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1.1 root 1: \input texinfo
2: @setfilename cpp.info
3: @settitle The C Preprocessor
4: @ifinfo
5: This file documents the GNU C Preprocessor.
6:
7: Copyright (C) 1987 Richard M. Stallman.
8:
9: Permission is granted to make and distribute verbatim copies of
10: this manual provided the copyright notice and this permission notice
11: are preserved on all copies.
12:
13: @ignore
14: Permission is granted to process this file through Tex and print the
15: results, provided the printed document carries copying permission
16: notice identical to this one except for the removal of this paragraph
17: (this paragraph not being relevant to the printed manual).
18:
19: @end ignore
20: Permission is granted to copy and distribute modified versions of this
21: manual under the conditions for verbatim copying, provided also that
22: the entire resulting derived work is distributed under the terms of a
23: permission notice identical to this one.
24:
25: Permission is granted to copy and distribute translations of this manual
26: into another language, under the above conditions for modified versions.
27: @end ifinfo
28:
29: @titlepage
30: @sp 6
31: @center @titlefont{The C Preprocessor}
32: @sp 4
33: @center First Edition
34: @sp 1
35: @center January 1987
36: @sp 5
37: @center Richard M. Stallman
38: @page
39: @vskip 0pt plus 1filll
40: Copyright @copyright{} 1987 Richard M. Stallman.
41:
42: Permission is granted to make and distribute verbatim copies of
43: this manual provided the copyright notice and this permission notice
44: are preserved on all copies.
45:
46: Permission is granted to copy and distribute modified versions of this
47: manual under the conditions for verbatim copying, provided also that
48: the entire resulting derived work is distributed under the terms of a
49: permission notice identical to this one.
50:
51: Permission is granted to copy and distribute translations of this manual
52: into another language, under the above conditions for modified versions.
53: @end titlepage
54: @page
55:
56: @node Top, Global Actions,, (DIR)
57: @chapter The C Preprocessor
58:
59: The C preprocessor is a @dfn{macro processor} that is used automatically by
60: the C compiler to transform your program before actual compilation. It is
61: called a macro processor because it allows you to define @dfn{macros},
62: which are brief abbreviations for longer constructs.
63:
64: The C preprocessor provides four separate facilities that you can use as
65: you see fit:
66:
67: @itemize @bullet
68: @item
69: Inclusion of header files. These are files of declarations that can be
70: substituted into your program.
71:
72: @item
73: Macro expansion. You can define @dfn{macros}, which are abbreviations
74: for arbitrary fragments of C code, and then the C preprocessor will
75: replace the macros with their definitions throughout the program.
76:
77: @item
78: Conditional compilation. Using special preprocessor commands, you
79: can include or exclude parts of the program according to various
80: conditions.
81:
82: @item
83: Line control. If you use a program to combine or rearrange source files into
84: an intermediate file which is then compiled, you can use line control
85: to inform the compiler of where each source line originally came from.
86: @end itemize
87:
88: C preprocessors vary in some details. This manual discusses the GNU C
89: preprocessor, the C Compatible Compiler Preprocessor. The GNU C
90: preprocessor provides a superset of the features of ANSI Standard C.
91:
92: ANSI Standard C requires the rejection of many harmless constructs commonly
93: used by today's C programs. Such incompatibility would be inconvenient for
94: users, so the GNU C preprocessor is configured to accept these constructs
95: by default. Strictly speaking, to get ANSI Standard C, you must use the
96: options @samp{-T}, @samp{-undef} and @samp{-pedantic}, but in practice the
97: consequences of having strict ANSI Standard C make it undesirable to do
98: this. @xref{Invocation}.
99:
100: @menu
101: * Global Actions:: Actions made uniformly on all input files.
102: * Commands:: General syntax of preprocessor commands.
103: * Header Files:: How and why to use header files.
104: * Macros:: How and why to use macros.
105: * Conditionals:: How and why to use conditionals.
106: * Combining Sources:: Use of line control when you combine source files.
107: * Other Commands:: Miscellaneous preprocessor commands.
108: * Output:: Format of output from the C preprocessor.
109: * Invocation:: How to invoke the preprocessor; command options.
110: * Concept Index:: Index of concepts and terms.
111: * Index:: Index of commands, predefined macros and options.
112: @end menu
113:
114: @node Global Actions, Commands, Top, Top
115: @section Transformations Made Globally
116:
117: Most C preprocessor features are inactive unless you give specific commands
118: to request their use. (Preprocessor commands are lines starting with
119: @samp{#}; @pxref{Commands}). But there are three transformations that the
120: preprocessor always makes on all the input it receives, even in the absence
121: of commands.
122:
123: @itemize @bullet
124: @item
125: All C comments are replaced with single spaces.
126:
127: @item
128: Backslash-Newline sequences are deleted, no matter where. This
129: feature allows you to break long lines for cosmetic purposes without
130: changing their meaning.
131:
132: @item
133: Predefined macro names are replaced with their expansions
134: (@pxref{Predefined}).
135: @end itemize
136:
137: The first two transformations are done @emph{before} nearly all other parsing
138: and before preprocessor commands are recognized. Thus, for example, you
139: can split a line cosmetically with Backslash-Newline anywhere (except
140: when trigraphs are in use; see below).
141:
142: @example
143: /*
144: */ # /*
145: */ defi\
146: ne FO\
147: O 10\
148: 20
149: @end example
150:
151: @noindent
152: is equivalent into @samp{#define FOO 1020}. You can split even an escape
153: sequence with Backslash-Newline. For example, you can split @code{"foo\bar"}
154: between the @samp{\} and the @samp{b} to get
155:
156: @example
157: "foo\\
158: bar"
159: @end example
160:
161: @noindent
162: This behavior is unclean: in all other contexts, a Backslash can be
163: inserted in a string constant as an ordinary character by writing a double
164: Backslash, and this creates an exception. But the ANSI C standard requires
165: it. (Strict ANSI C does not allow Newlines in string constants, so they
166: do not consider this a problem.)
167:
168: But there are a few exceptions to all three transformations.
169:
170: @itemize @bullet
171: @item
172: C comments and predefined macro names are not recognized inside a
173: @samp{#include} command in which the file name is delimited with
174: @samp{<} and @samp{>}.
175:
176: @item
177: C comments and predefined macro names are never recognized within a
178: character or string constant. (Strictly speaking, this is the rule,
179: not an exception, but it is worth noting here anyway.)
180:
181: @item
182: Backslash-Newline may not safely be used within an ANSI ``trigraph''.
183: Trigraphs are converted before Backslash-Newline is deleted. If you
184: write what looks like a trigraph with a Backslash-Newline inside, the
185: Backslash-Newline is deleted as usual, but it is then too late to
186: recognize the trigraph.
187:
188: This exception is relevant only if you use the @samp{-T} option to
189: enable trigraph processing. @xref{Invocation}.
190: @end itemize
191:
192: @node Commands, Header Files, Global Actions, Top
193: @section Preprocessor Commands
194:
195: @cindex preprocessor commands
196: @cindex commands
197: Most preprocessor features are active only if you use preprocessor commands
198: to request their use.
199:
200: Preprocessor commands are lines in your program that start with @samp{#}.
201: The @samp{#} is followed by an identifier that is the @dfn{command name}.
202: For example, @samp{#define} is the command that defines a macro.
203: Whitespace is also allowed before and after the @samp{#}.
204:
205: The set of valid command names is fixed. Programs cannot define new
206: preprocessor commands.
207:
208: Some command names require arguments; these make up the rest of the command
209: line and must be separated from the command name by whitespace. For example,
210: @samp{#define} must be followed by a macro name and the intended expansion
211: of the macro.
212:
213: A preprocessor command cannot be more than one line in normal circumstances.
214: It may be split cosmetically with Backslash-Newline, but that has no effect
215: on its meaning. Comments containing Newlines can also divide the command into
216: multiple lines, but the comments are changed to Spaces before the command
217: is interpreted. The only way a significant Newline can occur in a preprocessor
218: command is within a string constant or character constant. Note that
219: most C compilers that might be applied to the output from the preprocessor
220: do not accept string or character constants containing Newlines.
221:
222: The @samp{#} and the command name cannot come from a macro expansion. For
223: example, if @samp{foo} is defined as a macro expanding to @samp{define},
224: that does not make @samp{#foo} a valid preprocessor command.
225:
226: @node Header Files, Macros, Commands, Top
227: @section Header Files
228:
229: @cindex header file
230: A header file is a file containing C declarations and macro definitions
231: (@pxref{Macros}) to be shared between several source files. You request
232: the use of a header file in your program with the C preprocessor command
233: @samp{#include}.
234:
235: @node Header Uses, Include Syntax, Header Files, Header Files
236: @subsection Uses of Header Files
237:
238: Header files serve two kinds of purposes.
239:
240: @itemize @bullet
241: @item
242: @findex system header files
243: System header files declare the interfaces to parts of the operating
244: system. You include them in your program to supply the definitions
245: you need to invoke system calls and libraries.
246:
247: @item
248: Your own header files contain declarations for interfaces between the
249: source files of your program. Each time you have a group of related
250: declarations and macro definitions all or most of which are needed in
251: several different source files, it is a good idea to create a header
252: file for them.
253: @end itemize
254:
255: Including a header file produces the same results in C compilation as
256: copying the header file into each source file that needs it. But such
257: copying would be time-consuming and error-prone. With a header file, the
258: related declarations appear in only one place. If they need to be changed,
259: they can be changed in one place, and programs that include the header file
260: will automatically use the new version when next recompiled. The header
261: file eliminates the labor of finding and changing all the copies as well as
262: the risk that a failure to find one copy will result in inconsistencies
263: within a program.
264:
265: The usual convention is to give header files names that end with @file{.h}.
266:
267: @node Include Syntax, Include Operation, Header Uses, Header Files
268: @subsection The @samp{#include} Command
269:
270: @findex #include
271: Both user and system header files are included using the preprocessor
272: command @samp{#include}. It has three variants:
273:
274: @table @code
275: @item #include <@var{file}>
276: This variant is used for system header files. It searches for a file
277: named @var{file} in a list of directories specified by you, then in a
278: standard list of system directories. You specify directories to
279: search for header files with the command option @samp{-I}
280: (@pxref{Invocation}). The option @samp{-nostdinc} inhibits searching
281: the standard system directories; in this case only the directories
282: you specify are searched.
283:
284: The parsing of this form of @samp{#include} is slightly special
285: because comments are not recognized within the @samp{<@dots{}>}.
286: Thus, in @samp{#include <x/*y>} the @samp{/*} does not start a comment
287: and the command specifies inclusion of a system header file named
288: @file{x/*y}. Of course, a header file with such a name is unlikely to
289: exist on Unix, where shell wildcard features would make it hard to
290: manipulate.@refill
291:
292: The argument @var{file} may not contain a @samp{>} character. It may,
293: however, contain a @samp{<} character.
294:
295: @item #include "@var{file}"
296: This variant is used for header files of your own program. It
297: searches for a file named @var{file} first in the current
298: directory, then in the same directories used for system header
299: files. The current directory is tried first because it is
300: presumed to be the location of the files of the program being
301: compiled. (If the @samp{-I-} option is used, the special treatment
302: of the current directory is inhibited.)
303:
304: The argument @var{file} may not contain @samp{"} characters. If
305: backslashes occur within @var{file}, they are considered ordinary text
306: characters, not escape characters. None of the character escape
307: sequences appropriate to string constants in C are processed. Thus,
308: @samp{#include "x\n\\y"} specifies a filename containing three
309: backslashes. It is not clear why this behavior is ever useful, but
310: the ANSI standard specifies it.
311:
312: @item #include @var{anything else}
313: This variant is called a @dfn{computed #include}. Any @samp{#include}
314: command whose argument does not fit the above two forms is a computed
315: include. The text @var{anything else} is checked for macro calls,
316: which are expanded (@pxref{Macros}). When this is done, the result
317: must fit one of the above two variants.
318:
319: This feature allows you to define a macro which controls the file name
320: to be used at a later point in the program. One application of this
321: is to allow a site-configuration file for your program to specify the
322: names of the system include files to be used. This can help in
323: porting the program to various operating systems in which the
324: necessary system header files are found in different places.
325: @end table
326:
327: @node Include Operation,, Include Syntax, Header Files
328: @subsection How @samp{#include} Works
329:
330: The @samp{#include} command works by directing the C preprocessor to scan
331: the specified file as input before continuing with the rest of the current
332: file. The output from the preprocessor contains the output already
333: generated, followed by the output resulting from the included file,
334: followed by the output that comes from the text after the @samp{#include}
335: command. For example, given two files as follows:
336:
337: @example
338: /* File program.c */
339: int x;
340: #include "header.h"
341:
342: main ()
343: @{
344: printf (test ());
345: @}
346:
347:
348: /* File header.h */
349: char *test ();
350: @end example
351:
352: @noindent
353: the output generated by the C preprocessor for @file{program.c} as input
354: would be
355:
356: @example
357: int x;
358: char *test ();
359:
360: main ()
361: @{
362: printf (test ());
363: @}
364: @end example
365:
366: Included files are not limited to declarations and macro definitions; they
367: are merely the typical use. Any fragment of a C program can be included
368: from another file. The include file could even contain the beginning of a
369: statement that is concluded in the containing file, or the end of a
370: statement that was started in the including file. However, a comment or a
371: string or character constant may not start in the included file and finish
372: in the including file. An unterminated comment, string constant or
373: character constant in an included file is considered to end (with an error
374: message) at the end of the file.
375:
376: The line following the @samp{#include} command is always treated as a
377: separate line by the C preprocessor even if the included file lacks a final
378: newline.
379:
380: @node Macros, Conditionals, Header Files, Top
381: @section Macros
382:
383: A macro is a sort of abbreviation which you can define once and then
384: use later. There are many complicated features associated with macros
385: in the C preprocessor.
386:
387: @menu
388: * Simple Macros:: Macros that always expand the same way.
389: * Argument Macros:: Macros that accept arguments that are substituted
390: into the macro expansion.
391: * Predefined:: Predefined macros that are always available.
392: * Stringification:: Macro arguments converted into string constants.
393: * Concatenation:: Building tokens from parts taken from macro arguments.
394: * Undefining:: Cancelling a macro's definition.
395: * Redefining:: Changing a macro's definition.
396: * Macro Pitfalls:: Macros can confuse the unwary. Here we explain
397: several common problems and strange features.
398: @end menu
399:
400: @node Simple Macros, Argument Macros, Macros, Macros
401: @subsection Simple Macros
402:
403: A @dfn{simple macro} is a kind of abbreviation. It is a name which stands
404: for a fragment of code.
405:
406: Before you can use a macro, you must @dfn{define} it explicitly with the
407: @samp{#define} command. @samp{#define} is followed by the name of the
408: macro and then the code it should be an abbreviation for. For example,
409:
410: @example
411: #define BUFFER_SIZE 1020
412: @end example
413:
414: @noindent
415: defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the text
416: @samp{1020}. Therefore, if somewhere after this @samp{#define} command
417: there comes a C statement of the form
418:
419: @example
420: foo = (char *) xmalloc (BUFFER_SIZE);
421: @end example
422:
423: @noindent
424: then the C preprocessor will recognize and @dfn{expand} the macro
425: @samp{BUFFER_SIZE}, resulting in
426:
427: @example
428: foo = (char *) xmalloc (1020);
429: @end example
430:
431: @noindent
432: the definition must be a single line; however, it may not end in the
433: middle of a multi-line string constant or character constant.
434:
435: The use of all upper case for macro names is a standard convention.
436: Programs are easier to read when it is possible to tell at a glance which
437: names are macros.
438:
439: Normally, a macro definition must be a single line, like all C preprocessor
440: commands. (You can split a long macro definition cosmetically with
441: Backslash-Newline.) There is one exception: Newlines can be included in
442: the macro definition if within a string or character constant. By the same
443: token, it is not possible for a macro definition to contain an unbalanced
444: quote character; the definition automatically extends to include the
445: matching quote character that ends the string or character constant.
446: Comments within a macro definition may contain Newlines, which make no
447: difference since the comments are entirely replaced with Spaces regardless
448: of their contents.
449:
450: Aside from the above, there is no restriction on what can go in a macro
451: body. Parentheses need not balance. The body need not resemble valid C
452: code. (Of course, you might get error messages from the C compiler when
453: you use the macro.)
454:
455: The C preprocessor scans your program sequentially, so macro definitions
456: take effect at the place you write them. Therefore, the following input to
457: the C preprocessor
458:
459: @example
460: foo = X;
461: #define X 4
462: bar = X;
463: @end example
464:
465: @noindent
466: produces as output
467:
468: @example
469: foo = X;
470:
471: bar = 4;
472: @end example
473:
474: After the preprocessor expands a macro name, the macro's definition body is
475: appended to the front of the remaining input, and the check for macro calls
476: continues. Therefore, the macro body can contain calls to other macros.
477: For example, after
478:
479: @example
480: #define BUFSIZE 1020
481: #define TABLESIZE BUFSIZE
482: @end example
483:
484: @noindent
485: the name @samp{TABLESIZE} when used in the program would go through two
486: stages of expansion, resulting ultimately in @samp{1020}.
487:
488: This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
489: The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
490: specify---in this case, @samp{BUFSIZE}---and does not check to see whether
491: it too is the name of a macro. It's only when you @emph{use} @samp{TABLESIZE}
492: that the result of its expansion is checked for more macro names.
493: @xref{Cascaded Macros}.
494:
495: @node Argument Macros, Predefined, Simple Macros, Macros
496: @subsection Macros with Arguments
497:
498: A simple macro always stands for exactly the same text, each time it is
499: used. Macros can be more flexible when they accept @dfn{arguments}.
500: Arguments are fragments of code that you supply each time the macro is
501: used. These fragments are included in the expansion of the macro according
502: to the directions in the macro definition.
503:
504: To define a macro that uses arguments, you write a @samp{#define} command
505: with a list of @dfn{argument names} in parentheses after the name of the
506: macro. The argument names may be any valid C identifiers, separated by
507: commas and optionally whitespace. The open-parenthesis must follow the
508: macro name immediately, with no space in between.
509:
510: For example, here is a macro that computes the minimum of two numeric
511: values, as it is defined in many C programs:
512:
513: @example
514: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
515: @end example
516:
517: @noindent
518: (This is not the best way to define a ``minimum'' macro in GNU C.
519: @xref{Side Effects}, for more information.)
520:
521: To use a macro that expects arguments, you write the name of the macro
522: followed by a list of @dfn{actual arguments} in parentheses. separated by
523: commas. The number of actual arguments you give must match the number of
524: arguments the macro expects. Examples of use of the macro @samp{min}
525: include @samp{min (1, 2)} and @samp{min (x + 28, *p)}.
526:
527: The expansion text of the macro depends on the arguments you use.
528: Each of the argument names of the macro is replaced, throughout the
529: macro definition, with the corresponding actual argument. Using the
530: same macro @samp{min} defined above, @samp{min (1, 2)} expands into
531:
532: @example
533: ((1) < (2) ? (1) : (2))
534: @end example
535:
536: @noindent
537: where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.
538:
539: Likewise, @samp{min (x + 28, *p)} expands into
540:
541: @example
542: ((x + 28) < (*p) ? (x + 28) : (*p))
543: @end example
544:
545: Parentheses in the actual arguments must balance; a comma within
546: parentheses does not end an argument. However, there is no requirement for
547: brackets or braces to balance; thus, if you want to supply @samp{array[x =
548: y, x + 1]} as an argument, you must write it as @samp{array[(x = y, x +
549: 1)]}, which is equivalent C code.
550:
551: After the actual arguments are substituted into the macro body, the entire
552: result is appended to the front of the remaining input, and the check for
553: macro calls continues. Therefore, the actual arguments can contain calls
554: to other macros, either with or without arguments, or even to the same
555: macro. The macro body can also contain calls to other macros. For
556: example, @samp{min (min (a, b), c)} expands into
557:
558: @example
559: ((((a) < (b) ? (a) : (b))) < (c)
560: ? (((a) < (b) ? (a) : (b)))
561: : (c))
562: @end example
563:
564: @noindent
565: (Line breaks shown here for clarity would not actually be generated.)
566:
567: If you use the macro name followed by something other than an
568: open-parenthesis (after ignoring any spaces, tabs and comments that
569: follow), it is not a call to the macro, and the preprocessor does not
570: change what you have written. Therefore, it is possible for the same name
571: to be a variable or function in your program as well as a macro, and you
572: can choose in each instance whether to refer to the macro (if an actual
573: argument list follows) or the variable or function (if an argument list
574: does not follow).
575:
576: Such dual use of one name could be confusing and should be avoided
577: except when the two meanings are effectively synonymous: that is, when the
578: name is both a macro and a function and the two have similar effects. You
579: can think of the name simply as a function; use of the name for purposes
580: other than calling it (such as, to take the address) will refer to the
581: function, while calls will expand the macro and generate better but
582: equivalent code. For example, you can use a function named @samp{min} in
583: the same source file that defines the macro. If you write @samp{&min} with
584: no argument list, you refer to the function. If you write @samp{min (x,
585: bb)}, with an argument list, the macro is expanded. If you write
586: @samp{(min) (a, bb)}, where the name @samp{min} is not followed by an
587: open-parenthesis, the macro is not expanded, so you wind up with a call to
588: the function @samp{min}.
589:
590: It is not allowed to define the same name as both a simple macro and
591: a macro with arguments.
592:
593: In the definition of a macro with arguments, the list of argument names
594: must follow the macro name immediately with no space in between. If there
595: is a space after the macro name, the macro is defined as taking no
596: arguments, and all the rest of the name is taken to be the expansion. The
597: reason for this is that it is often useful to define a macro that takes no
598: arguments and whose definition begins with an identifier in parentheses.
599: This rule about spaces makes it possible for you to do either this:
600:
601: @example
602: #define FOO(x) - 1 / (x)
603: @end example
604:
605: @noindent
606: (which defines @samp{FOO} to take an argument and expand into minus the
607: reciprocal of that argument) or this:
608:
609: @example
610: #define BAR (x) - 1 / (x)
611: @end example
612:
613: @noindent
614: (which defines @samp{BAR} to take no argument and always expand into
615: @samp{(x) - 1 / (x)}).
616:
617: Note that the @emph{uses} of a macro with arguments can have spaces before
618: the left parenthesis; it's the @emph{definition} where it matters whether
619: there is a space.
620:
621: @node Predefined, Stringification, Argument Macros, Macros
622: @subsection Predefined Macros
623:
624: @cindex predefined macros
625: Several simple macros are predefined. You can use them without giving
626: definitions for them. They fall into two classes: standard macros and
627: system-specific macros.
628:
629: @menu
630: * Standard Predefined:: Standard predefined macros.
631: * Nonstandard Predefined:: Nonstandard predefined macros.
632: @end menu
633:
634: @node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
635: @subsubsection Standard Predefined Macros
636:
637: The standard predefined macros are available with the same meanings
638: regardless of the machine or operating system on which you are using GNU C.
639: Their names all start and end with double underscores. Those preceding
640: @code{__GNUC__} in this table are standardized by ANSI C; the rest are
641: GNU C extensions.
642:
643: @table @code
644: @item __FILE__
645: @findex __FILE__
646: This macro expands to the name of the current input file, in the form
647: of a C string constant.
648:
649: @item __LINE__
650: @findex __LINE__
651: This macro expands to the current input line number, in the form of a
652: decimal integer constant. While we call it a predefined macro, it's
653: a pretty strange macro, since its ``definition'' changes with each
654: new line of source code.
655:
656: This and @samp{__FILE__} are useful in generating an error message to
657: report an inconsistency detected by the program; the message can state
658: the source line at which the inconsistency was detected. For example,
659:
660: @example
661: fprintf (stderr, "Internal error: negative string length "
662: "%d at %s, line %d.",
663: length, __FILE__, __LINE__);
664: @end example
665:
666: A @samp{#include} command changes the expansions of @samp{__FILE__}
667: and @samp{__LINE__} to correspond to the included file. At the end of
668: that file, when processing resumes on the input file that contained
669: the @samp{#include} command, the expansions of @samp{__FILE__} and
670: @samp{__LINE__} revert to the values they had before the
671: @samp{#include} (but @samp{__LINE__} is then incremented by one as
672: processing moves to the line after the @samp{#include}).
673:
674: The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
675: if a @samp{#line} command is used. @xref{Combining Sources}.
676:
677: @item __DATE__
678: @findex __DATE__
679: This macro expands to a string constant that describes the date on
680: which the preprocessor is being run. The string constant contains
681: eleven characters and looks like @samp{"Jan 29 1987"} or @w{@samp{"Apr
682: 1 1905"}}.
683:
684: @item __TIME__
685: @findex __TIME__
686: This macro expands to a string constant that describes the time at
687: which the preprocessor is being run. The string constant contains
688: eight characters and looks like @samp{"23:59:01"}.
689:
690: @item __STDC__
691: @findex __STDC__
692: This macro expands to the constant 1, to signify that this is ANSI
693: Standard C. (Whether that is actually true depends on what C compiler
694: will operate on the output from the preprocessor.)
695:
696: @item __GNUC__
697: This macro is defined if and only if this is GNU C. This macro is
698: defined only when the entire GNU C compiler is in use; if you invoke
699: the preprocessor directly, @samp{__GNUC__} is undefined.
700:
701: @item __STRICT_ANSI__
702: This macro is defined if and only if the @samp{-ansi} switch was
703: specified when GNU C was invoked. Its definition is the null string.
704: This macro exists primarily to direct certain GNU header files not to
705: define certain traditional Unix constructs which are incompatible with
706: ANSI C.
707:
708: @item __VERSION__
709: This macro expands to a string which describes the version number of
710: GNU C. The string is normally a sequence of decimal numbers separated
711: by periods, such as @samp{"1.18"}. The only reasonable use of this
712: macro is to incorporate it into a string constant.
713:
714: @item __OPTIMIZE__
715: This macro is defined in optimizing compilations. It causes certain
716: GNU header files to define alternative macro definitions for some
717: system library functions. It is unwise to refer to or test the
718: definition of this macro unless you make very sure that programs will
719: execute with the same effect regardless.
720:
721: @item __CHAR_UNSIGNED__
722: This macro is defined if and only if the data type @code{char} is
723: unsigned on the target machine. It exists to cause the standard
724: header file @file{limit.h} to work correctly. It is bad practice
725: to refer to this macro yourself; instead, refer to the standard
726: macros defined in @file{limit.h}.
727: @end table
728:
729: @node Nonstandard Predefined,, Standard Predefined, Predefined
730: @subsubsection Nonstandard Predefined Macros
731:
732: The C preprocessor normally has several predefined macros that vary between
733: machines because their purpose is to indicate what type of system and
734: machine is in use. This manual, being for all systems and machines, cannot
735: tell you exactly what their names are; instead, we offer a list of some
736: typical ones.
737:
738: Some nonstandard predefined macros describe the operating system in use,
739: with more or less specificity. For example,
740:
741: @table @code
742: @item unix
743: @findex unix
744: @samp{unix} is normally predefined on all Unix systems.
745:
746: @item BSD
747: @findex BSD
748: @samp{BSD} is predefined on recent versions of Berkeley Unix
749: (perhaps only in version 4.3).
750: @end table
751:
752: Other nonstandard predefined macros describe the kind of CPU, with more or
753: less specificity. For example,
754:
755: @table @code
756: @item vax
757: @findex vax
758: @samp{vax} is predefined on Vax computers.
759:
760: @item mc68000
761: @findex mc68000
762: @samp{mc68000} is predefined on most computers whose CPU is a Motorola
763: 68000, 68010 or 68020.
764:
765: @item m68k
766: @findex m68k
767: @samp{m68k} is also predefined on most computers whose CPU is a 68000,
768: 68010 or 68020; however, some makers use @samp{mc68000} and some use
769: @samp{m68k}. Some predefine both names. What happens in GNU C
770: depends on the system you are using it on.
771:
772: @item M68020
773: @findex M68020
774: @samp{M68020} has been observed to be predefined on some systems that
775: use 68020 CPUs---in addition to @samp{mc68000} and @samp{m68k} that
776: are less specific.
777:
778: @item ns32000
779: @findex ns32000
780: @samp{ns32000} is predefined on computers which use the National
781: Semiconductor 32000 series CPU.
782: @end table
783:
784: Yet other nonstandard predefined macros describe the manufacturer of
785: the system. For example,
786:
787: @table @code
788: @item sun
789: @findex sun
790: @samp{sun} is predefined on all models of Sun computers.
791:
792: @item pyr
793: @findex pyr
794: @samp{pyr} is predefined on all models of Pyramid computers.
795:
796: @item sequent
797: @findex sequent
798: @samp{sequent} is predefined on all models of Sequent computers.
799: @end table
800:
801: These predefined symbols are not only nonstandard, they are contrary to the
802: ANSI standard because their names do not start with underscores. However,
803: the GNU C preprocessor would be useless if it did not predefine the same
804: names that are normally predefined on the system and machine you are using.
805: Even system header files check the predefined names and will generate
806: incorrect declarations if they do not find the names that are expected.
807:
808: The set of nonstandard predefined names in the GNU C preprocessor is
809: controlled by the macro @samp{CPP_PREDEFINES}, which should be a string
810: containing @samp{-D} options, separated by spaces. For example, on the Sun,
811: the definition
812:
813: @example
814: #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
815: @end example
816:
817: @noindent
818: is used.
819:
820: The @samp{-ansi} option which requests complete support for ANSI C
821: inhibits the definition of these predefined symbols.
822:
823: @node Stringification, Concatenation, Predefined, Macros
824: @subsection Stringification
825:
826: @cindex stringification
827: @dfn{Stringification} means turning a code fragment into a string constant
828: whose contents are the text for the code fragment. For example,
829: stringifying @samp{foo (z)} results in @samp{"foo (z)"}.
830:
831: In the C preprocessor, stringification is an option available when macro
832: arguments are substituted into the macro definition. In the body of the
833: definition, when an argument name appears, the character @samp{#} before
834: the name specifies stringification of the corresponding actual argument
835: when it is substituted at that point in the definition. The same argument
836: may be substituted in other places in the definition without
837: stringification if the argument name appears in those places with no
838: @samp{#}.
839:
840: Here is an example of a macro definition that uses stringification:
841:
842: @example
843: #define WARN_IF(EXP) \
844: do @{ if (EXP) fprintf (stderr, "Warning: " #EXP "\n"); @} while (0)
845: @end example
846:
847: @noindent
848: Here the actual argument for @samp{EXP} is substituted once as given,
849: into the @samp{if} statement, and once as stringified, into the
850: argument to @samp{fprintf}. The @samp{do} and @samp{while (0)} are
851: a kludge to make it possible to write @samp{WARN_IF (@var{arg});},
852: which the resemblance of @samp{WARN_IF} to a function would make
853: C programmers want to do; @pxref{Swallow Semicolon}).
854:
855: The stringification feature is limited to transforming one macro argument
856: into one string constant: there is no way to combine the argument with
857: other text and then stringify it all together. But the example above shows
858: how an equivalent result can be obtained in ANSI Standard C using the
859: feature that adjacent string constants are concatenated as one string
860: constant. The preprocessor stringifies @samp{EXP}'s actual argument
861: into a separate string constant, resulting in text like
862:
863: @example
864: do @{ if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0)
865: @end example
866:
867: @noindent
868: but the C compiler then sees three consecutive string constants and
869: concatenates them into one, producing effectively
870:
871: @example
872: do @{ if (x == 0) fprintf (stderr, "Warning: x == 0\n"); @} while (0)
873: @end example
874:
875: Stringification in C involves more than putting doublequote characters
876: around the fragment; it is necessary to put backslashes in front of all
877: doublequote characters, and all backslashes in string and character
878: constants, in order to get a valid C string constant with the proper
879: contents. Thus, stringifying @samp{p = "foo\n";} results in @samp{"p =
880: \"foo\\n\";"}. However, backslashes that are not inside of string or
881: character constants are not duplicated: @samp{\n} by itself stringifies to
882: @samp{"\n"}.
883:
884: Whitespace (including comments) in the text being stringified is handled
885: according to precise rules. All leading and trailing whitespace is ignored.
886: Any sequence of whitespace in the middle of the text is converted to
887: a single space in the stringified result.
888:
889: @node Concatenation, Undefining, Stringification, Macros
890: @subsection Concatenation
891:
892: @cindex concatenation
893: @dfn{Concatenation} means joining two strings into one. In the context
894: of macro expansion, concatenation refers to joining two lexical units
895: into one longer one. Specifically, an actual argument to the macro can be
896: concatenated with another actual argument or with fixed text to produce
897: a longer name. The longer name might be the name of a function,
898: variable or type, or a C keyword; it might even be the name of another
899: macro, in which case it will be expanded.
900:
901: When you define a macro, you request concatenation with the special
902: operator @samp{##} in the macro body. When the macro is called,
903: after actual arguments are substituted, all @samp{##} operators are
904: deleted, and so is any whitespace next to them (including whitespace
905: that was part of an actual argument). The result is to concatenate
906: the syntactic tokens on either side of the @samp{##}.
907:
908: Consider a C program that interprets named commands. There probably needs
909: to be a table of commands, perhaps an array of structures declared as
910: follows:
911:
912: @example
913: struct command
914: @{
915: char *name;
916: void (*function) ();
917: @};
918:
919: struct command commands[] =
920: @{
921: @{ "quit", quit_command@},
922: @{ "help", help_command@},
923: @dots{}
924: @};
925: @end example
926:
927: It would be cleaner not to have to give each command name twice, once in
928: the string constant and once in the function name. A macro which takes the
929: name of a command as an argument can make this unnecessary. The string
930: constant can be created with stringification, and the function name by
931: concatenating the argument with @samp{_command}. Here is how it is done:
932:
933: @example
934: #define COMMAND(NAME) @{ #NAME, NAME ## _command @}
935:
936: struct command commands[] =
937: @{
938: COMMAND (quit),
939: COMMAND (help),
940: @dots{}
941: @};
942: @end example
943:
944: The usual case of concatenation is concatenating two names (or a name and a
945: number) into a longer name. But this isn't the only valid case. It is
946: also possible to concatenate two numbers (or a number and a name, such as
947: @samp{1.5} and @samp{e3}) into a number. Also, multi-character operators
948: such as @samp{+=} can be formed by concatenation. In some cases it is even
949: possible to piece together a string constant. However, two pieces of text
950: that don't together form a valid lexical unit cannot be concatenated. For
951: example, concatenation with @samp{x} on one side and @samp{+} on the other
952: is not meaningful because those two characters can't fit together in any
953: lexical unit of C. The ANSI standard says that such attempts at
954: concatenation are undefined, but in the GNU C preprocessor it is well
955: defined: it puts the @samp{x} and @samp{+} side by side with no particular
956: special results.
957:
958: Keep in mind that the C preprocessor converts comments to whitespace before
959: macros are even considered. Therefore, you cannot create a comment by
960: concatenating @samp{/} and @samp{*}: the @samp{/*} sequence that starts a
961: comment is not a lexical unit, but rather the beginning of a ``long'' space
962: character. Also, you can freely use comments next to a @samp{##} in a
963: macro definition, or in actual arguments that will be concatenated, because
964: the comments will be converted to spaces at first sight, and concatenation
965: will later discard the spaces.
966:
967: @node Undefining, Redefining, Concatenation, Macros
968: @subsection Undefining Macros
969:
970: @cindex undefining macros
971: To @dfn{undefine} a macro means to cancel its definition. This is done
972: with the @samp{#undef} command. @samp{#undef} is followed by the macro
973: name to be undefined.
974:
975: Like definition, undefinition occurs at a specific point in the source
976: file, and it applies starting from that point. The name ceases to be a
977: macro name, and from that point on it is treated by the preprocessor as if
978: it had never been a macro name.
979:
980: For example,
981:
982: @example
983: #define FOO 4
984: x = FOO;
985: #undef FOO
986: x = FOO;
987: @end example
988:
989: @noindent
990: expands into
991:
992: @example
993: x = 4;
994:
995: x = FOO;
996: @end example
997:
998: @noindent
999: In this example, @samp{FOO} had better be a variable or function as well
1000: as (temporarily) a macro, in order for the result of the expansion to be
1001: valid C code.
1002:
1003: The same form of @samp{#undef} command will cancel definitions with
1004: arguments or definitions that don't expect arguments. The @samp{#undef}
1005: command has no effect when used on a name not currently defined as a macro.
1006:
1007: @node Redefining, Macro Pitfalls, Undefining, Macros
1008: @subsection Redefining Macros
1009:
1010: @cindex redefining macros
1011: @dfn{Redefining} a macro means defining (with @samp{#define}) a name that
1012: is already defined as a macro.
1013:
1014: A redefinition is trivial if the new definition is transparently identical
1015: to the old one. You probably wouldn't deliberately write a trivial
1016: redefinition, but they can happen automatically when a header file is
1017: included more than once (@pxref{Header Files}), so they are accepted
1018: silently and without effect.
1019:
1020: Nontrivial redefinition is considered likely to be an error, so
1021: it provokes a warning message from the preprocessor. However, sometimes it
1022: is useful to change the definition of a macro in mid-compilation. You can
1023: inhibit the warning by undefining the macro with @samp{#undef} before the
1024: second definition.
1025:
1026: In order for a reefinition to be trivial, the new definition must exactly match
1027: the one already in effect, with two possible exceptions:
1028:
1029: @itemize @bullet
1030: @item
1031: Whitespace may be added or deleted at the beginning or the end.
1032:
1033: @item
1034: Whitespace may be changed in the middle (but not inside strings).
1035: However, it may not be eliminated entirely, and it may not be added
1036: where there was no whitespace at all.
1037: @end itemize
1038:
1039: Recall that a comment counts as whitespace.
1040:
1041: @node Macro Pitfalls,, Redefining, Macros
1042: @subsection Pitfalls and Subtleties of Macros
1043:
1044: In this section we describe some special rules that apply to macros and
1045: macro expansion, and point out certain cases in which the rules have
1046: counterintuitive consequences that you must watch out for.
1047:
1048: @menu
1049: * Misnesting:: Macros can contain unmatched parentheses.
1050: * Macro Parentheses:: Why apparently superfluous parentheses
1051: may be necessary to avoid incorrect grouping.
1052: * Swallow Semicolon:: Macros that look like functions
1053: but expand into compound statements.
1054: * Side Effects:: Unsafe macros that cause trouble when
1055: arguments contain side effects.
1056: * Self-Reference:: Macros whose definitions use the macros' own names.
1057: * Argument Prescan:: Actual arguments are checked for macro calls
1058: before they are substituted.
1059: * Cascaded Macros:: Macros whose definitions use other macros.
1060: @end menu
1061:
1062: @node Misnesting, Macro Parentheses, Macro Pitfalls, Macro Pitfalls
1063: @subsubsection Improperly Nested Constructs
1064:
1065: Recall that when a macro is called with arguments, the arguments are
1066: substituted into the macro body and the result is checked, together with
1067: the rest of the input file, for more macro calls.
1068:
1069: It is possible to piece together a macro call coming partially from the
1070: macro body and partially from the actual arguments. For example,
1071:
1072: @example
1073: #define double(x) (2*(x))
1074: #define call_with_1(x) x(1)
1075: @end example
1076:
1077: @noindent
1078: would expand @samp{call_with_1 (double)} into @samp{(2*(1))}.
1079:
1080: Macro definitions do not have to have balanced parentheses. By writing an
1081: unbalanced open parenthesis in a macro body, it is possible to create a
1082: macro call that begins inside the macro body but ends outside of it. For
1083: example,
1084:
1085: @example
1086: #define strange(file) fprintf (file, "%s %d",
1087: @dots{}
1088: strange(stderr) p, 35)
1089: @end example
1090:
1091: @noindent
1092: This bizarre example expands to @samp{fprintf (stderr, "%s %d", p, 35)}!
1093:
1094: @node Macro Parentheses, Swallow Semicolon, Misnesting, Macro Pitfalls
1095: @subsubsection Unintended Grouping of Arithmetic
1096:
1097: You may have noticed that in most of the macro definition examples shown
1098: above, each occurrence of a macro argument name had parentheses around it.
1099: In addition, another pair of parentheses usually surround the entire macro
1100: definition. Here is why it is best to write macros that way.
1101:
1102: Suppose you define a macro as follows
1103:
1104: @example
1105: #define ceil_div(x, y) (x + y - 1) / y
1106: @end example
1107:
1108: @noindent
1109: whose purpose is to divide, rounding up. (One use for this
1110: operation is to compute how many @samp{int}'s are needed to hold
1111: a certain number of @samp{char}'s.) Then suppose it is used as follows:
1112:
1113: @example
1114: a = ceil_div (b & c, sizeof (int));
1115: @end example
1116:
1117: @noindent
1118: This expands into
1119:
1120: @example
1121: a = (b & c + sizeof (int) - 1) / sizeof (int);
1122: @end example
1123:
1124: @noindent
1125: which does not do what is intended. The operator-precedence rules of
1126: C make it equivalent to this:
1127:
1128: @example
1129: a = (b & (c + sizeof (int) - 1)) / sizeof (int);
1130: @end example
1131:
1132: @noindent
1133: But what we want is this:
1134:
1135: @example
1136: a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
1137: @end example
1138:
1139: @noindent
1140: Defining the macro as
1141:
1142: @example
1143: #define ceil_div(x, y) ((x) + (y) - 1) / (y)
1144: @end example
1145:
1146: @noindent
1147: provides the desired result.
1148:
1149: However, unintended grouping can result in another way. Consider
1150: @samp{sizeof ceil_div(1, 2)}. That has the appearance of a C expression
1151: that would compute the size of the type of @samp{ceil_div (1, 2)}, but in
1152: fact it means something very different. Here is what it expands to:
1153:
1154: @example
1155: sizeof ((1) + (2) - 1) / (2)
1156: @end example
1157:
1158: @noindent
1159: This would take the size of an integer and divide it by two. The precedence
1160: rules have put the division outside the @samp{sizeof} when it was intended
1161: to be inside.
1162:
1163: Parentheses around the entire macro definition can prevent such problems.
1164: Here, then, is the recommended way to define @samp{ceil_div}:
1165:
1166: @example
1167: #define ceil_div(x, y) (((x) + (y) - 1) / (y))
1168: @end example
1169:
1170: @node Swallow Semicolon, Side Effects, Macro Parentheses, Macro Pitfalls
1171: @subsubsection Swallowing the Semicolon
1172:
1173: @cindex semicolons (after macro calls)
1174: Often it is desirable to define a macro that expands into a compound
1175: statement. Consider, for example, the following macro, that advances a
1176: pointer (the argument @samp{p} says where to find it) across whitespace
1177: characters:
1178:
1179: @example
1180: #define SKIP_SPACES (p, limit) \
1181: @{ register char *lim = (limit); \
1182: while (p != lim) @{ \
1183: if (*p++ != ' ') @{ \
1184: p--; break; @}@}@}
1185: @end example
1186:
1187: @noindent
1188: Here Backslash-Newline is used to split the macro definition, which must
1189: be a single line, so that it resembles the way such C code would be
1190: layed out if not part of a macro definition.
1191:
1192: A call to this macro might be @samp{SKIP_SPACES (p, lim)}. Strictly
1193: speaking, the call expands to a compound statement, which is a complete
1194: statement with no need for a semicolon to end it. But it looks like a
1195: function call. So it minimizes confusion if you can use it like a function
1196: call, writing a semicolon afterward, as in @samp{SKIP_SPACES (p, lim);}
1197:
1198: But this can cause trouble before @samp{else} statements, because the
1199: semicolon is actually a null statement. Suppose you write
1200:
1201: @example
1202: if (*p != 0)
1203: SKIP_SPACES (p, lim);
1204: else @dots{}
1205: @end example
1206:
1207: @noindent
1208: The presence of two statements---the compound statement and a null
1209: statement---in between the @samp{if} condition and the @samp{else}
1210: makes invalid C code.
1211:
1212: The definition of the macro @samp{SKIP_SPACES} can be altered to solve
1213: this problem, using a @samp{do @dots{} while} statement. Here is how:
1214:
1215: @example
1216: #define SKIP_SPACES (p, limit) \
1217: do @{ register char *lim = (limit); \
1218: while (p != lim) @{ \
1219: if (*p++ != ' ') @{ \
1220: p--; break; @}@}@} \
1221: while (0)
1222: @end example
1223:
1224: Now @samp{SKIP_SPACES (p, lim);} expands into
1225:
1226: @example
1227: do @{@dots{}@} while (0);
1228: @end example
1229:
1230: @noindent
1231: which is one statement.
1232:
1233: @node Side Effects, Self-Reference, Swallow Semicolon, Macro Pitfalls
1234: @subsubsection Duplication of Side Effects
1235:
1236: @cindex side effects (in macro arguments)
1237: @cindex unsafe macros
1238: Many C programs define a macro @samp{min}, for ``minimum'', like this:
1239:
1240: @example
1241: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
1242: @end example
1243:
1244: When you use this macro with an argument containing a side effect,
1245: as shown here,
1246:
1247: @example
1248: next = min (x + y, foo (z));
1249: @end example
1250:
1251: @noindent
1252: it expands as follows:
1253:
1254: @example
1255: next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
1256: @end example
1257:
1258: @noindent
1259: where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
1260: for @samp{Y}.
1261:
1262: The function @samp{foo} is used only once in the statement as it appears
1263: in the program, but the expression @samp{foo (z)} has been substituted
1264: twice into the macro expansion. As a result, @samp{foo} might be called
1265: two times when the statement is executed. If it has side effects or
1266: if it takes a long time to compute, the results might not be what you
1267: intended. We say that @samp{min} is an @dfn{unsafe} macro.
1268:
1269: The best solution to this problem is to define @samp{min} in a way that
1270: computes the value of @samp{foo (z)} only once. The C language offers no
1271: way standard way to do this, but it can be done with GNU C extensions as
1272: follows:
1273:
1274: @example
1275: #define min(X, Y) \
1276: (@{ typeof (X) __x = (X), __y = (Y); \
1277: (__x < __y) ? __x : __y; @})
1278: @end example
1279:
1280: If you do not wish to use GNU C extensions, the only solution is to be
1281: careful when @emph{using} the macro @samp{min}. For example, you can
1282: calculate the value of @samp{foo (z)}, save it in a variable, and use that
1283: variable in @samp{min}:
1284:
1285: @example
1286: #define min(X, Y) ((X) < (Y) ? (X) : (Y))
1287: @dots{}
1288: @{
1289: int tem = foo (z);
1290: next = min (x + y, tem);
1291: @}
1292: @end example
1293:
1294: @noindent
1295: (where I assume that @samp{foo} returns type @samp{int}).
1296:
1297: @node Self-Reference, Argument Prescan, Side Effects, Macro Pitfalls
1298: @subsubsection Self-Referential Macros
1299:
1300: @cindex self-reference
1301: A @dfn{self-referential} macro is one whose name appears in its definition.
1302: A special feature of ANSI Standard C is that the self-reference is not
1303: considered a macro call. It is passed into the preprocessor output
1304: unchanged.
1305:
1306: Let's consider an example:
1307:
1308: @example
1309: #define foo (4 + foo)
1310: @end example
1311:
1312: @noindent
1313: where @samp{foo} is also a variable in your program.
1314:
1315: Following the ordinary rules, each reference to @samp{foo} will expand into
1316: @samp{(4 + foo)}; then this will be rescanned and will expand into @samp{(4
1317: + (4 + foo))}; and so on until it causes a fatal error (memory full) in the
1318: preprocessor.
1319:
1320: However, the special rule about self-reference cuts this process short
1321: after one step, at @samp{(4 + foo)}. Therefore, this macro definition
1322: has the possibly useful effect of causing the program to add 4 to
1323: the value of @samp{foo} wherever @samp{foo} is referred to.
1324:
1325: In most cases, it is a bad idea to take advantage of this feature.
1326: A person reading the program who sees that @samp{foo} is a variable will
1327: not expect that it is a macro as well. The reader will come across a
1328: the identifier @samp{foo} in the program and think its value should be
1329: that of the variable @samp{foo}, whereas in fact the value is four
1330: greater.
1331:
1332: The special rule for self-reference applies also to @dfn{indirect}
1333: self-reference. This is the case where a macro @var{x} expands to use a
1334: macro @samp{y}, and @samp{y}'s expansion refers to the macro @samp{x}. The
1335: resulting reference to @samp{x} comes indirectly from the expansion of
1336: @samp{x}, so it is a self-reference and is not further expanded. Thus,
1337: after
1338:
1339: @example
1340: #define x (4 + y)
1341: #define y (2 * x)
1342: @end example
1343:
1344: @noindent
1345: @samp{x} would expand into @samp{(4 + (2 * x))}. Clear?
1346:
1347: But suppose @samp{y} is used elsewhere, not from the definition of @samp{x}.
1348: Then the use of @samp{x} in the expansion of @samp{y} is not a self-reference
1349: because @samp{x} is not ``in progress''. So it does expand. However,
1350: the expansion of @samp{x} contains a reference to @samp{y}, and that
1351: is an indirect self-reference now because @samp{y} is ``in progress''.
1352: The result is that @samp{y} expands to @samp{(2 * (4 + y))}.
1353:
1354: It is not clear that this behavior would ever be useful, but it is specified
1355: by the ANSI C standard, so you need to understand it.
1356:
1357: @node Argument Prescan, Cascaded Macros, Self-Reference, Macro Pitfalls
1358: @subsubsection Separate Expansion of Macro Arguments
1359:
1360: We have explained that the expansion of a macro, including the substituted
1361: actual arguments, is scanned over again for macro calls to be expanded.
1362:
1363: What really happens is more subtle: first each actual argument text is scanned
1364: separately for macro calls. Then the results of this are substituted into
1365: the macro body to produce the macro expansion, and the macro expansion
1366: is scanned again for macros to expand.
1367:
1368: The result is that the actual arguments are scanned @emph{twice} to expand
1369: macro calls in them.
1370:
1371: Most of the time, this has no effect. If the actual argument contained
1372: any macro calls, they are expanded during the first scan. The result
1373: therefore contains no macro calls, so the second scan does not change it.
1374: If the actual argument were substituted as given, with no prescan,
1375: the single remaining scan would find the same macro calls and produce
1376: the same results.
1377:
1378: You might expect the double scan to change the results when a
1379: self-referential macro is used in an actual argument of another macro
1380: (@pxref{Self-Reference}): the self-referential macro would be expanded once
1381: in the first scan, and a second time in the second scan. But this is not
1382: what happens. The self-references that do not expand in the first scan are
1383: marked so that they will not expand in the second scan either.
1384:
1385: The prescan is not done when an argument is stringified or concatenated.
1386: (More precisely, stringification and concatenation use the argument as
1387: written, in un-prescanned form. The same actual argument would be used in
1388: prescanned form if it is substituted elsewhere without stringification or
1389: concatenation.) Thus,
1390:
1391: @example
1392: #define str(s) #s
1393: #define foo 4
1394: str (foo)
1395: @end example
1396:
1397: @noindent
1398: expands to @samp{"foo"}. Once more, prescan has been prevented from
1399: having any noticeable effect.
1400:
1401: You might now ask, ``Why mention the prescan, if it makes no difference?
1402: Why not skip it and make the preprocessor faster?'' The answer is that the
1403: prescan does make a difference in two special cases:
1404:
1405: @itemize @bullet
1406: @item
1407: Nested calls to a macro.
1408:
1409: @item
1410: Macros that call other macros that stringify or concatenate.
1411: @end itemize
1412:
1413: We say that @dfn{nested} calls to a macro occur when a macro's actual
1414: argument contains a call to that very macro. For example, if @samp{f}
1415: is a macro that expects one argument, @samp{f (f (1))} is a nested
1416: pair of calls to @samp{f}. The desired expansion is made by
1417: expanding @samp{f (1)} and substituting that into the definition of
1418: @samp{f}. The prescan causes the expected result to happen.
1419: Without the prescan, @samp{f (1)} itself would be substituted as
1420: an actual argument, and the inner use of @samp{f} would appear
1421: during the main scan as an indirect self-reference and would not
1422: be expanded. Here, the prescan cancels an undesirable side effect
1423: (in the medical, not computational, sense of the term) of the special
1424: rule for self-referential macros.
1425:
1426: There is also one case where prescan is useful. It is possible
1427: to use prescan to expand an argument and then stringify it---if you use
1428: two levels of macros. Let's add a new macro @samp{xstr} to the
1429: example shown above:
1430:
1431: @example
1432: #define xstr(s) str(s)
1433: #define str(s) #s
1434: #define foo 4
1435: xstr (foo)
1436: @end example
1437:
1438: This expands into @samp{"4"}, not @samp{"foo"}. The reason for the
1439: difference is that the argument of @samp{xstr} is expanded at prescan
1440: (because @samp{xstr} does not specify stringification or concatenation of
1441: the argument). The result of prescan then forms the actual argument for
1442: @samp{str}. @samp{str} uses its argument without prescan because it
1443: performs strigification; but it cannot prevent or undo the prescanning
1444: already done by @samp{xstr}.
1445:
1446: @node Cascaded Macros,, Argument Prescan, Macro Pitfalls
1447: @subsubsection Cascaded Use of Macros
1448:
1449: @cindex cascaded macros
1450: @cindex macro body uses macro
1451: A @dfn{cascade} of macros is when one macro's body contains a reference
1452: to another macro. This is very common practice. For example,
1453:
1454: @example
1455: #define BUFSIZE 1020
1456: #define TABLESIZE BUFSIZE
1457: @end example
1458:
1459: This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
1460: The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
1461: specify---in this case, @samp{BUFSIZE}---and does not check to see whether
1462: it too is the name of a macro.
1463:
1464: It's only when you @emph{use} @samp{TABLESIZE} that the result of its expansion
1465: is checked for more macro names.
1466:
1467: This makes a difference if you change the definition of @samp{BUFSIZE}
1468: at some point in the source file. @samp{TABLESIZE}, defined as shown,
1469: will always expand using the definition of @samp{BUFSIZE} that is
1470: currently in effect:
1471:
1472: @example
1473: #define BUFSIZE 1020
1474: #define TABLESIZE BUFSIZE
1475: #undef BUFSIZE
1476: #define BUFSIZE 37
1477: @end example
1478:
1479: @noindent
1480: Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.
1481:
1482: @node Conditionals, Combining Sources, Macros, Top
1483: @section Conditionals
1484:
1485: @cindex conditionals
1486: In a macro processor, a @dfn{conditional} is a command that allows a part
1487: of the program to be ignored during compilation, on some conditions.
1488: In the C preprocessor, a conditional can test either an arithmetic expression
1489: or whether a name is defined as a macro.
1490:
1491: A conditional in the C preprocessor resembles in some ways an @samp{if}
1492: statement in C, but it is important to understand the difference between
1493: them. The condition in an @samp{if} statement is tested during the execution
1494: of your program. Its purpose is to allow your program to behave differently
1495: from run to run, depending on the data it is operating on. The condition
1496: in a preprocessor conditional command is tested when your program is compiled.
1497: Its purpose is to allow different code to be included in the program depending
1498: on the situation at the time of compilation.
1499:
1500: @menu
1501: * Uses: Conditional Uses. What conditionals are for.
1502: * Syntax: Conditional Syntax. How conditionals are written.
1503: * Deletion: Deleted Code. Making code into a comment.
1504: * Macros: Conditionals-Macros. Why conditionals are used with macros.
1505: * Errors: #error Command. Detecting inconsistent compilation parameters.
1506: @end menu
1507:
1508: @node Conditional Uses, Conditional Syntax, Conditionals, Conditionals
1509: Generally there are three kinds of reason to use a conditional.
1510:
1511: @itemize @bullet
1512: @item
1513: A program may need to use different code depending on the machine or
1514: operating system it is to run on. In some cases the code for one
1515: operating system may be erroneous on another operating system; for
1516: example, it might refer to library routines that do not exist on the
1517: other system. When this happens, it is not enough to avoid executing
1518: the invalid code: merely having it in the program makes it impossible
1519: to link the program and run it. With a preprocessor conditional, the
1520: offending code can be effectively excised from the program when it is
1521: not valid.
1522:
1523: @item
1524: You may want to be able to compile the same source file into two
1525: different programs. Sometimes the difference between the programs is
1526: that one makes frequent time-consuming consistency checks on its
1527: intermediate data while the other does not.
1528:
1529: @item
1530: A conditional whose condition is always false is a good way to exclude
1531: code from the program but keep it as a sort of comment for future
1532: reference.
1533: @end itemize
1534:
1535: Most simple programs that are intended to run on only one machine will
1536: not need to use preprocessor conditionals.
1537:
1538: @node Conditional Syntax, Conditionals-Macros, Conditional Uses, Conditionals
1539: @subsection Syntax of Conditionals
1540:
1541: @findex #if
1542: A conditional in the C preprocessor begins with a @dfn{conditional
1543: command}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.@xref{Conditionals},
1544: for info on @samp{#ifdef} and @samp{#ifndef}; only @samp{#if} is explained
1545: here.
1546:
1547: @menu
1548: * If: #if Command. Basic conditionals using @samp{#if} and @samp{#endif}.
1549: * Else: #else Command. Including some text if the condition fails.
1550: * Elif: #elif Command. Testing several alternative possibilities.
1551: @end menu
1552:
1553: @node #if Command, #else Command, Conditional Syntax, Conditional Syntax
1554: @subsubsection The @samp{#if} Command
1555:
1556: The @samp{#if} command in its simplest form consists of
1557:
1558: @example
1559: #if @var{expression}
1560: @var{controlled text}
1561: #endif /* @var{expression} */
1562: @end example
1563:
1564: The comment following the @samp{#endif} is not required, but it is a good
1565: practice because it helps people match the @samp{#endif} to the
1566: corresponding @samp{#if}. Such comments should always be used, except in
1567: short conditionals that are not nested. In fact, you can put anything at
1568: all after the @samp{#endif} and it will be ignored by the GNU C preprocessor,
1569: but only comments are acceptable in ANSI Standard C.
1570:
1571: @var{expression} is a C expression of integer type, subject to stringent
1572: restrictions. It may contain
1573:
1574: @itemize @bullet
1575: @item
1576: Integer constants, which are all regarded as @code{long} or
1577: @code{unsigned long}.
1578:
1579: @item
1580: Character constants, which are interpreted according to the character
1581: set and conventions of the machine and operating system on which the
1582: preprocessor is running. The GNU C preprocessor uses the C data type
1583: @samp{char} for these character constants; therefore, whether some
1584: character codes are negative is determined by the C compiler used to
1585: compile the preprocessor. If it treats @samp{char} as signed, then
1586: character codes large enough to set the sign bit will be considered
1587: negative; otherwise, no character code is considered negative.
1588:
1589: @item
1590: Arithmetic operators for addition, subtraction, multiplication,
1591: division, bitwise operations, shifts, comparisons, and @samp{&&} and
1592: @samp{||}.
1593:
1594: @item
1595: Identifiers that are not macros, which are all treated as zero(!).
1596:
1597: @item
1598: Macro calls. All macro calls in the expression are expanded before
1599: actual computation of the expression's value begins.
1600: @end itemize
1601:
1602: Note that @samp{sizeof} operators and @code{enum}-type values are not allowed.
1603: @code{enum}-type values, like all other identifiers that are not taken
1604: as macro calls and expanded, are treated as zero.
1605:
1606: The text inside of a conditional can include preprocessor commands. Then
1607: the commands inside the conditional are obeyed only if that branch of the
1608: conditional succeeds. The text can also contain other conditional groups.
1609: However, the @samp{#if}'s and @samp{#endif}'s must balance.
1610:
1611: @node #else Command, #elif Command, #if Command, Conditional Syntax
1612: @subsubsection The @samp{#else} Command
1613:
1614: @findex #else
1615: The @samp{#else} command can be added a conditional to provide alternative
1616: text to be used if the condition is false. This looks like
1617:
1618: @example
1619: #if @var{expression}
1620: @var{text-if-true}
1621: #else /* Not @var{expression} */
1622: @var{text-if-false}
1623: #endif /* Not @var{expression} */
1624: @end example
1625:
1626: If @var{expression} is nonzero, and the @var{text-if-true} is considered
1627: included, then @samp{#else} acts like a failing conditional and the
1628: @var{text-if-false} is ignored. Contrariwise, if the @samp{#if}
1629: conditional fails, the @var{text-if-false} is considered included.
1630:
1631: @node #elif Command,, #else Command, Conditional Syntax
1632: @subsubsection The @samp{#elif} Command
1633:
1634: @findex #elif
1635: One common case of nested conditionals is used to check for more than two
1636: possible alternatives. For example, you might have
1637:
1638: @example
1639: #if X == 1
1640: @dots{}
1641: #else /* X != 1 */
1642: #if X == 2
1643: @dots{}
1644: #else /* X != 2 */
1645: @dots{}
1646: #endif /* X != 2 */
1647: #endif /* X != 1 */
1648: @end example
1649:
1650: Another conditional command, @samp{#elif}, allows this to be abbreviated
1651: as follows:
1652:
1653: @example
1654: #if X == 1
1655: @dots{}
1656: #elif X == 2
1657: @dots{}
1658: #else /* X != 2 and X != 1*/
1659: @dots{}
1660: #endif /* X != 2 and X != 1*/
1661: @end example
1662:
1663: @samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the
1664: middle of a @samp{#if}-@samp{#endif} pair and subdivides it; it does not
1665: require a matching @samp{#endif} of its own. Like @samp{#if}, the
1666: @samp{#elif} command includes an expression to be tested.
1667:
1668: The text following the @samp{#elif} is processed only if the original
1669: @samp{#if}-condition failed and the @samp{#elif} condition succeeeds. More
1670: than one @samp{#elif} can go in the same @samp{#if}-@samp{#endif} group.
1671: Then the text after each @samp{#elif} is processed only if the @samp{#elif}
1672: condition succeeds after the original @samp{#if} and any previous
1673: @samp{#elif}'s within it have failed. @samp{#else} is equivalent to
1674: @samp{#elif 1}, and @samp{#else} is allowed after any number of
1675: @samp{#elif}'s, but @samp{#elif} may not follow a @samp{#else}.
1676:
1677: @node Deleted Code, Conditionals-Macros, Conditional Syntax, Conditionals
1678: @subsection Keeping Deleted Code for Future Reference
1679:
1680: If you replace or delete a part of the program but want to keep the old
1681: code around as a comment for future reference, the easy way to do this is
1682: to put @samp{#if 0} before it and @samp{#endif} after it.
1683:
1684: This works even if the code being turned off contains conditionals, but
1685: they must be entire conditionals (balanced @samp{#if} and @samp{#endif}).
1686:
1687: @node Conditionals-Macros, #error Command, Deleted Code, Conditionals
1688: @subsection Conditionals and Macros
1689:
1690: Conditionals are rarely useful except in connection with macros. A
1691: @samp{#if} command whose expression uses no macros is equivalent to
1692: @samp{#if 1} or @samp{#if 0}; you might as well determine which one, by
1693: computing the value of the expression yourself, and then simplify the
1694: program. But when the expression uses macros, its value can vary from
1695: compilation to compilation.
1696:
1697: For example, here is a conditional that tests the expression
1698: @samp{BUFSIZE == 1020}, where @samp{BUFSIZE} must be a macro.
1699:
1700: @example
1701: #if BUFSIZE == 1020
1702: printf ("Large buffers!\n");
1703: #endif /* BUFSIZE is large */
1704: @end example
1705:
1706: @findex defined
1707: The special operator @samp{defined} may be used in @samp{#if} expressions
1708: to test whether a certain name is defined as a macro. Either @samp{defined
1709: @var{name}} or @samp{defined (@var{name})} is an expression whose value is
1710: 1 if @var{name} is defined as macro at the current point in the program,
1711: and 0 otherwise. For the @samp{defined} operator it makes no difference
1712: what the definition of the macro is; all that matters is whether there is a
1713: definition. Thus, for example,@refill
1714:
1715: @example
1716: #if defined (vax) || defined (ns16000)
1717: @end example
1718:
1719: @noindent
1720: would include the following code if either of the names @samp{vax} and
1721: @samp{ns16000} is defined as a macro.
1722:
1723: If a macro is defined and later undefined with @samp{#undef},
1724: subsequent use of the @samp{defined} operator will return 0, because
1725: the name is no longer defined. If the macro is defined again with
1726: another @samp{#define}, @samp{defined} will recommence returning 1.
1727:
1728: @findex #ifdef
1729: @findex #ifndef
1730: Conditionals that test just the definedness of one name are very common, so
1731: there are two special short conditional commands for this case. They are
1732:
1733: @table @code
1734: @item #ifdef @var{name}
1735: is equivalent to @samp{#if defined (@var{name})}.
1736:
1737: @item #ifndef @var{name}
1738: is equivalent to @samp{#if ! defined (@var{name})}.
1739: @end table
1740:
1741: Macro definitions can vary between compilations for several reasons.
1742:
1743: @itemize @bullet
1744: @item
1745: Some macros are predefined on each kind of machine. For example, on a
1746: Vax, the name @samp{vax} is a predefined macro. On other machines, it
1747: would not be defined.
1748:
1749: @item
1750: Many more macros are defined by system header files. Different
1751: systems and machines define different macros, or give them different
1752: values. It is useful to test these macros with conditionals to avoid
1753: using a system feature on a machine where it is not implemented.
1754:
1755: @item
1756: Macros are a common way of allowing users to customize a program for
1757: different machines or applications. For example, the macro
1758: @samp{BUFSIZE} might be defined in a configuration file for your
1759: program that is included as a header file in each source file. You
1760: would use @samp{BUFSIZE} in a preprocessor conditional in order to
1761: generate different code depending on the chosen configuration.
1762:
1763: @item
1764: Macros can be defined or undefined with @samp{-D} and @samp{-U}
1765: command options when you compile the program. You can arrange to
1766: compile the same source file into two different programs by choosing
1767: a macro name to specify which program you want, writing conditionals
1768: to test whether or how this macro is defined, and then controlling
1769: the state of the macro with compiler command options.
1770: @xref{Invocation}.
1771: @end itemize
1772:
1773: @node #error Command,, Conditionals-Macros, Conditionals
1774: @subsection The @samp{#error} Command
1775:
1776: @findex #error
1777: The command @samp{#error} causes the preprocessor to report a fatal
1778: error. The rest of the line that follows @samp{#error} is used as the
1779: error message.
1780:
1781: You would use @samp{#error} inside of a conditional that detects a
1782: combination of parameters which you know the program does not properly
1783: support. For example, if you know that the program will not run
1784: properly on a Vax, you might write
1785:
1786: @example
1787: #ifdef vax
1788: #error Won't work on Vaxen. See comments at get_last_object.
1789: #endif
1790: @end example
1791:
1792: @noindent
1793: @xref{Nonstandard Predefined}, for why this works.
1794:
1795: If you have several configuration parameters that must be set up by
1796: the installation in a consistent way, you can use conditionals to detect
1797: an inconsistency and report it with @samp{#error}. For example,
1798:
1799: @example
1800: #if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
1801: || HASH_TABLE_SIZE % 5 == 0
1802: #error HASH_TABLE_SIZE should not be divisible by a small prime
1803: #endif
1804: @end example
1805:
1806: @node Combining Sources, Other Commands, Conditionals, Top
1807: @section Combining Source Files
1808:
1809: @cindex line control
1810: @findex #line
1811: One of the jobs of the C preprocessor is to inform the C compiler of where
1812: each line of C code came from: which source file and which line number.
1813:
1814: C code can come from multiple source files if you use @samp{#include};
1815: both @samp{#include} and the use of conditionals and macros can cause
1816: the line number of a line in the preprocessor output to be different
1817: from the line's number in the original source file. You will appreciate
1818: the value of making both the C compiler (in error messages) and symbolic
1819: debuggers such as GDB use the line numbers in your source file.
1820:
1821: The C preprocessor builds on this feature by offering a command by which
1822: you can control the feature explicitly. This is useful when a file for
1823: input to the C preprocessor is the output from another program such as the
1824: @code{bison} parser generator, which operates on another file that is the
1825: true source file. Parts of the output from @code{bison} are generated from
1826: scratch, other parts come from a standard parser file. The rest are copied
1827: nearly verbatim from the source file, but their line numbers in the
1828: @code{bison} output are not the same as their original line numbers.
1829: Naturally you would like compiler error messages and symbolic debuggers to
1830: know the original source file and line number of each line in the
1831: @code{bison} output.
1832:
1833: @code{bison} arranges this by writing @samp{#line} commands into the output
1834: file. @samp{#line} is a command that specifies the original line number
1835: and source file name for subsequent input in the current preprocessor input
1836: file. @samp{#line} has three variants:
1837:
1838: @table @code
1839: @item #line @var{linenum}
1840: Here @var{linenum} is a decimal integer constant. This specifies that
1841: the line number of the following line of input, in its original source file,
1842: was @var{linenum}.
1843:
1844: @item #line @var{linenum} @var{filename}
1845: Here @var{linenum} is a decimal integer constant and @var{filename}
1846: is a string constant. This specifies that the following line of input
1847: came originally from source file @var{filename} and its line number there
1848: was @var{linenum}. Keep in mind that @var{filename} is not just a
1849: file name; it is surrounded by doublequote characters so that it looks
1850: like a string constant.
1851:
1852: @item #line @var{anything else}
1853: @var{anything else} is checked for macro calls, which are expanded.
1854: The result should be a decimal integer constant followed optionally
1855: by a string constant, as described above.
1856: @end table
1857:
1858: @samp{#line} commands alter the results of the @samp{__FILE__} and
1859: @samp{__LINE__} predefined macros from that point on. @xref{Standard
1860: Predefined}.
1861:
1862: @node Other Commands, Output, Combining Sources, Top
1863: @section Miscellaneous Preprocessor Commands
1864:
1865: @findex #pragma
1866: @cindex null command
1867: This section describes two additional preprocesor commands. They are
1868: not very useful, but are mentioned for completeness.
1869:
1870: The @dfn{null command} consists of a @samp{#} followed by a Newline, with
1871: only whitespace (including comments) in between. A null command is
1872: understood as a preprocessor command but has no effect on the preprocessor
1873: output. The primary significance of the existence of the null command is
1874: that an input line consisting of just a @samp{#} will produce no output,
1875: rather than a line of output containing just a @samp{#}. Supposedly
1876: some old C programs contain such lines.
1877:
1878: The @samp{#pragma} command is specified in the ANSI standard to have an
1879: arbitrary implementation-defined effect. In the GNU C preprocessor,
1880: @samp{#pragma} first attempts to run the game @code{rogue}; if that fails,
1881: it tries to run the game @code{hack}; if that fails, it tries to run
1882: GNU Emacs displaying the Tower of Hanoi; if that fails, it reports a
1883: fatal error. In any case, preprocessing does not continue.
1884:
1885: @node Output, Invocation, Other Commands, Top
1886: @section C Preprocessor Output
1887:
1888: @cindex output format
1889: The output from the C preprocessor looks much like the input, except
1890: that all preprocessor command lines have been replaced with blank lines
1891: and all comments with spaces. Whitespace within a line is not altered;
1892: however, a space is inserted after the expansions of most macro calls.
1893:
1894: Source file name and line number information is conveyed by lines of
1895: the form
1896:
1897: @example
1898: # @var{linenum} @var{filename}
1899: @end example
1900:
1901: @noindent
1902: which are inserted as needed into the middle of the input (but never within
1903: a string or character constant). Such a line means that the following line
1904: originated in file @var{filename} at line @var{linenum}.
1905:
1906: @node Invocation,, Output, Top
1907: @section Invoking the C Preprocessor
1908:
1909: Most often when you use the C preprocessor you will not have to invoke it
1910: explicitly: the C compiler will do so automatically. However, the
1911: preprocessor is sometimes useful individually.
1912:
1913: The C preprocessor expects two file names as arguments, @var{infile} and
1914: @var{outfile}. The preprocessor reads @var{infile} together with any other
1915: files it specifies with @samp{#include}. All the output generated by the
1916: combined input files is written in @var{outfile}.
1917:
1918: Either @var{infile} or @var{outfile} may be @samp{-}, which as @var{infile}
1919: means to read from standard input and as @var{outfile} means to write to
1920: standard output. Also, if @var{outfile} or both file names are omitted,
1921: the standard output and standard input are used for the omitted file names.
1922:
1923: @cindex options
1924: Here is a table of command options accepted by the C preprocessor. Most
1925: of them can also be given when compiling a C program; they are passed along
1926: automatically to the preprocessor when it is invoked by the compiler.
1927:
1928: @table @samp
1929: @item -P
1930: @findex -P
1931: Inhibit generation of @samp{#}-lines with line-number information in
1932: the output from the preprocessor (@pxref{Output}). This might be
1933: useful when running the preprocessor on something that is not C code
1934: and will be sent to a program which might be confused by the
1935: @samp{#}-lines
1936:
1937: @item -C
1938: @findex -C
1939: Do not discard comments: pass them through to the output file.
1940: Comments appearing in arguments of a macro call will be copied to the
1941: output before the expansion of the macro call.
1942:
1943: @item -T
1944: @findex -T
1945: Process ANSI standard trigraph sequences. These are three-character
1946: sequences, all starting with @samp{??}, that are defined by ANSI C to
1947: stand for single characters. For example, @samp{??/} stands for
1948: @samp{\}, so @samp{'??/n'} is a character constant for Newline.
1949: Strictly speaking, the GNU C preprocessor does not support all
1950: programs in ANSI Standard C unless @samp{-T} is used, but if you
1951: ever notice the difference it will be with relief.
1952:
1953: You don't want to know any more about trigraphs.
1954:
1955: @item -pedantic
1956: @findex -pedantic
1957: Issue warnings required by the ANSI C standard in certain cases such
1958: as when text other than a comment follows @samp{#else} or @samp{#endif}.
1959:
1960: @item -I @var{directory}
1961: @findex -I
1962: Add the directory @var{directory} to the end of the list of
1963: directories to be searched for header files (@pxref{Include Syntax}).
1964: This can be used to override a system header file, substituting your
1965: own version, since these directories are searched before the system
1966: header file directories. If you use more than one @samp{-I} option,
1967: the directories are scanned in left-to-right order; the standard
1968: system directories come after.
1969:
1970: @item -I-
1971: Any directories specified with @samp{-I} options before the @samp{-I-}
1972: option are searched only for the case of @samp{#include "@var{file}"};
1973: they are not searched for @samp{#include <@var{file}>}.
1974:
1975: If additional directories are specified with @samp{-I} options after
1976: the @samp{-I-}, these directories are searched for all @samp{#include}
1977: directives.
1978:
1979: In addition, the @samp{-I-} option inhibits the use of the current
1980: directory as the first search directory for @samp{#include "@var{file}"}.
1981: Therefore, the current directory is searched only if it is requested
1982: explicitly with @samp{-I.}. Specifying both @samp{-I-} and @samp{-I.}
1983: allows you to control precisely which directories are searched before
1984: the current one and which are searched after.
1985:
1986: @item -nostdinc
1987: Do not search the standard system directories for header files.
1988: Only the directories you have specified with @samp{-I} options
1989: (and the current directory, if appropriate) are searched.
1990:
1991: @item -D @var{name}
1992: @findex -D
1993: Predefine @var{name} as a macro, with definition @samp{1}.
1994:
1995: @item -D @var{name}=@var{definition}
1996: Predefine @var{name} as a macro, with definition @var{definition}.
1997: There are no restrictions on the contents of @var{definition}, but if
1998: you are invoking the preprocessor from a shell or shell-like program
1999: you may need to use the shell's quoting syntax to protect characters
2000: such as spaces that have a meaning in the shell syntax.
2001:
2002: @item -U @var{name}
2003: @findex -U
2004: Do not predefine @var{name}. If both @samp{-U} and @samp{-D} are
2005: specified for one name, the @samp{-U} beats the @samp{-D} and the name
2006: is not predefined.
2007:
2008: @item -undef
2009: @findex -undef
2010: Do not predefine any nonstandard macros.
2011:
2012: @item -d
2013: @findex -d
2014: Instead of outputting the result of preprocessing, output a list of
2015: @samp{#define} commands for all the macros defined during the
2016: execution of the preprocessor.
2017:
2018: @item -M
2019: @findex -M
2020: Instead of outputting the result of preprocessing, output a rule
2021: suitable for @code{make} describing the dependencies of the main
2022: source file. The preprocessor outputs one @code{make} rule containing
2023: the object file name for that source file, a colon, and the names of
2024: all the included files. If there are many included files then the
2025: rule is split into several lines using @samp{\}-newline.
2026:
2027: This feature is used in automatic updating of makefiles.
2028:
2029: @item -MM
2030: @findex -MM
2031: Like @samp{-M} but mention only the files included with @samp{#include
2032: "@var{file}"}. System header files included with @samp{#include
2033: <@var{file}>} are omitted.
2034:
2035: @item -i @var{file}
2036: @findex -i
2037: Process @var{file} as input, discarding the resulting output, before
2038: processing the regular input file. Because the output generated from
2039: @var{file} is discarded, the only effect of @samp{-i @var{file}} is to
2040: make the macros defined in @var{file} available for use in the main
2041: input.
2042: @end table
2043:
2044: @node Concept Index, Index, Invocation, Top
2045: @unnumbered Concept Index
2046: @printindex cp
2047:
2048: @node Index,, Concept Index, Top
2049: @unnumbered Index of Commands, Macros and Options
2050: @printindex fn
2051:
2052: @contents
2053: @bye
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