013: and so on.
1014:
1015: The set of nonstandard predefined names in the GNU C preprocessor is
1016: controlled (when @code{cpp} is itself compiled) by the macro
1017: @samp{CPP_PREDEFINES}, which should be a string containing @samp{-D}
1018: options, separated by spaces. For example, on the Sun 3, we use the
1019: following definition:
1020:
1021: @example
1022: #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
1023: @end example
1024:
1025: @noindent
1026: This macro is usually specified in @file{tm.h}.
1027:
1028: @node Stringification, Concatenation, Predefined, Macros
1029: @subsection Stringification
1030:
1031: @cindex stringification
1032: @dfn{Stringification} means turning a code fragment into a string constant
1033: whose contents are the text for the code fragment. For example,
1034: stringifying @samp{foo (z)} results in @samp{"foo (z)"}.
1035:
1036: In the C preprocessor, stringification is an option available when macro
1037: arguments are substituted into the macro definition. In the body of the
1038: definition, when an argument name appears, the character @samp{#} before
1039: the name specifies stringification of the corresponding actual argument
1040: when it is substituted at that point in the definition. The same argument
1041: may be substituted in other places in the definition without
1042: stringification if the argument name appears in those places with no
1043: @samp{#}.
1044:
1045: Here is an example of a macro definition that uses stringification:
1046:
1047: @smallexample
1048: #define WARN_IF(EXP) \
1049: do @{ if (EXP) \
1050: fprintf (stderr, "Warning: " #EXP "\n"); @} \
1051: while (0)
1052: @end smallexample
1053:
1054: @noindent
1055: Here the actual argument for @samp{EXP} is substituted once as given,
1056: into the @samp{if} statement, and once as stringified, into the
1057: argument to @samp{fprintf}. The @samp{do} and @samp{while (0)} are
1058: a kludge to make it possible to write @samp{WARN_IF (@var{arg});},
1059: which the resemblance of @samp{WARN_IF} to a function would make
1060: C programmers want to do; @pxref{Swallow Semicolon}).
1061:
1062: The stringification feature is limited to transforming one macro argument
1063: into one string constant: there is no way to combine the argument with
1064: other text and then stringify it all together. But the example above shows
1065: how an equivalent result can be obtained in ANSI Standard C using the
1066: feature that adjacent string constants are concatenated as one string
1067: constant. The preprocessor stringifies the actual value of @samp{EXP}
1068: into a separate string constant, resulting in text like
1069:
1070: @smallexample
1071: do @{ if (x == 0) \
1072: fprintf (stderr, "Warning: " "x == 0" "\n"); @} \
1073: while (0)
1074: @end smallexample
1075:
1076: @noindent
1077: but the C compiler then sees three consecutive string constants and
1078: concatenates them into one, producing effectively
1079:
1080: @smallexample
1081: do @{ if (x == 0) \
1082: fprintf (stderr, "Warning: x == 0\n"); @} \
1083: while (0)
1084: @end smallexample
1085:
1086: Stringification in C involves more than putting doublequote characters
1087: around the fragment; it is necessary to put backslashes in front of all
1088: doublequote characters, and all backslashes in string and character
1089: constants, in order to get a valid C string constant with the proper
1090: contents. Thus, stringifying @samp{p = "foo\n";} results in @samp{"p =
1091: \"foo\\n\";"}. However, backslashes that are not inside of string or
1092: character constants are not duplicated: @samp{\n} by itself stringifies to
1093: @samp{"\n"}.
1094:
1095: Whitespace (including comments) in the text being stringified is handled
1096: according to precise rules. All leading and trailing whitespace is ignored.
1097: Any sequence of whitespace in the middle of the text is converted to
1098: a single space in the stringified result.
1099:
1100: @node Concatenation, Undefining, Stringification, Macros
1101: @subsection Concatenation
1102:
1103: @cindex concatenation
1104: @dfn{Concatenation} means joining two strings into one. In the context
1105: of macro expansion, concatenation refers to joining two lexical units
1106: into one longer one. Specifically, an actual argument to the macro can be
1107: concatenated with another actual argument or with fixed text to produce
1108: a longer name. The longer name might be the name of a function,
1109: variable or type, or a C keyword; it might even be the name of another
1110: macro, in which case it will be expanded.
1111:
1112: When you define a macro, you request concatenation with the special
1113: operator @samp{##} in the macro body. When the macro is called,
1114: after actual arguments are substituted, all @samp{##} operators are
1115: deleted, and so is any whitespace next to them (including whitespace
1116: that was part of an actual argument). The result is to concatenate
1117: the syntactic tokens on either side of the @samp{##}.
1118:
1119: Consider a C program that interprets named commands. There probably needs
1120: to be a table of commands, perhaps an array of structures declared as
1121: follows:
1122:
1123: @example
1124: struct command
1125: @{
1126: char *name;
1127: void (*function) ();
1128: @};
1129:
1130: struct command commands[] =
1131: @{
1132: @{ "quit", quit_command@},
1133: @{ "help", help_command@},
1134: @dots{}
1135: @};
1136: @end example
1137:
1138: It would be cleaner not to have to give each command name twice, once in
1139: the string constant and once in the function name. A macro which takes the
1140: name of a command as an argument can make this unnecessary. The string
1141: constant can be created with stringification, and the function name by
1142: concatenating the argument with @samp{_command}. Here is how it is done:
1143:
1144: @example
1145: #define COMMAND(NAME) @{ #NAME, NAME ## _command @}
1146:
1147: struct command commands[] =
1148: @{
1149: COMMAND (quit),
1150: COMMAND (help),
1151: @dots{}
1152: @};
1153: @end example
1154:
1155: The usual case of concatenation is concatenating two names (or a name and a
1156: number) into a longer name. But this isn't the only valid case. It is
1157: also possible to concatenate two numbers (or a number and a name, such as
1158: @samp{1.5} and @samp{e3}) into a number. Also, multi-character operators
1159: such as @samp{+=} can be formed by concatenation. In some cases it is even
1160: possible to piece together a string constant. However, two pieces of text
1161: that don't together form a valid lexical unit cannot be concatenated. For
1162: example, concatenation with @samp{x} on one side and @samp{+} on the other
1163: is not meaningful because those two characters can't fit together in any
1164: lexical unit of C. The ANSI standard says that such attempts at
1165: concatenation are undefined, but in the GNU C preprocessor it is well
1166: defined: it puts the @samp{x} and @samp{+} side by side with no particular
1167: special results.
1168:
1169: Keep in mind that the C preprocessor converts comments to whitespace before
1170: macros are even considered. Therefore, you cannot create a comment by
1171: concatenating @samp{/} and @samp{*}: the @samp{/*} sequence that starts a
1172: comment is not a lexical unit, but rather the beginning of a ``long'' space
1173: character. Also, you can freely use comments next to a @samp{##} in a
1174: macro definition, or in actual arguments that will be concatenated, because
1175: the comments will be converted to spaces at first sight, and concatenation
1176: will later discard the spaces.
1177:
1178: @node Undefining, Redefining, Concatenation, Macros
1179: @subsection Undefining Macros
1180:
1181: @cindex undefining macros
1182: To @dfn{undefine} a macro means to cancel its definition. This is done
1183: with the @samp{#undef} command. @samp{#undef} is followed by the macro
1184: name to be undefined.
1185:
1186: Like definition, undefinition occurs at a specific point in the source
1187: file, and it applies starting from that point. The name ceases to be a
1188: macro name, and from that point on it is treated by the preprocessor as if
1189: it had never been a macro name.
1190:
1191: For example,
1192:
1193: @example
1194: #define FOO 4
1195: x = FOO;
1196: #undef FOO
1197: x = FOO;
1198: @end example
1199:
1200: @noindent
1201: expands into
1202:
1203: @example
1204: x = 4;
1205:
1206: x = FOO;
1207: @end example
1208:
1209: @noindent
1210: In this example, @samp{FOO} had better be a variable or function as well
1211: as (temporarily) a macro, in order for the result of the expansion to be
1212: valid C code.
1213:
1214: The same form of @samp{#undef} command will cancel definitions with
1215: arguments or definitions that don't expect arguments. The @samp{#undef}
1216: command has no effect when used on a name not currently defined as a macro.
1217:
1218: @node Redefining, Macro Pitfalls, Undefining, Macros
1219: @subsection Redefining Macros
1220:
1221: @cindex redefining macros
1222: @dfn{Redefining} a macro means defining (with @samp{#define}) a name that
1223: is already defined as a macro.
1224:
1225: A redefinition is trivial if the new definition is transparently identical
1226: to the old one. You probably wouldn't deliberately write a trivial
1227: redefinition, but they can happen automatically when a header file is
1228: included more than once (@pxref{Header Files}), so they are accepted
1229: silently and without effect.
1230:
1231: Nontrivial redefinition is considered likely to be an error, so
1232: it provokes a warning message from the preprocessor. However, sometimes it
1233: is useful to change the definition of a macro in mid-compilation. You can
1234: inhibit the warning by undefining the macro with @samp{#undef} before the
1235: second definition.
1236:
1237: In order for a redefinition to be trivial, the new definition must
1238: exactly match the one already in effect, with two possible exceptions:
1239:
1240: @itemize @bullet
1241: @item
1242: Whitespace may be added or deleted at the beginning or the end.
1243:
1244: @item
1245: Whitespace may be changed in the middle (but not inside strings).
1246: However, it may not be eliminated entirely, and it may not be added
1247: where there was no whitespace at all.
1248: @end itemize
1249:
1250: Recall that a comment counts as whitespace.
1251:
1252: @node Macro Pitfalls,, Redefining, Macros
1253: @subsection Pitfalls and Subtleties of Macros
1254:
1255: In this section we describe some special rules that apply to macros and
1256: macro expansion, and point out certain cases in which the rules have
1257: counterintuitive consequences that you must watch out for.
1258:
1259: @menu
1260: * Misnesting:: Macros can contain unmatched parentheses.
1261: * Macro Parentheses:: Why apparently superfluous parentheses
1262: may be necessary to avoid incorrect grouping.
1263: * Swallow Semicolon:: Macros that look like functions
1264: but expand into compound statements.
1265: * Side Effects:: Unsafe macros that cause trouble when
1266: arguments contain side effects.
1267: * Self-Reference:: Macros whose definitions use the macros' own names.
1268: * Argument Prescan:: Actual arguments are checked for macro calls
1269: before they are substituted.
1270: * Cascaded Macros:: Macros whose definitions use other macros.
1271: * Newlines in Args:: Sometimes line numbers get confused.
1272: @end menu
1273:
1274: @node Misnesting, Macro Parentheses, Macro Pitfalls, Macro Pitfalls
1275: @subsubsection Improperly Nested Constructs
1276:
1277: Recall that when a macro is called with arguments, the arguments are
1278: substituted into the macro body and the result is checked, together with
1279: the rest of the input file, for more macro calls.
1280:
1281: It is possible to piece together a macro call coming partially from the
1282: macro body and partially from the actual arguments. For example,
1283:
1284: @example
1285: #define double(x) (2*(x))
1286: #define call_with_1(x) x(1)
1287: @end example
1288:
1289: @noindent
1290: would expand @samp{call_with_1 (double)} into @samp{(2*(1))}.
1291:
1292: Macro definitions do not have to have balanced parentheses. By writing an
1293: unbalanced open parenthesis in a macro body, it is possible to create a
1294: macro call that begins inside the macro body but ends outside of it. For
1295: example,
1296:
1297: @example
1298: #define strange(file) fprintf (file, "%s %d",
1299: @dots{}
1300: strange(stderr) p, 35)
1301: @end example
1302:
1303: @noindent
1304: This bizarre example expands to @samp{fprintf (stderr, "%s %d", p, 35)}!
1305:
1306: @node Macro Parentheses, Swallow Semicolon, Misnesting, Macro Pitfalls
1307: @subsubsection Unintended Grouping of Arithmetic
1308:
1309: You may have noticed that in most of the macro definition examples shown
1310: above, each occurrence of a macro argument name had parentheses around it.
1311: In addition, another pair of parentheses usually surround the entire macro
1312: definition. Here is why it is best to write macros that way.
1313:
1314: Suppose you define a macro as follows,
1315:
1316: @example
1317: #define ceil_div(x, y) (x + y - 1) / y
1318: @end example
1319:
1320: @noindent
1321: whose purpose is to divide, rounding up. (One use for this operation is
1322: to compute how many @samp{int} objects are needed to hold a certain
1323: number of @samp{char} objects.) Then suppose it is used as follows:
1324:
1325: @example
1326: a = ceil_div (b & c, sizeof (int));
1327: @end example
1328:
1329: @noindent
1330: This expands into
1331:
1332: @example
1333: a = (b & c + sizeof (int) - 1) / sizeof (int);
1334: @end example
1335:
1336: @noindent
1337: which does not do what is intended. The operator-precedence rules of
1338: C make it equivalent to this:
1339:
1340: @example
1341: a = (b & (c + sizeof (int) - 1)) / sizeof (int);
1342: @end example
1343:
1344: @noindent
1345: But what we want is this:
1346:
1347: @example
1348: a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
1349: @end example
1350:
1351: @noindent
1352: Defining the macro as
1353:
1354: @example
1355: #define ceil_div(x, y) ((x) + (y) - 1) / (y)
1356: @end example
1357:
1358: @noindent
1359: provides the desired result.
1360:
1361: However, unintended grouping can result in another way. Consider
1362: @samp{sizeof ceil_div(1, 2)}. That has the appearance of a C expression
1363: that would compute the size of the type of @samp{ceil_div (1, 2)}, but in
1364: fact it means something very different. Here is what it expands to:
1365:
1366: @example
1367: sizeof ((1) + (2) - 1) / (2)
1368: @end example
1369:
1370: @noindent
1371: This would take the size of an integer and divide it by two. The precedence
1372: rules have put the division outside the @samp{sizeof} when it was intended
1373: to be inside.
1374:
1375: Parentheses around the entire macro definition can prevent such problems.
1376: Here, then, is the recommended way to define @samp{ceil_div}:
1377:
1378: @example
1379: #define ceil_div(x, y) (((x) + (y) - 1) / (y))
1380: @end example
1381:
1382: @node Swallow Semicolon, Side Effects, Macro Parentheses, Macro Pitfalls
1383: @subsubsection Swallowing the Semicolon
1384:
1385: @cindex semicolons (after macro calls)
1386: Often it is desirable to define a macro that expands into a compound
1387: statement. Consider, for example, the following macro, that advances a
1388: pointer (the argument @samp{p} says where to find it) across whitespace
1389: characters:
1390:
1391: @example
1392: #define SKIP_SPACES (p, limit) \
1393: @{ register char *lim = (limit); \
1394: while (p != lim) @{ \
1395: if (*p++ != ' ') @{ \
1396: p--; break; @}@}@}
1397: @end example
1398:
1399: @noindent
1400: Here Backslash-Newline is used to split the macro definition, which must
1401: be a single line, so that it resembles the way such C code would be
1402: laid out if not part of a macro definition.
1403:
1404: A call to this macro might be @samp{SKIP_SPACES (p, lim)}. Strictly
1405: speaking, the call expands to a compound statement, which is a complete
1406: statement with no need for a semicolon to end it. But it looks like a
1407: function call. So it minimizes confusion if you can use it like a function
1408: call, writing a semicolon afterward, as in @samp{SKIP_SPACES (p, lim);}
1409:
1410: But this can cause trouble before @samp{else} statements, because the
1411: semicolon is actually a null statement. Suppose you write
1412:
1413: @example
1414: if (*p != 0)
1415: SKIP_SPACES (p, lim);
1416: else @dots{}
1417: @end example
1418:
1419: @noindent
1420: The presence of two statements---the compound statement and a null
1421: statement---in between the @samp{if} condition and the @samp{else}
1422: makes invalid C code.
1423:
1424: The definition of the macro @samp{SKIP_SPACES} can be altered to solve
1425: this problem, using a @samp{do @dots{} while} statement. Here is how:
1426:
1427: @example
1428: #define SKIP_SPACES (p, limit) \
1429: do @{ register char *lim = (limit); \
1430: while (p != lim) @{ \
1431: if (*p++ != ' ') @{ \
1432: p--; break; @}@}@} \
1433: while (0)
1434: @end example
1435:
1436: Now @samp{SKIP_SPACES (p, lim);} expands into
1437:
1438: @example
1439: do @{@dots{}@} while (0);
1440: @end example
1441:
1442: @noindent
1443: which is one statement.
1444:
1445: @node Side Effects, Self-Reference, Swallow Semicolon, Macro Pitfalls
1446: @subsubsection Duplication of Side Effects
1447:
1448: @cindex side effects (in macro arguments)
1449: @cindex unsafe macros
1450: Many C programs define a macro @samp{min}, for ``minimum'', like this:
1451:
1452: @example
1453: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
1454: @end example
1455:
1456: When you use this macro with an argument containing a side effect,
1457: as shown here,
1458:
1459: @example
1460: next = min (x + y, foo (z));
1461: @end example
1462:
1463: @noindent
1464: it expands as follows:
1465:
1466: @example
1467: next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
1468: @end example
1469:
1470: @noindent
1471: where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
1472: for @samp{Y}.
1473:
1474: The function @samp{foo} is used only once in the statement as it appears
1475: in the program, but the expression @samp{foo (z)} has been substituted
1476: twice into the macro expansion. As a result, @samp{foo} might be called
1477: two times when the statement is executed. If it has side effects or
1478: if it takes a long time to compute, the results might not be what you
1479: intended. We say that @samp{min} is an @dfn{unsafe} macro.
1480:
1481: The best solution to this problem is to define @samp{min} in a way that
1482: computes the value of @samp{foo (z)} only once. The C language offers no
1483: standard way to do this, but it can be done with GNU C extensions as
1484: follows:
1485:
1486: @example
1487: #define min(X, Y) \
1488: (@{ typeof (X) __x = (X), __y = (Y); \
1489: (__x < __y) ? __x : __y; @})
1490: @end example
1491:
1492: If you do not wish to use GNU C extensions, the only solution is to be
1493: careful when @emph{using} the macro @samp{min}. For example, you can
1494: calculate the value of @samp{foo (z)}, save it in a variable, and use that
1495: variable in @samp{min}:
1496:
1497: @example
1498: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
1499: @dots{}
1500: @{
1501: int tem = foo (z);
1502: next = min (x + y, tem);
1503: @}
1504: @end example
1505:
1506: @noindent
1507: (where we assume that @samp{foo} returns type @samp{int}).
1508:
1509: @node Self-Reference, Argument Prescan, Side Effects, Macro Pitfalls
1510: @subsubsection Self-Referential Macros
1511:
1512: @cindex self-reference
1513: A @dfn{self-referential} macro is one whose name appears in its definition.
1514: A special feature of ANSI Standard C is that the self-reference is not
1515: considered a macro call. It is passed into the preprocessor output
1516: unchanged.
1517:
1518: Let's consider an example:
1519:
1520: @example
1521: #define foo (4 + foo)
1522: @end example
1523:
1524: @noindent
1525: where @samp{foo} is also a variable in your program.
1526:
1527: Following the ordinary rules, each reference to @samp{foo} will expand into
1528: @samp{(4 + foo)}; then this will be rescanned and will expand into @samp{(4
1529: + (4 + foo))}; and so on until it causes a fatal error (memory full) in the
1530: preprocessor.
1531:
1532: However, the special rule about self-reference cuts this process short
1533: after one step, at @samp{(4 + foo)}. Therefore, this macro definition
1534: has the possibly useful effect of causing the program to add 4 to
1535: the value of @samp{foo} wherever @samp{foo} is referred to.
1536:
1537: In most cases, it is a bad idea to take advantage of this feature. A
1538: person reading the program who sees that @samp{foo} is a variable will
1539: not expect that it is a macro as well. The reader will come across the
1540: identifier @samp{foo} in the program and think its value should be that
1541: of the variable @samp{foo}, whereas in fact the value is four greater.
1542:
1543: The special rule for self-reference applies also to @dfn{indirect}
1544: self-reference. This is the case where a macro @var{x} expands to use a
1545: macro @samp{y}, and the expansion of @samp{y} refers to the macro
1546: @samp{x}. The resulting reference to @samp{x} comes indirectly from the
1547: expansion of @samp{x}, so it is a self-reference and is not further
1548: expanded. Thus, after
1549:
1550: @example
1551: #define x (4 + y)
1552: #define y (2 * x)
1553: @end example
1554:
1555: @noindent
1556: @samp{x} would expand into @samp{(4 + (2 * x))}. Clear?
1557:
1558: But suppose @samp{y} is used elsewhere, not from the definition of @samp{x}.
1559: Then the use of @samp{x} in the expansion of @samp{y} is not a self-reference
1560: because @samp{x} is not ``in progress''. So it does expand. However,
1561: the expansion of @samp{x} contains a reference to @samp{y}, and that
1562: is an indirect self-reference now because @samp{y} is ``in progress''.
1563: The result is that @samp{y} expands to @samp{(2 * (4 + y))}.
1564:
1565: It is not clear that this behavior would ever be useful, but it is specified
1566: by the ANSI C standard, so you may need to understand it.
1567:
1568: @node Argument Prescan, Cascaded Macros, Self-Reference, Macro Pitfalls
1569: @subsubsection Separate Expansion of Macro Arguments
1570:
1571: We have explained that the expansion of a macro, including the substituted
1572: actual arguments, is scanned over again for macro calls to be expanded.
1573:
1574: What really happens is more subtle: first each actual argument text is scanned
1575: separately for macro calls. Then the results of this are substituted into
1576: the macro body to produce the macro expansion, and the macro expansion
1577: is scanned again for macros to expand.
1578:
1579: The result is that the actual arguments are scanned @emph{twice} to expand
1580: macro calls in them.
1581:
1582: Most of the time, this has no effect. If the actual argument contained
1583: any macro calls, they are expanded during the first scan. The result
1584: therefore contains no macro calls, so the second scan does not change it.
1585: If the actual argument were substituted as given, with no prescan,
1586: the single remaining scan would find the same macro calls and produce
1587: the same results.
1588:
1589: You might expect the double scan to change the results when a
1590: self-referential macro is used in an actual argument of another macro
1591: (@pxref{Self-Reference}): the self-referential macro would be expanded once
1592: in the first scan, and a second time in the second scan. But this is not
1593: what happens. The self-references that do not expand in the first scan are
1594: marked so that they will not expand in the second scan either.
1595:
1596: The prescan is not done when an argument is stringified or concatenated.
1597: Thus,
1598:
1599: @example
1600: #define str(s) #s
1601: #define foo 4
1602: str (foo)
1603: @end example
1604:
1605: @noindent
1606: expands to @samp{"foo"}. Once more, prescan has been prevented from
1607: having any noticeable effect.
1608:
1609: More precisely, stringification and concatenation use the argument as
1610: written, in un-prescanned form. The same actual argument would be used in
1611: prescanned form if it is substituted elsewhere without stringification or
1612: concatenation.
1613:
1614: @example
1615: #define str(s) #s lose(s)
1616: #define foo 4
1617: str (foo)
1618: @end example
1619:
1620: expands to @samp{"foo" lose(4)}.
1621:
1622: You might now ask, ``Why mention the prescan, if it makes no difference?
1623: And why not skip it and make the preprocessor faster?'' The answer is
1624: that the prescan does make a difference in three special cases:
1625:
1626: @itemize @bullet
1627: @item
1628: Nested calls to a macro.
1629:
1630: @item
1631: Macros that call other macros that stringify or concatenate.
1632:
1633: @item
1634: Macros whose expansions contain unshielded commas.
1635: @end itemize
1636:
1637: We say that @dfn{nested} calls to a macro occur when a macro's actual
1638: argument contains a call to that very macro. For example, if @samp{f}
1639: is a macro that expects one argument, @samp{f (f (1))} is a nested
1640: pair of calls to @samp{f}. The desired expansion is made by
1641: expanding @samp{f (1)} and substituting that into the definition of
1642: @samp{f}. The prescan causes the expected result to happen.
1643: Without the prescan, @samp{f (1)} itself would be substituted as
1644: an actual argument, and the inner use of @samp{f} would appear
1645: during the main scan as an indirect self-reference and would not
1646: be expanded. Here, the prescan cancels an undesirable side effect
1647: (in the medical, not computational, sense of the term) of the special
1648: rule for self-referential macros.
1649:
1650: But prescan causes trouble in certain other cases of nested macro calls.
1651: Here is an example:
1652:
1653: @example
1654: #define foo a,b
1655: #define bar(x) lose(x)
1656: #define lose(x) (1 + (x))
1657:
1658: bar(foo)
1659: @end example
1660:
1661: @noindent
1662: We would like @samp{bar(foo)} to turn into @samp{(1 + (foo))}, which
1663: would then turn into @samp{(1 + (a,b))}. But instead, @samp{bar(foo)}
1664: expands into @samp{lose(a,b)}, and you get an error because @code{lose}
1665: requires a single argument. In this case, the problem is easily solved
1666: by the same parentheses that ought to be used to prevent misnesting of
1667: arithmetic operations:
1668:
1669: @example
1670: #define foo (a,b)
1671: #define bar(x) lose((x))
1672: @end example
1673:
1674: The problem is more serious when the operands of the macro are not
1675: expressions; for example, when they are statements. Then parentheses
1676: are unacceptable because they would make for invalid C code:
1677:
1678: @example
1679: #define foo @{ int a, b; @dots{} @}
1680: @end example
1681:
1682: @noindent
1683: In GNU C you can shield the commas using the @samp{(@{@dots{}@})}
1684: construct which turns a compound statement into an expression:
1685:
1686: @example
1687: #define foo (@{ int a, b; @dots{} @})
1688: @end example
1689:
1690: Or you can rewrite the macro definition to avoid such commas:
1691:
1692: @example
1693: #define foo @{ int a; int b; @dots{} @}
1694: @end example
1695:
1696: There is also one case where prescan is useful. It is possible
1697: to use prescan to expand an argument and then stringify it---if you use
1698: two levels of macros. Let's add a new macro @samp{xstr} to the
1699: example shown above:
1700:
1701: @example
1702: #define xstr(s) str(s)
1703: #define str(s) #s
1704: #define foo 4
1705: xstr (foo)
1706: @end example
1707:
1708: This expands into @samp{"4"}, not @samp{"foo"}. The reason for the
1709: difference is that the argument of @samp{xstr} is expanded at prescan
1710: (because @samp{xstr} does not specify stringification or concatenation of
1711: the argument). The result of prescan then forms the actual argument for
1712: @samp{str}. @samp{str} uses its argument without prescan because it
1713: performs stringification; but it cannot prevent or undo the prescanning
1714: already done by @samp{xstr}.
1715:
1716: @node Cascaded Macros, Newlines in Args, Argument Prescan, Macro Pitfalls
1717: @subsubsection Cascaded Use of Macros
1718:
1719: @cindex cascaded macros
1720: @cindex macro body uses macro
1721: A @dfn{cascade} of macros is when one macro's body contains a reference
1722: to another macro. This is very common practice. For example,
1723:
1724: @example
1725: #define BUFSIZE 1020
1726: #define TABLESIZE BUFSIZE
1727: @end example
1728:
1729: This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
1730: The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
1731: specify---in this case, @samp{BUFSIZE}---and does not check to see whether
1732: it too is the name of a macro.
1733:
1734: It's only when you @emph{use} @samp{TABLESIZE} that the result of its expansion
1735: is checked for more macro names.
1736:
1737: This makes a difference if you change the definition of @samp{BUFSIZE}
1738: at some point in the source file. @samp{TABLESIZE}, defined as shown,
1739: will always expand using the definition of @samp{BUFSIZE} that is
1740: currently in effect:
1741:
1742: @example
1743: #define BUFSIZE 1020
1744: #define TABLESIZE BUFSIZE
1745: #undef BUFSIZE
1746: #define BUFSIZE 37
1747: @end example
1748:
1749: @noindent
1750: Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.
1751:
1752: @node Newlines in Args,, Cascaded Macros, Macro Pitfalls
1753: @subsection Newlines in Macro Arguments
1754:
1755: Traditional macro processing carries forward all newlines in macro
1756: arguments into the expansion of the macro. This means that, if some of
1757: the arguments are substituted more than once, or not at all, or out of
1758: order, newlines can be duplicated, lost, or moved around within the
1759: expansion. If the expansion consists of multiple statements, then the
1760: effect is to distort the line numbers of some of these statements. The
1761: result can be incorrect line numbers, in error messages or displayed in
1762: a debugger.
1763:
1764: The GNU C preprocessor operating in ANSI C mode adjusts appropriately
1765: for multiple use of an argument---the first use expands all the
1766: newlines, and subsequent uses of the same argument produce no newlines.
1767: But even in this mode, it can produce incorrect line numbering if
1768: arguments are used out of order, or not used at all.
1769:
1770: Here is an example illustrating this problem:
1771:
1772: @example
1773: #define ignore_second_arg(a,b,c) a; c
1774:
1775: ignore_second_arg (foo (),
1776: ignored (),
1777: syntax error);
1778: @end example
1779:
1780: @noindent
1781: The syntax error triggered by the tokens @samp{syntax error} results
1782: in an error message citing line four, even though the statement text
1783: comes from line five.
1784:
1785: @node Conditionals, Combining Sources, Macros, Top
1786: @section Conditionals
1787:
1788: @cindex conditionals
1789: In a macro processor, a @dfn{conditional} is a command that allows a part
1790: of the program to be ignored during compilation, on some conditions.
1791: In the C preprocessor, a conditional can test either an arithmetic expression
1792: or whether a name is defined as a macro.
1793:
1794: A conditional in the C preprocessor resembles in some ways an @samp{if}
1795: statement in C, but it is important to understand the difference between
1796: them. The condition in an @samp{if} statement is tested during the execution
1797: of your program. Its purpose is to allow your program to behave differently
1798: from run to run, depending on the data it is operating on. The condition
1799: in a preprocessor conditional command is tested when your program is compiled.
1800: Its purpose is to allow different code to be included in the program depending
1801: on the situation at the time of compilation.
1802:
1803: @menu
1804: * Uses: Conditional Uses. What conditionals are for.
1805: * Syntax: Conditional Syntax. How conditionals are written.
1806: * Deletion: Deleted Code. Making code into a comment.
1807: * Macros: Conditionals-Macros. Why conditionals are used with macros.
1808: * Assertions:: How and why to use assertions.
1809: * Errors: #error Command. Detecting inconsistent compilation parameters.
1810: @end menu
1811:
1812: @node Conditional Uses
1813: @subsection Why Conditionals are Used
1814:
1815: Generally there are three kinds of reason to use a conditional.
1816:
1817: @itemize @bullet
1818: @item
1819: A program may need to use different code depending on the machine or
1820: operating system it is to run on. In some cases the code for one
1821: operating system may be erroneous on another operating system; for
1822: example, it might refer to library routines that do not exist on the
1823: other system. When this happens, it is not enough to avoid executing
1824: the invalid code: merely having it in the program makes it impossible
1825: to link the program and run it. With a preprocessor conditional, the
1826: offending code can be effectively excised from the program when it is
1827: not valid.
1828:
1829: @item
1830: You may want to be able to compile the same source file into two
1831: different programs. Sometimes the difference between the programs is
1832: that one makes frequent time-consuming consistency checks on its
1833: intermediate data while the other does not.
1834:
1835: @item
1836: A conditional whose condition is always false is a good way to exclude
1837: code from the program but keep it as a sort of comment for future
1838: reference.
1839: @end itemize
1840:
1841: Most simple programs that are intended to run on only one machine will
1842: not need to use preprocessor conditionals.
1843:
1844: @node Conditional Syntax
1845: @subsection Syntax of Conditionals
1846:
1847: @findex #if
1848: A conditional in the C preprocessor begins with a @dfn{conditional
1849: command}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.
1850: @xref{Conditionals-Macros}, for information on @samp{#ifdef} and
1851: @samp{#ifndef}; only @samp{#if} is explained here.
1852:
1853: @menu
1854: * If: #if Command. Basic conditionals using @samp{#if} and @samp{#endif}.
1855: * Else: #else Command. Including some text if the condition fails.
1856: * Elif: #elif Command. Testing several alternative possibilities.
1857: @end menu
1858:
1859: @node #if Command
1860: @subsubsection The @samp{#if} Command
1861:
1862: The @samp{#if} command in its simplest form consists of
1863:
1864: @example
1865: #if @var{expression}
1866: @var{controlled text}
1867: #endif /* @var{expression} */
1868: @end example
1869:
1870: The comment following the @samp{#endif} is not required, but it is a good
1871: practice because it helps people match the @samp{#endif} to the
1872: corresponding @samp{#if}. Such comments should always be used, except in
1873: short conditionals that are not nested. In fact, you can put anything at
1874: all after the @samp{#endif} and it will be ignored by the GNU C preprocessor,
1875: but only comments are acceptable in ANSI Standard C.
1876:
1877: @var{expression} is a C expression of integer type, subject to stringent
1878: restrictions. It may contain
1879:
1880: @itemize @bullet
1881: @item
1882: Integer constants, which are all regarded as @code{long} or
1883: @code{unsigned long}.
1884:
1885: @item
1886: Character constants, which are interpreted according to the character
1887: set and conventions of the machine and operating system on which the
1888: preprocessor is running. The GNU C preprocessor uses the C data type
1889: @samp{char} for these character constants; therefore, whether some
1890: character codes are negative is determined by the C compiler used to
1891: compile the preprocessor. If it treats @samp{char} as signed, then
1892: character codes large enough to set the sign bit will be considered
1893: negative; otherwise, no character code is considered negative.
1894:
1895: @item
1896: Arithmetic operators for addition, subtraction, multiplication,
1897: division, bitwise operations, shifts, comparisons, and @samp{&&} and
1898: @samp{||}.
1899:
1900: @item
1901: Identifiers that are not macros, which are all treated as zero(!).
1902:
1903: @item
1904: Macro calls. All macro calls in the expression are expanded before
1905: actual computation of the expression's value begins.
1906: @end itemize
1907:
1908: Note that @samp{sizeof} operators and @code{enum}-type values are not allowed.
1909: @code{enum}-type values, like all other identifiers that are not taken
1910: as macro calls and expanded, are treated as zero.
1911:
1912: The @var{controlled text} inside of a conditional can include
1913: preprocessor commands. Then the commands inside the conditional are
1914: obeyed only if that branch of the conditional succeeds. The text can
1915: also contain other conditional groups. However, the @samp{#if} and
1916: @samp{#endif} commands must balance.
1917:
1918: @node #else Command
1919: @subsubsection The @samp{#else} Command
1920:
1921: @findex #else
1922: The @samp{#else} command can be added to a conditional to provide
1923: alternative text to be used if the condition is false. This is what
1924: it looks like:
1925:
1926: @example
1927: #if @var{expression}
1928: @var{text-if-true}
1929: #else /* Not @var{expression} */
1930: @var{text-if-false}
1931: #endif /* Not @var{expression} */
1932: @end example
1933:
1934: If @var{expression} is nonzero, and thus the @var{text-if-true} is
1935: active, then @samp{#else} acts like a failing conditional and the
1936: @var{text-if-false} is ignored. Contrariwise, if the @samp{#if}
1937: conditional fails, the @var{text-if-false} is considered included.
1938:
1939: @node #elif Command
1940: @subsubsection The @samp{#elif} Command
1941:
1942: @findex #elif
1943: One common case of nested conditionals is used to check for more than two
1944: possible alternatives. For example, you might have
1945:
1946: @example
1947: #if X == 1
1948: @dots{}
1949: #else /* X != 1 */
1950: #if X == 2
1951: @dots{}
1952: #else /* X != 2 */
1953: @dots{}
1954: #endif /* X != 2 */
1955: #endif /* X != 1 */
1956: @end example
1957:
1958: Another conditional command, @samp{#elif}, allows this to be abbreviated
1959: as follows:
1960:
1961: @example
1962: #if X == 1
1963: @dots{}
1964: #elif X == 2
1965: @dots{}
1966: #else /* X != 2 and X != 1*/
1967: @dots{}
1968: #endif /* X != 2 and X != 1*/
1969: @end example
1970:
1971: @samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the
1972: middle of a @samp{#if}-@samp{#endif} pair and subdivides it; it does not
1973: require a matching @samp{#endif} of its own. Like @samp{#if}, the
1974: @samp{#elif} command includes an expression to be tested.
1975:
1976: The text following the @samp{#elif} is processed only if the original
1977: @samp{#if}-condition failed and the @samp{#elif} condition succeeds.
1978: More than one @samp{#elif} can go in the same @samp{#if}-@samp{#endif}
1979: group. Then the text after each @samp{#elif} is processed only if the
1980: @samp{#elif} condition succeeds after the original @samp{#if} and any
1981: previous @samp{#elif} commands within it have failed. @samp{#else} is
1982: equivalent to @samp{#elif 1}, and @samp{#else} is allowed after any
1983: number of @samp{#elif} commands, but @samp{#elif} may not follow
1984: @samp{#else}.
1985:
1986: @node Deleted Code
1987: @subsection Keeping Deleted Code for Future Reference
1988:
1989: If you replace or delete a part of the program but want to keep the old
1990: code around as a comment for future reference, the easy way to do this is
1991: to put @samp{#if 0} before it and @samp{#endif} after it.
1992:
1993: This works even if the code being turned off contains conditionals, but
1994: they must be entire conditionals (balanced @samp{#if} and @samp{#endif}).
1995:
1996: @node Conditionals-Macros
1997: @subsection Conditionals and Macros
1998:
1999: Conditionals are useful in connection with macros or assertions, because
2000: those are the only ways that an expression's value can vary from one
2001: compilation to another. A @samp{#if} command whose expression uses no
2002: macros or assertions is equivalent to @samp{#if 1} or @samp{#if 0}; you
2003: might as well determine which one, by computing the value of the
2004: expression yourself, and then simplify the program.
2005:
2006: For example, here is a conditional that tests the expression
2007: @samp{BUFSIZE == 1020}, where @samp{BUFSIZE} must be a macro.
2008:
2009: @example
2010: #if BUFSIZE == 1020
2011: printf ("Large buffers!\n");
2012: #endif /* BUFSIZE is large */
2013: @end example
2014:
2015: (Programmers often wish they could test the size of a variable or data
2016: type in @samp{#if}, but this does not work. The preprocessor does not
2017: understand @code{sizeof}, or typedef names, or even the type keywords
2018: such as @code{int}.)
2019:
2020: @findex defined
2021: The special operator @samp{defined} is used in @samp{#if} expressions to
2022: test whether a certain name is defined as a macro. Either @samp{defined
2023: @var{name}} or @samp{defined (@var{name})} is an expression whose value
2024: is 1 if @var{name} is defined as macro at the current point in the
2025: program, and 0 otherwise. For the @samp{defined} operator it makes no
2026: difference what the definition of the macro is; all that matters is
2027: whether there is a definition. Thus, for example,@refill
2028:
2029: @example
2030: #if defined (vax) || defined (ns16000)
2031: @end example
2032:
2033: @noindent
2034: would include the following code if either of the names @samp{vax} and
2035: @samp{ns16000} is defined as a macro. You can test the same condition
2036: using assertions (@pxref{Assertions}), like this:
2037:
2038: @example
2039: #if #cpu (vax) || #cpu (ns16000)
2040: @end example
2041:
2042: If a macro is defined and later undefined with @samp{#undef},
2043: subsequent use of the @samp{defined} operator returns 0, because
2044: the name is no longer defined. If the macro is defined again with
2045: another @samp{#define}, @samp{defined} will recommence returning 1.
2046:
2047: @findex #ifdef
2048: @findex #ifndef
2049: Conditionals that test just the definedness of one name are very common, so
2050: there are two special short conditional commands for this case.
2051:
2052: @table @code
2053: @item #ifdef @var{name}
2054: is equivalent to @samp{#if defined (@var{name})}.
2055:
2056: @item #ifndef @var{name}
2057: is equivalent to @samp{#if ! defined (@var{name})}.
2058: @end table
2059:
2060: Macro definitions can vary between compilations for several reasons.
2061:
2062: @itemize @bullet
2063: @item
2064: Some macros are predefined on each kind of machine. For example, on a
2065: Vax, the name @samp{vax} is a predefined macro. On other machines, it
2066: would not be defined.
2067:
2068: @item
2069: Many more macros are defined by system header files. Different
2070: systems and machines define different macros, or give them different
2071: values. It is useful to test these macros with conditionals to avoid
2072: using a system feature on a machine where it is not implemented.
2073:
2074: @item
2075: Macros are a common way of allowing users to customize a program for
2076: different machines or applications. For example, the macro
2077: @samp{BUFSIZE} might be defined in a configuration file for your
2078: program that is included as a header file in each source file. You
2079: would use @samp{BUFSIZE} in a preprocessor conditional in order to
2080: generate different code depending on the chosen configuration.
2081:
2082: @item
2083: Macros can be defined or undefined with @samp{-D} and @samp{-U}
2084: command options when you compile the program. You can arrange to
2085: compile the same source file into two different programs by choosing
2086: a macro name to specify which program you want, writing conditionals
2087: to test whether or how this macro is defined, and then controlling
2088: the state of the macro with compiler command options.
2089: @xref{Invocation}.
2090: @end itemize
2091:
2092: @ifinfo
2093: Assertions are usually predefined, but can be defined with preprocessor
2094: commands or command-line options.
2095: @end ifinfo
2096:
2097: @node Assertions
2098: @subsection Assertions
2099:
2100: @cindex assertions
2101: @dfn{Assertions} are a more systematic alternative to macros in writing
2102: conditionals to test what sort of computer or system the compiled
2103: program will run on. Assertions are usually predefined, but you can
2104: define them with preprocessor commands or command-line options.
2105:
2106: @cindex predicates
2107: The macros traditionally used to describe the type of target are not
2108: classified in any way according to which question they answer; they may
2109: indicate a hardware architecture, a particular hardware model, an
2110: operating system, a particular version of an operating system, or
2111: specific configuration options. These are jumbled together in a single
2112: namespace. In contrast, each assertion consists of a named question and
2113: an answer. The question is usually called the @dfn{predicate}.
2114: An assertion looks like this:
2115:
2116: @example
2117: #@var{predicate} (@var{answer})
2118: @end example
2119:
2120: @noindent
2121: You must use a properly formed identifier for @var{predicate}. The
2122: value of @var{answer} can be any sequence of words; all characters are
2123: significant except for leading and trailing whitespace, and differences
2124: in internal whitespace sequences are ignored. Thus, @samp{x + y} is
2125: different from @samp{x+y} but equivalent to @samp{x + y}. @samp{)} is
2126: not allowed in an answer.
2127:
2128: @cindex testing predicates
2129: Here is a conditional to test whether the answer @var{answer} is asserted
2130: for the predicate @var{predicate}:
2131:
2132: @example
2133: #if #@var{predicate} (@var{answer})
2134: @end example
2135:
2136: @noindent
2137: There may be more than one answer asserted for a given predicate. If
2138: you omit the answer, you can test whether @emph{any} answer is asserted
2139: for @var{predicate}:
2140:
2141: @example
2142: #if #@var{predicate}
2143: @end example
2144:
2145: Most of the time, the assertions you test will be predefined assertions.
2146: GNU C provides three predefined predicates: @code{system}, @code{cpu},
2147: and @code{machine}. @code{system} is for assertions about the type of
2148: software, @code{cpu} describes the type of computer architecture, and
2149: @code{machine} gives more information about the computer. For example,
2150: on a GNU system, the following assertions would be true:
2151:
2152: @example
2153: #system (gnu)
2154: #system (mach)
2155: #system (mach 3)
2156: #system (mach 3.@var{subversion})
2157: #system (hurd)
2158: #system (hurd @var{version})
2159: @end example
2160:
2161: @noindent
2162: and perhaps others. The alternatives with
2163: more or less version information let you ask more or less detailed
2164: questions about the type of system software.
2165:
2166: On a Unix system, you would find @code{#system (unix)} and perhaps one of:
2167: @code{#system (aix)}, @code{#system (bsd)}, @code{#system (hpux)},
2168: @code{#system (lynx)}, @code{#system (mach)}, @code{#system (posix)},
2169: @code{#system (svr3)}, @code{#system (svr4)}, or @code{#system (xpg4)}
2170: with possible version numbers following.
2171:
2172: Other values for @code{system} are @code{#system (mvs)}
2173: and @code{#system (vms)}.
2174:
2175: @strong{Portability note:} Many Unix C compilers provide only one answer
2176: for the @code{system} assertion: @code{#system (unix)}, if they support
2177: assertions at all. This is less than useful.
2178:
2179: An assertion with a multi-word answer is completely different from several
2180: assertions with individual single-word answers. For example, the presence
2181: of @code{system (mach 3.0)} does not mean that @code{system (3.0)} is true.
2182: It also does not directly imply @code{system (mach)}, but in GNU C, that
2183: last will normally be asserted as well.
2184:
2185: The current list of possible assertion values for @code{cpu} is:
2186: @code{#cpu (a29k)}, @code{#cpu (alpha)}, @code{#cpu (arm)}, @code{#cpu
2187: (clipper)}, @code{#cpu (convex)}, @code{#cpu (elxsi)}, @code{#cpu
2188: (tron)}, @code{#cpu (h8300)}, @code{#cpu (i370)}, @code{#cpu (i386)},
2189: @code{#cpu (i860)}, @code{#cpu (i960)}, @code{#cpu (m68k)}, @code{#cpu
2190: (m88k)}, @code{#cpu (mips)}, @code{#cpu (ns32k)}, @code{#cpu (hppa)},
2191: @code{#cpu (pyr)}, @code{#cpu (ibm032)}, @code{#cpu (rs6000)},
2192: @code{#cpu (sh)}, @code{#cpu (sparc)}, @code{#cpu (spur)}, @code{#cpu
2193: (tahoe)}, @code{#cpu (vax)}, @code{#cpu (we32000)}.
2194:
2195: @findex #assert
2196: You can create assertions within a C program using @samp{#assert}, like
2197: this:
2198:
2199: @example
2200: #assert @var{predicate} (@var{answer})
2201: @end example
2202:
2203: @noindent
2204: (Note the absence of a @samp{#} before @var{predicate}.)
2205:
2206: @cindex unassert
2207: @cindex assertions, undoing
2208: @cindex retracting assertions
2209: @findex #unassert
2210: Each time you do this, you assert a new true answer for @var{predicate}.
2211: Asserting one answer does not invalidate previously asserted answers;
2212: they all remain true. The only way to remove an assertion is with
2213: @samp{#unassert}. @samp{#unassert} has the same syntax as
2214: @samp{#assert}. You can also remove all assertions about
2215: @var{predicate} like this:
2216:
2217: @example
2218: #unassert @var{predicate}
2219: @end example
2220:
2221: You can also add or cancel assertions using command options
2222: when you run @code{gcc} or @code{cpp}. @xref{Invocation}.
2223:
2224: @node #error Command
2225: @subsection The @samp{#error} and @samp{#warning} Commands
2226:
2227: @findex #error
2228: The command @samp{#error} causes the preprocessor to report a fatal
2229: error. The rest of the line that follows @samp{#error} is used as the
2230: error message.
2231:
2232: You would use @samp{#error} inside of a conditional that detects a
2233: combination of parameters which you know the program does not properly
2234: support. For example, if you know that the program will not run
2235: properly on a Vax, you might write
2236:
2237: @smallexample
2238: #ifdef vax
2239: #error Won't work on Vaxen. See comments at get_last_object.
2240: #endif
2241: @end smallexample
2242:
2243: @noindent
2244: @xref{Nonstandard Predefined}, for why this works.
2245:
2246: If you have several configuration parameters that must be set up by
2247: the installation in a consistent way, you can use conditionals to detect
2248: an inconsistency and report it with @samp{#error}. For example,
2249:
2250: @smallexample
2251: #if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
2252: || HASH_TABLE_SIZE % 5 == 0
2253: #error HASH_TABLE_SIZE should not be divisible by a small prime
2254: #endif
2255: @end smallexample
2256:
2257: @findex #warning
2258: The command @samp{#warning} is like the command @samp{#error}, but causes
2259: the preprocessor to issue a warning and continue preprocessing. The rest of
2260: the line that follows @samp{#warning} is used as the warning message.
2261:
2262: You might use @samp{#warning} in obsolete header files, with a message
2263: directing the user to the header file which should be used instead.
2264:
2265: @node Combining Sources, Other Commands, Conditionals, Top
2266: @section Combining Source Files
2267:
2268: @cindex line control
2269: @findex #line
2270: One of the jobs of the C preprocessor is to inform the C compiler of where
2271: each line of C code came from: which source file and which line number.
2272:
2273: C code can come from multiple source files if you use @samp{#include};
2274: both @samp{#include} and the use of conditionals and macros can cause
2275: the line number of a line in the preprocessor output to be different
2276: from the line's number in the original source file. You will appreciate
2277: the value of making both the C compiler (in error messages) and symbolic
2278: debuggers such as GDB use the line numbers in your source file.
2279:
2280: The C preprocessor builds on this feature by offering a command by which
2281: you can control the feature explicitly. This is useful when a file for
2282: input to the C preprocessor is the output from another program such as the
2283: @code{bison} parser generator, which operates on another file that is the
2284: true source file. Parts of the output from @code{bison} are generated from
2285: scratch, other parts come from a standard parser file. The rest are copied
2286: nearly verbatim from the source file, but their line numbers in the
2287: @code{bison} output are not the same as their original line numbers.
2288: Naturally you would like compiler error messages and symbolic debuggers to
2289: know the original source file and line number of each line in the
2290: @code{bison} input.
2291:
2292: @code{bison} arranges this by writing @samp{#line} commands into the output
2293: file. @samp{#line} is a command that specifies the original line number
2294: and source file name for subsequent input in the current preprocessor input
2295: file. @samp{#line} has three variants:
2296:
2297: @table @code
2298: @item #line @var{linenum}
2299: Here @var{linenum} is a decimal integer constant. This specifies that
2300: the line number of the following line of input, in its original source file,
2301: was @var{linenum}.
2302:
2303: @item #line @var{linenum} @var{filename}
2304: Here @var{linenum} is a decimal integer constant and @var{filename}
2305: is a string constant. This specifies that the following line of input
2306: came originally from source file @var{filename} and its line number there
2307: was @var{linenum}. Keep in mind that @var{filename} is not just a
2308: file name; it is surrounded by doublequote characters so that it looks
2309: like a string constant.
2310:
2311: @item #line @var{anything else}
2312: @var{anything else} is checked for macro calls, which are expanded.
2313: The result should be a decimal integer constant followed optionally
2314: by a string constant, as described above.
2315: @end table
2316:
2317: @samp{#line} commands alter the results of the @samp{__FILE__} and
2318: @samp{__LINE__} predefined macros from that point on. @xref{Standard
2319: Predefined}.
2320:
2321: The output of the preprocessor (which is the input for the rest of the
2322: compiler) contains commands that look much like @samp{#line} commands.
2323: They start with just @samp{#} instead of @samp{#line}, but this is
2324: followed by a line number and file name as in @samp{#line}. @xref{Output}.
2325:
2326: @node Other Commands, Output, Combining Sources, Top
2327: @section Miscellaneous Preprocessor Commands
2328:
2329: @findex #pragma
2330: @findex #ident
2331: @cindex null command
2332: This section describes three additional preprocessor commands. They are
2333: not very useful, but are mentioned for completeness.
2334:
2335: The @dfn{null command} consists of a @samp{#} followed by a Newline, with
2336: only whitespace (including comments) in between. A null command is
2337: understood as a preprocessor command but has no effect on the preprocessor
2338: output. The primary significance of the existence of the null command is
2339: that an input line consisting of just a @samp{#} will produce no output,
2340: rather than a line of output containing just a @samp{#}. Supposedly
2341: some old C programs contain such lines.
2342:
2343: The ANSI standard specifies that the @samp{#pragma} command has an
2344: arbitrary, implementation-defined effect. In the GNU C preprocessor,
2345: @samp{#pragma} commands are not used, except for @samp{#pragma once}
2346: (@pxref{Once-Only}). However, they are left in the preprocessor output,
2347: so they are available to the compilation pass.
2348:
2349: The @samp{#ident} command is supported for compatibility with certain
2350: other systems. It is followed by a line of text. On some systems, the
2351: text is copied into a special place in the object file; on most systems,
2352: the text is ignored and this command has no effect. Typically
2353: @samp{#ident} is only used in header files supplied with those systems
2354: where it is meaningful.
2355:
2356: @node Output, Invocation, Other Commands, Top
2357: @section C Preprocessor Output
2358:
2359: @cindex output format
2360: The output from the C preprocessor looks much like the input, except
2361: that all preprocessor command lines have been replaced with blank lines
2362: and all comments with spaces. Whitespace within a line is not altered;
2363: however, a space is inserted after the expansions of most macro calls.
2364:
2365: Source file name and line number information is conveyed by lines of
2366: the form
2367:
2368: @example
2369: # @var{linenum} @var{filename} @var{flags}
2370: @end example
2371:
2372: @noindent
2373: which are inserted as needed into the middle of the input (but never
2374: within a string or character constant). Such a line means that the
2375: following line originated in file @var{filename} at line @var{linenum}.
2376:
2377: After the file name comes zero or more flags, which are @samp{1},
2378: @samp{2} or @samp{3}. If there are multiple flags, spaces separate
2379: them. Here is what the flags mean:
2380:
2381: @table @samp
2382: @item 1
2383: This indicates the start of a new file.
2384: @item 2
2385: This indicates returning to a file (after having included another file).
2386: @item 3
2387: This indicates that the following text comes from a system header file,
2388: so certain warnings should be suppressed.
2389: @end table
2390:
2391: @node Invocation, Concept Index, Output, Top
2392: @section Invoking the C Preprocessor
2393:
2394: Most often when you use the C preprocessor you will not have to invoke it
2395: explicitly: the C compiler will do so automatically. However, the
2396: preprocessor is sometimes useful individually.
2397:
2398: The C preprocessor expects two file names as arguments, @var{infile} and
2399: @var{outfile}. The preprocessor reads @var{infile} together with any other
2400: files it specifies with @samp{#include}. All the output generated by the
2401: combined input files is written in @var{outfile}.
2402:
2403: Either @var{infile} or @var{outfile} may be @samp{-}, which as @var{infile}
2404: means to read from standard input and as @var{outfile} means to write to
2405: standard output. Also, if @var{outfile} or both file names are omitted,
2406: the standard output and standard input are used for the omitted file names.
2407:
2408: @cindex options
2409: Here is a table of command options accepted by the C preprocessor.
2410: These options can also be given when compiling a C program; they are
2411: passed along automatically to the preprocessor when it is invoked by the
2412: compiler.
2413:
2414: @table @samp
2415: @item -P
2416: @findex -P
2417: Inhibit generation of @samp{#}-lines with line-number information in
2418: the output from the preprocessor (@pxref{Output}). This might be
2419: useful when running the preprocessor on something that is not C code
2420: and will be sent to a program which might be confused by the
2421: @samp{#}-lines.
2422:
2423: @item -C
2424: @findex -C
2425: Do not discard comments: pass them through to the output file.
2426: Comments appearing in arguments of a macro call will be copied to the
2427: output before the expansion of the macro call.
2428:
2429: @item -traditional
2430: @findex -traditional
2431: Try to imitate the behavior of old-fashioned C, as opposed to ANSI C.
2432:
2433: @itemize @bullet
2434: @item
2435: Traditional macro expansion pays no attention to singlequote or
2436: doublequote characters; macro argument symbols are replaced by the
2437: argument values even when they appear within apparent string or
2438: character constants.
2439:
2440: @item
2441: Traditionally, it is permissible for a macro expansion to end in the
2442: middle of a string or character constant. The constant continues into
2443: the text surrounding the macro call.
2444:
2445: @item
2446: However, traditionally the end of the line terminates a string or
2447: character constant, with no error.
2448:
2449: @item
2450: In traditional C, a comment is equivalent to no text at all. (In ANSI
2451: C, a comment counts as whitespace.)
2452:
2453: @item
2454: Traditional C does not have the concept of a ``preprocessing number''.
2455: It considers @samp{1.0e+4} to be three tokens: @samp{1.0e}, @samp{+},
2456: and @samp{4}.
2457:
2458: @item
2459: A macro is not suppressed within its own definition, in traditional C.
2460: Thus, any macro that is used recursively inevitably causes an error.
2461:
2462: @item
2463: The character @samp{#} has no special meaning within a macro definition
2464: in traditional C.
2465:
2466: @item
2467: In traditional C, the text at the end of a macro expansion can run
2468: together with the text after the macro call, to produce a single token.
2469: (This is impossible in ANSI C.)
2470:
2471: @item
2472: Traditionally, @samp{\} inside a macro argument suppresses the syntactic
2473: significance of the following character.
2474: @end itemize
2475:
2476: @item -trigraphs
2477: @findex -trigraphs
2478: Process ANSI standard trigraph sequences. These are three-character
2479: sequences, all starting with @samp{??}, that are defined by ANSI C to
2480: stand for single characters. For example, @samp{??/} stands for
2481: @samp{\}, so @samp{'??/n'} is a character constant for a newline.
2482: Strictly speaking, the GNU C preprocessor does not support all
2483: programs in ANSI Standard C unless @samp{-trigraphs} is used, but if
2484: you ever notice the difference it will be with relief.
2485:
2486: You don't want to know any more about trigraphs.
2487:
2488: @item -pedantic
2489: @findex -pedantic
2490: Issue warnings required by the ANSI C standard in certain cases such
2491: as when text other than a comment follows @samp{#else} or @samp{#endif}.
2492:
2493: @item -pedantic-errors
2494: @findex -pedantic-errors
2495: Like @samp{-pedantic}, except that errors are produced rather than
2496: warnings.
2497:
2498: @item -Wtrigraphs
2499: @findex -Wtrigraphs
2500: Warn if any trigraphs are encountered (assuming they are enabled).
2501:
2502: @item -Wcomment
2503: @findex -Wcomment
2504: @ignore
2505: @c "Not worth documenting" both singular and plural forms of this
2506: @c option, per RMS. But also unclear which is better; hence may need to
2507: @c switch this at some future date. [email protected], 2jan92.
2508: @itemx -Wcomments
2509: (Both forms have the same effect).
2510: @end ignore
2511: Warn whenever a comment-start sequence @samp{/*} appears in a comment.
2512:
2513: @item -Wall
2514: @findex -Wall
2515: Requests both @samp{-Wtrigraphs} and @samp{-Wcomment} (but not
2516: @samp{-Wtraditional}).
2517:
2518: @item -Wtraditional
2519: @findex -Wtraditional
2520: Warn about certain constructs that behave differently in traditional and
2521: ANSI C.
2522:
2523: @item -I @var{directory}
2524: @findex -I
2525: Add the directory @var{directory} to the end of the list of
2526: directories to be searched for header files (@pxref{Include Syntax}).
2527: This can be used to override a system header file, substituting your
2528: own version, since these directories are searched before the system
2529: header file directories. If you use more than one @samp{-I} option,
2530: the directories are scanned in left-to-right order; the standard
2531: system directories come after.
2532:
2533: @item -I-
2534: Any directories specified with @samp{-I} options before the @samp{-I-}
2535: option are searched only for the case of @samp{#include "@var{file}"};
2536: they are not searched for @samp{#include <@var{file}>}.
2537:
2538: If additional directories are specified with @samp{-I} options after
2539: the @samp{-I-}, these directories are searched for all @samp{#include}
2540: commands.
2541:
2542: In addition, the @samp{-I-} option inhibits the use of the current
2543: directory as the first search directory for @samp{#include "@var{file}"}.
2544: Therefore, the current directory is searched only if it is requested
2545: explicitly with @samp{-I.}. Specifying both @samp{-I-} and @samp{-I.}
2546: allows you to control precisely which directories are searched before
2547: the current one and which are searched after.
2548:
2549: @item -nostdinc
2550: @findex -nostdinc
2551: Do not search the standard system directories for header files.
2552: Only the directories you have specified with @samp{-I} options
2553: (and the current directory, if appropriate) are searched.
2554:
2555: @item -nostdinc++
2556: @findex -nostdinc++
2557: Do not search for header files in the C++-specific standard directories,
2558: but do still search the other standard directories.
2559: (This option is used when building libg++.)
2560:
2561: @item -D @var{name}
2562: @findex -D
2563: Predefine @var{name} as a macro, with definition @samp{1}.
2564:
2565: @item -D @var{name}=@var{definition}
2566: Predefine @var{name} as a macro, with definition @var{definition}.
2567: There are no restrictions on the contents of @var{definition}, but if
2568: you are invoking the preprocessor from a shell or shell-like program you
2569: may need to use the shell's quoting syntax to protect characters such as
2570: spaces that have a meaning in the shell syntax. If you use more than
2571: one @samp{-D} for the same @var{name}, the rightmost definition takes
2572: effect.
2573:
2574: @item -U @var{name}
2575: @findex -U
2576: Do not predefine @var{name}. If both @samp{-U} and @samp{-D} are
2577: specified for one name, the @samp{-U} beats the @samp{-D} and the name
2578: is not predefined.
2579:
2580: @item -undef
2581: @findex -undef
2582: Do not predefine any nonstandard macros.
2583:
2584: @item -A @var{predicate}(@var{answer})
2585: @findex -A
2586: Make an assertion with the predicate @var{predicate} and answer
2587: @var{answer}. @xref{Assertions}.
2588:
2589: @noindent
2590: You can use @samp{-A-} to disable all predefined assertions; it also
2591: undefines all predefined macros that identify the type of target system.
2592:
2593: @item -dM
2594: @findex -dM
2595: Instead of outputting the result of preprocessing, output a list of
2596: @samp{#define} commands for all the macros defined during the
2597: execution of the preprocessor, including predefined macros. This gives
2598: you a way of finding out what is predefined in your version of the
2599: preprocessor; assuming you have no file @samp{foo.h}, the command
2600:
2601: @example
2602: touch foo.h; cpp -dM foo.h
2603: @end example
2604:
2605: @noindent
2606: will show the values of any predefined macros.
2607:
2608: @item -dD
2609: @findex -dD
2610: Like @samp{-dM} except in two respects: it does @emph{not} include the
2611: predefined macros, and it outputs @emph{both} the @samp{#define}
2612: commands and the result of preprocessing. Both kinds of output go to
2613: the standard output file.
2614:
2615: @item -M
2616: @findex -M
2617: Instead of outputting the result of preprocessing, output a rule
2618: suitable for @code{make} describing the dependencies of the main
2619: source file. The preprocessor outputs one @code{make} rule containing
2620: the object file name for that source file, a colon, and the names of
2621: all the included files. If there are many included files then the
2622: rule is split into several lines using @samp{\}-newline.
2623:
2624: This feature is used in automatic updating of makefiles.
2625:
2626: @item -MM
2627: @findex -MM
2628: Like @samp{-M} but mention only the files included with @samp{#include
2629: "@var{file}"}. System header files included with @samp{#include
2630: <@var{file}>} are omitted.
2631:
2632: @item -MD
2633: @findex -MD
2634: Like @samp{-M} but the dependency information is written to files with
2635: names made by replacing @samp{.c} with @samp{.d} at the end of the
2636: input file names. This is in addition to compiling the file as
2637: specified---@samp{-MD} does not inhibit ordinary compilation the way
2638: @samp{-M} does.
2639:
2640: In Mach, you can use the utility @code{md} to merge the @samp{.d} files
2641: into a single dependency file suitable for using with the @samp{make}
2642: command.
2643:
2644: @item -MMD
2645: @findex -MMD
2646: Like @samp{-MD} except mention only user header files, not system
2647: header files.
2648:
2649: @item -H
2650: @findex -H
2651: Print the name of each header file used, in addition to other normal
2652: activities.
2653:
2654: @item -imacros @var{file}
2655: @findex -imacros
2656: Process @var{file} as input, discarding the resulting output, before
2657: processing the regular input file. Because the output generated from
2658: @var{file} is discarded, the only effect of @samp{-imacros @var{file}}
2659: is to make the macros defined in @var{file} available for use in the
2660: main input.
2661:
2662: @item -include @var{file}
2663: @findex -include
2664: Process @var{file} as input, and include all the resulting output,
2665: before processing the regular input file.
2666:
2667: @item -idirafter @var{dir}
2668: @findex -idirafter
2669: @cindex second include path
2670: Add the directory @var{dir} to the second include path. The directories
2671: on the second include path are searched when a header file is not found
2672: in any of the directories in the main include path (the one that
2673: @samp{-I} adds to).
2674:
2675: @item -iprefix @var{prefix}
2676: @findex -iprefix
2677: Specify @var{prefix} as the prefix for subsequent @samp{-iwithprefix}
2678: options.
2679:
2680: @item -iwithprefix @var{dir}
2681: @findex -iwithprefix
2682: Add a directory to the second include path. The directory's name is
2683: made by concatenating @var{prefix} and @var{dir}, where @var{prefix}
2684: was specified previously with @samp{-iprefix}.
2685:
2686: @item -lang-c
2687: @itemx -lang-c++
2688: @itemx -lang-objc
2689: @itemx -lang-objc++
2690: @findex -lang-c
2691: @findex -lang-c++
2692: @findex -lang-objc
2693: @findex -lang-objc++
2694: @findex #import
2695: Specify the source language. @samp{-lang-c++} makes the preprocessor
2696: handle C++ comment syntax (comments may begin with @samp{//}, in which
2697: case they end at end of line), and includes extra default include
2698: directories for C++; and @samp{-lang-objc} enables the Objective C
2699: @samp{#import} command. @samp{-lang-c} explicitly turns off both of
2700: these extensions, and @samp{-lang-objc++} enables both.
2701:
2702: These options are generated by the compiler driver @code{gcc}, but not
2703: passed from the @samp{gcc} command line.
2704:
2705: @item -lint
2706: Look for commands to the program checker @code{lint} embedded in
2707: comments, and emit them preceded by @samp{#pragma lint}. For example,
2708: the comment @samp{/* NOTREACHED */} becomes @samp{#pragma lint
2709: NOTREACHED}.
2710:
2711: This option is available only when you call @code{cpp} directly;
2712: @code{gcc} will not pass it from its command line.
2713:
2714: @item -$
2715: @findex -$
2716: Forbid the use of @samp{$} in identifiers. This is required for ANSI
2717: conformance. @code{gcc} automatically supplies this option to the
2718: preprocessor if you specify @samp{-ansi}, but @code{gcc} doesn't
2719: recognize the @samp{-$} option itself---to use it without the other
2720: effects of @samp{-ansi}, you must call the preprocessor directly.
2721:
2722: @end table
2723:
2724: @node Concept Index, Index, Invocation, Top
2725: @unnumbered Concept Index
2726: @printindex cp
2727:
2728: @node Index,, Concept Index, Top
2729: @unnumbered Index of Commands, Macros and Options
2730: @printindex fn
2731:
2732: @contents
2733: @bye
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