![[BACK]](/cvsweb/icons/back.gif){kind=icon}
Return to cpp.info-1 CVS log ![[TXT]](/cvsweb/icons/text.gif){kind=icon}
![[DIR]](/cvsweb/icons/dir.gif){kind=icon}
Up to [GCC 2.x] / gcc|
Return to cpp.info-1 CVS log | Up to [GCC 2.x] / gcc |
1.1.1.6 ! root 1: This is Info file cpp.info, produced by Makeinfo version 1.67 from the ! 2: input file cpp.texi. 1.1 root 3: 4: This file documents the GNU C Preprocessor. 5: 1.1.1.5 root 6: Copyright 1987, 1989, 1991, 1992, 1993, 1994, 1995 Free Software 1.1.1.4 root 7: Foundation, Inc. 1.1 root 8: 1.1.1.2 root 9: Permission is granted to make and distribute verbatim copies of this 10: manual provided the copyright notice and this permission notice are 11: preserved on all copies. 1.1 root 12: 13: Permission is granted to copy and distribute modified versions of 14: this manual under the conditions for verbatim copying, provided also 15: that the entire resulting derived work is distributed under the terms 16: of a permission notice identical to this one. 17: 18: Permission is granted to copy and distribute translations of this 19: manual into another language, under the above conditions for modified 20: versions. 21: 22: 23: File: cpp.info, Node: Top, Next: Global Actions, Up: (DIR) 24: 25: The C Preprocessor 26: ****************** 27: 1.1.1.2 root 28: The C preprocessor is a "macro processor" that is used automatically 1.1.1.3 root 29: by the C compiler to transform your program before actual compilation. 1.1.1.2 root 30: It is called a macro processor because it allows you to define "macros", 31: which are brief abbreviations for longer constructs. 1.1 root 32: 33: The C preprocessor provides four separate facilities that you can 34: use as you see fit: 35: 36: * Inclusion of header files. These are files of declarations that 37: can be substituted into your program. 38: 39: * Macro expansion. You can define "macros", which are abbreviations 1.1.1.2 root 40: for arbitrary fragments of C code, and then the C preprocessor will 41: replace the macros with their definitions throughout the program. 1.1 root 42: 1.1.1.5 root 43: * Conditional compilation. Using special preprocessing directives, 44: you can include or exclude parts of the program according to 45: various conditions. 1.1 root 46: 1.1.1.2 root 47: * Line control. If you use a program to combine or rearrange source 48: files into an intermediate file which is then compiled, you can 49: use line control to inform the compiler of where each source line 50: originally came from. 1.1 root 51: 1.1.1.2 root 52: C preprocessors vary in some details. This manual discusses the GNU 53: C preprocessor, the C Compatible Compiler Preprocessor. The GNU C 1.1 root 54: preprocessor provides a superset of the features of ANSI Standard C. 55: 56: ANSI Standard C requires the rejection of many harmless constructs 57: commonly used by today's C programs. Such incompatibility would be 58: inconvenient for users, so the GNU C preprocessor is configured to 59: accept these constructs by default. Strictly speaking, to get ANSI 60: Standard C, you must use the options `-trigraphs', `-undef' and 61: `-pedantic', but in practice the consequences of having strict ANSI 62: Standard C make it undesirable to do this. *Note Invocation::. 63: 64: * Menu: 65: 66: * Global Actions:: Actions made uniformly on all input files. 1.1.1.5 root 67: * Directives:: General syntax of preprocessing directives. 1.1 root 68: * Header Files:: How and why to use header files. 69: * Macros:: How and why to use macros. 70: * Conditionals:: How and why to use conditionals. 71: * Combining Sources:: Use of line control when you combine source files. 1.1.1.5 root 72: * Other Directives:: Miscellaneous preprocessing directives. 1.1 root 73: * Output:: Format of output from the C preprocessor. 74: * Invocation:: How to invoke the preprocessor; command options. 75: * Concept Index:: Index of concepts and terms. 1.1.1.5 root 76: * Index:: Index of directives, predefined macros and options. 1.1 root 77: 78: 1.1.1.5 root 79: File: cpp.info, Node: Global Actions, Next: Directives, Prev: Top, Up: Top 1.1 root 80: 81: Transformations Made Globally 82: ============================= 83: 84: Most C preprocessor features are inactive unless you give specific 1.1.1.5 root 85: directives to request their use. (Preprocessing directives are lines 86: starting with `#'; *note Directives::.). But there are three 1.1 root 87: transformations that the preprocessor always makes on all the input it 1.1.1.5 root 88: receives, even in the absence of directives. 1.1 root 89: 90: * All C comments are replaced with single spaces. 91: 92: * Backslash-Newline sequences are deleted, no matter where. This 93: feature allows you to break long lines for cosmetic purposes 94: without changing their meaning. 95: 96: * Predefined macro names are replaced with their expansions (*note 97: Predefined::.). 98: 99: The first two transformations are done *before* nearly all other 1.1.1.5 root 100: parsing and before preprocessing directives are recognized. Thus, for 1.1 root 101: example, you can split a line cosmetically with Backslash-Newline 102: anywhere (except when trigraphs are in use; see below). 103: 104: /* 105: */ # /* 106: */ defi\ 107: ne FO\ 108: O 10\ 109: 20 110: 111: is equivalent into `#define FOO 1020'. You can split even an escape 1.1.1.2 root 112: sequence with Backslash-Newline. For example, you can split `"foo\bar"' 113: between the `\' and the `b' to get 1.1 root 114: 115: "foo\\ 116: bar" 117: 118: This behavior is unclean: in all other contexts, a Backslash can be 119: inserted in a string constant as an ordinary character by writing a 120: double Backslash, and this creates an exception. But the ANSI C 1.1.1.2 root 121: standard requires it. (Strict ANSI C does not allow Newlines in string 122: constants, so they do not consider this a problem.) 1.1 root 123: 124: But there are a few exceptions to all three transformations. 125: 126: * C comments and predefined macro names are not recognized inside a 1.1.1.5 root 127: `#include' directive in which the file name is delimited with `<' 1.1 root 128: and `>'. 129: 1.1.1.2 root 130: * C comments and predefined macro names are never recognized within a 131: character or string constant. (Strictly speaking, this is the 1.1 root 132: rule, not an exception, but it is worth noting here anyway.) 133: 1.1.1.2 root 134: * Backslash-Newline may not safely be used within an ANSI "trigraph". 135: Trigraphs are converted before Backslash-Newline is deleted. If 136: you write what looks like a trigraph with a Backslash-Newline 137: inside, the Backslash-Newline is deleted as usual, but it is then 138: too late to recognize the trigraph. 1.1 root 139: 1.1.1.2 root 140: This exception is relevant only if you use the `-trigraphs' option 141: to enable trigraph processing. *Note Invocation::. 1.1 root 142: 143: 1.1.1.5 root 144: File: cpp.info, Node: Directives, Next: Header Files, Prev: Global Actions, Up: Top 1.1 root 145: 1.1.1.5 root 146: Preprocessing Directives 147: ======================== 1.1 root 148: 1.1.1.5 root 149: Most preprocessor features are active only if you use preprocessing 150: directives to request their use. 1.1 root 151: 1.1.1.5 root 152: Preprocessing directives are lines in your program that start with 153: `#'. The `#' is followed by an identifier that is the "directive name". 154: For example, `#define' is the directive that defines a macro. 155: Whitespace is also allowed before and after the `#'. 1.1 root 156: 1.1.1.5 root 157: The set of valid directive names is fixed. Programs cannot define 158: new preprocessing directives. 1.1 root 159: 1.1.1.5 root 160: Some directive names require arguments; these make up the rest of 161: the directive line and must be separated from the directive name by 162: whitespace. For example, `#define' must be followed by a macro name 163: and the intended expansion of the macro. *Note Simple Macros::. 1.1 root 164: 1.1.1.5 root 165: A preprocessing directive cannot be more than one line in normal 1.1.1.3 root 166: circumstances. It may be split cosmetically with Backslash-Newline, 167: but that has no effect on its meaning. Comments containing Newlines 1.1.1.5 root 168: can also divide the directive into multiple lines, but the comments are 169: changed to Spaces before the directive is interpreted. The only way a 170: significant Newline can occur in a preprocessing directive is within a 1.1.1.2 root 171: string constant or character constant. Note that most C compilers that 172: might be applied to the output from the preprocessor do not accept 173: string or character constants containing Newlines. 174: 1.1.1.5 root 175: The `#' and the directive name cannot come from a macro expansion. 176: For example, if `foo' is defined as a macro expanding to `define', that 177: does not make `#foo' a valid preprocessing directive. 1.1 root 178: 179: 1.1.1.5 root 180: File: cpp.info, Node: Header Files, Next: Macros, Prev: Directives, Up: Top 1.1 root 181: 182: Header Files 183: ============ 184: 185: A header file is a file containing C declarations and macro 186: definitions (*note Macros::.) to be shared between several source 1.1.1.2 root 187: files. You request the use of a header file in your program with the C 1.1.1.5 root 188: preprocessing directive `#include'. 1.1 root 189: 190: * Menu: 191: 192: * Header Uses:: What header files are used for. 1.1.1.5 root 193: * Include Syntax:: How to write `#include' directives. 1.1 root 194: * Include Operation:: What `#include' does. 195: * Once-Only:: Preventing multiple inclusion of one header file. 196: * Inheritance:: Including one header file in another header file. 197: 198: 199: File: cpp.info, Node: Header Uses, Next: Include Syntax, Prev: Header Files, Up: Header Files 200: 201: Uses of Header Files 202: -------------------- 203: 204: Header files serve two kinds of purposes. 205: 206: * System header files declare the interfaces to parts of the 207: operating system. You include them in your program to supply the 208: definitions and declarations you need to invoke system calls and 209: libraries. 210: 211: * Your own header files contain declarations for interfaces between 212: the source files of your program. Each time you have a group of 213: related declarations and macro definitions all or most of which 1.1.1.2 root 214: are needed in several different source files, it is a good idea to 215: create a header file for them. 1.1 root 216: 1.1.1.2 root 217: Including a header file produces the same results in C compilation as 218: copying the header file into each source file that needs it. But such 219: copying would be time-consuming and error-prone. With a header file, 220: the related declarations appear in only one place. If they need to be 221: changed, they can be changed in one place, and programs that include 222: the header file will automatically use the new version when next 223: recompiled. The header file eliminates the labor of finding and 1.1 root 224: changing all the copies as well as the risk that a failure to find one 225: copy will result in inconsistencies within a program. 226: 227: The usual convention is to give header files names that end with 1.1.1.4 root 228: `.h'. Avoid unusual characters in header file names, as they reduce 229: portability. 1.1 root 230: 231: 232: File: cpp.info, Node: Include Syntax, Next: Include Operation, Prev: Header Uses, Up: Header Files 233: 1.1.1.5 root 234: The `#include' Directive 235: ------------------------ 1.1 root 236: 1.1.1.5 root 237: Both user and system header files are included using the 238: preprocessing directive `#include'. It has three variants: 1.1 root 239: 240: `#include <FILE>' 241: This variant is used for system header files. It searches for a 1.1.1.2 root 242: file named FILE in a list of directories specified by you, then in 243: a standard list of system directories. You specify directories to 244: search for header files with the command option `-I' (*note 245: Invocation::.). The option `-nostdinc' inhibits searching the 246: standard system directories; in this case only the directories you 247: specify are searched. 248: 249: The parsing of this form of `#include' is slightly special because 1.1.1.3 root 250: comments are not recognized within the `<...>'. Thus, in 251: `#include <x/*y>' the `/*' does not start a comment and the 1.1.1.5 root 252: directive specifies inclusion of a system header file named 253: `x/*y'. Of course, a header file with such a name is unlikely to 254: exist on Unix, where shell wildcard features would make it hard to 1.1 root 255: manipulate. 256: 257: The argument FILE may not contain a `>' character. It may, 258: however, contain a `<' character. 259: 260: `#include "FILE"' 261: This variant is used for header files of your own program. It 262: searches for a file named FILE first in the current directory, 263: then in the same directories used for system header files. The 264: current directory is the directory of the current input file. It 265: is tried first because it is presumed to be the location of the 1.1.1.2 root 266: files that the current input file refers to. (If the `-I-' option 267: is used, the special treatment of the current directory is 1.1 root 268: inhibited.) 269: 270: The argument FILE may not contain `"' characters. If backslashes 271: occur within FILE, they are considered ordinary text characters, 272: not escape characters. None of the character escape sequences 273: appropriate to string constants in C are processed. Thus, 274: `#include "x\n\\y"' specifies a filename containing three 1.1.1.2 root 275: backslashes. It is not clear why this behavior is ever useful, but 276: the ANSI standard specifies it. 1.1 root 277: 278: `#include ANYTHING ELSE' 279: This variant is called a "computed #include". Any `#include' 1.1.1.5 root 280: directive whose argument does not fit the above two forms is a 1.1 root 281: computed include. The text ANYTHING ELSE is checked for macro 282: calls, which are expanded (*note Macros::.). When this is done, 283: the result must fit one of the above two variants--in particular, 284: the expanded text must in the end be surrounded by either quotes 285: or angle braces. 286: 287: This feature allows you to define a macro which controls the file 288: name to be used at a later point in the program. One application 1.1.1.4 root 289: of this is to allow a site-specific configuration file for your 290: program to specify the names of the system include files to be 291: used. This can help in porting the program to various operating 292: systems in which the necessary system header files are found in 293: different places. 1.1 root 294: 295: 296: File: cpp.info, Node: Include Operation, Next: Once-Only, Prev: Include Syntax, Up: Header Files 297: 298: How `#include' Works 299: -------------------- 300: 1.1.1.5 root 301: The `#include' directive works by directing the C preprocessor to 302: scan the specified file as input before continuing with the rest of the 1.1 root 303: current file. The output from the preprocessor contains the output 304: already generated, followed by the output resulting from the included 305: file, followed by the output that comes from the text after the 1.1.1.5 root 306: `#include' directive. For example, given a header file `header.h' as 1.1.1.4 root 307: follows, 308: 309: char *test (); 310: 311: and a main program called `program.c' that uses the header file, like 312: this, 1.1 root 313: 314: int x; 315: #include "header.h" 316: 317: main () 318: { 319: printf (test ()); 320: } 321: 322: the output generated by the C preprocessor for `program.c' as input 323: would be 324: 325: int x; 326: char *test (); 327: 328: main () 329: { 330: printf (test ()); 331: } 332: 333: Included files are not limited to declarations and macro 334: definitions; those are merely the typical uses. Any fragment of a C 1.1.1.2 root 335: program can be included from another file. The include file could even 336: contain the beginning of a statement that is concluded in the 1.1 root 337: containing file, or the end of a statement that was started in the 338: including file. However, a comment or a string or character constant 1.1.1.3 root 339: may not start in the included file and finish in the including file. 1.1 root 340: An unterminated comment, string constant or character constant in an 341: included file is considered to end (with an error message) at the end 342: of the file. 343: 1.1.1.4 root 344: It is possible for a header file to begin or end a syntactic unit 345: such as a function definition, but that would be very confusing, so 346: don't do it. 347: 1.1.1.5 root 348: The line following the `#include' directive is always treated as a 1.1 root 349: separate line by the C preprocessor even if the included file lacks a 350: final newline. 351: 352: 353: File: cpp.info, Node: Once-Only, Next: Inheritance, Prev: Include Operation, Up: Header Files 354: 355: Once-Only Include Files 356: ----------------------- 357: 358: Very often, one header file includes another. It can easily result 359: that a certain header file is included more than once. This may lead 360: to errors, if the header file defines structure types or typedefs, and 361: is certainly wasteful. Therefore, we often wish to prevent multiple 362: inclusion of a header file. 363: 364: The standard way to do this is to enclose the entire real contents 365: of the file in a conditional, like this: 366: 1.1.1.4 root 367: #ifndef FILE_FOO_SEEN 368: #define FILE_FOO_SEEN 1.1 root 369: 370: THE ENTIRE FILE 371: 1.1.1.4 root 372: #endif /* FILE_FOO_SEEN */ 1.1 root 373: 1.1.1.4 root 374: The macro `FILE_FOO_SEEN' indicates that the file has been included 375: once already. In a user header file, the macro name should not begin 376: with `_'. In a system header file, this name should begin with `__' to 377: avoid conflicts with user programs. In any kind of header file, the 378: macro name should contain the name of the file and some additional 379: text, to avoid conflicts with other header files. 1.1 root 380: 381: The GNU C preprocessor is programmed to notice when a header file 382: uses this particular construct and handle it efficiently. If a header 383: file is contained entirely in a `#ifndef' conditional, then it records 1.1.1.2 root 384: that fact. If a subsequent `#include' specifies the same file, and the 385: macro in the `#ifndef' is already defined, then the file is entirely 386: skipped, without even reading it. 1.1 root 387: 1.1.1.5 root 388: There is also an explicit directive to tell the preprocessor that it 1.1 root 389: need not include a file more than once. This is called `#pragma once', 390: and was used *in addition to* the `#ifndef' conditional around the 1.1.1.2 root 391: contents of the header file. `#pragma once' is now obsolete and should 392: not be used at all. 1.1 root 393: 1.1.1.2 root 394: In the Objective C language, there is a variant of `#include' called 1.1.1.3 root 395: `#import' which includes a file, but does so at most once. If you use 1.1.1.2 root 396: `#import' *instead of* `#include', then you don't need the conditionals 397: inside the header file to prevent multiple execution of the contents. 398: 1.1.1.4 root 399: `#import' is obsolete because it is not a well designed feature. It 1.1.1.2 root 400: requires the users of a header file--the applications programmers--to 401: know that a certain header file should only be included once. It is 402: much better for the header file's implementor to write the file so that 403: users don't need to know this. Using `#ifndef' accomplishes this goal. 1.1 root 404: 405: 406: File: cpp.info, Node: Inheritance, Prev: Once-Only, Up: Header Files 407: 408: Inheritance and Header Files 1.1.1.4 root 409: ---------------------------- 1.1 root 410: 411: "Inheritance" is what happens when one object or file derives some 1.1.1.2 root 412: of its contents by virtual copying from another object or file. In the 413: case of C header files, inheritance means that one header file includes 414: another header file and then replaces or adds something. 415: 416: If the inheriting header file and the base header file have different 417: names, then inheritance is straightforward: simply write `#include 418: "BASE"' in the inheriting file. 1.1 root 419: 420: Sometimes it is necessary to give the inheriting file the same name 421: as the base file. This is less straightforward. 422: 423: For example, suppose an application program uses the system header 424: file `sys/signal.h', but the version of `/usr/include/sys/signal.h' on 1.1.1.3 root 425: a particular system doesn't do what the application program expects. 426: It might be convenient to define a "local" version, perhaps under the 427: name `/usr/local/include/sys/signal.h', to override or add to the one 1.1 root 428: supplied by the system. 429: 430: You can do this by using the option `-I.' for compilation, and 431: writing a file `sys/signal.h' that does what the application program 432: expects. But making this file include the standard `sys/signal.h' is 433: not so easy--writing `#include <sys/signal.h>' in that file doesn't 434: work, because it includes your own version of the file, not the 435: standard system version. Used in that file itself, this leads to an 436: infinite recursion and a fatal error in compilation. 437: 438: `#include </usr/include/sys/signal.h>' would find the proper file, 439: but that is not clean, since it makes an assumption about where the 440: system header file is found. This is bad for maintenance, since it 441: means that any change in where the system's header files are kept 442: requires a change somewhere else. 443: 1.1.1.2 root 444: The clean way to solve this problem is to use `#include_next', which 1.1.1.5 root 445: means, "Include the *next* file with this name." This directive works 1.1.1.2 root 446: like `#include' except in searching for the specified file: it starts 447: searching the list of header file directories *after* the directory in 448: which the current file was found. 1.1 root 449: 450: Suppose you specify `-I /usr/local/include', and the list of 451: directories to search also includes `/usr/include'; and suppose that 452: both directories contain a file named `sys/signal.h'. Ordinary 1.1.1.3 root 453: `#include <sys/signal.h>' finds the file under `/usr/local/include'. 1.1 root 454: If that file contains `#include_next <sys/signal.h>', it starts 455: searching after that directory, and finds the file in `/usr/include'. 456: 457: 458: File: cpp.info, Node: Macros, Next: Conditionals, Prev: Header Files, Up: Top 459: 460: Macros 461: ====== 462: 463: A macro is a sort of abbreviation which you can define once and then 464: use later. There are many complicated features associated with macros 465: in the C preprocessor. 466: 467: * Menu: 468: 469: * Simple Macros:: Macros that always expand the same way. 470: * Argument Macros:: Macros that accept arguments that are substituted 471: into the macro expansion. 472: * Predefined:: Predefined macros that are always available. 473: * Stringification:: Macro arguments converted into string constants. 474: * Concatenation:: Building tokens from parts taken from macro arguments. 475: * Undefining:: Cancelling a macro's definition. 476: * Redefining:: Changing a macro's definition. 477: * Macro Pitfalls:: Macros can confuse the unwary. Here we explain 478: several common problems and strange features. 479: 480: 481: File: cpp.info, Node: Simple Macros, Next: Argument Macros, Prev: Macros, Up: Macros 482: 483: Simple Macros 484: ------------- 485: 486: A "simple macro" is a kind of abbreviation. It is a name which 1.1.1.2 root 487: stands for a fragment of code. Some people refer to these as "manifest 488: constants". 1.1 root 489: 490: Before you can use a macro, you must "define" it explicitly with the 1.1.1.5 root 491: `#define' directive. `#define' is followed by the name of the macro 492: and then the code it should be an abbreviation for. For example, 1.1 root 493: 494: #define BUFFER_SIZE 1020 495: 496: defines a macro named `BUFFER_SIZE' as an abbreviation for the text 1.1.1.5 root 497: `1020'. If somewhere after this `#define' directive there comes a C 1.1.1.4 root 498: statement of the form 1.1 root 499: 500: foo = (char *) xmalloc (BUFFER_SIZE); 501: 502: then the C preprocessor will recognize and "expand" the macro 503: `BUFFER_SIZE', resulting in 504: 505: foo = (char *) xmalloc (1020); 506: 1.1.1.2 root 507: The use of all upper case for macro names is a standard convention. 1.1 root 508: Programs are easier to read when it is possible to tell at a glance 509: which names are macros. 510: 511: Normally, a macro definition must be a single line, like all C 1.1.1.5 root 512: preprocessing directives. (You can split a long macro definition 1.1.1.2 root 513: cosmetically with Backslash-Newline.) There is one exception: Newlines 514: can be included in the macro definition if within a string or character 1.1.1.4 root 515: constant. This is because it is not possible for a macro definition to 516: contain an unbalanced quote character; the definition automatically 1.1.1.2 root 517: extends to include the matching quote character that ends the string or 1.1.1.3 root 518: character constant. Comments within a macro definition may contain 1.1.1.2 root 519: Newlines, which make no difference since the comments are entirely 520: replaced with Spaces regardless of their contents. 1.1 root 521: 522: Aside from the above, there is no restriction on what can go in a 523: macro body. Parentheses need not balance. The body need not resemble 1.1.1.4 root 524: valid C code. (But if it does not, you may get error messages from the 525: C compiler when you use the macro.) 1.1 root 526: 527: The C preprocessor scans your program sequentially, so macro 528: definitions take effect at the place you write them. Therefore, the 529: following input to the C preprocessor 530: 531: foo = X; 532: #define X 4 533: bar = X; 534: 535: produces as output 536: 537: foo = X; 538: 539: bar = 4; 540: 541: After the preprocessor expands a macro name, the macro's definition 1.1.1.2 root 542: body is appended to the front of the remaining input, and the check for 543: macro calls continues. Therefore, the macro body can contain calls to 1.1.1.3 root 544: other macros. For example, after 1.1 root 545: 546: #define BUFSIZE 1020 547: #define TABLESIZE BUFSIZE 548: 549: the name `TABLESIZE' when used in the program would go through two 550: stages of expansion, resulting ultimately in `1020'. 551: 1.1.1.2 root 552: This is not at all the same as defining `TABLESIZE' to be `1020'. 1.1 root 553: The `#define' for `TABLESIZE' uses exactly the body you specify--in 554: this case, `BUFSIZE'--and does not check to see whether it too is the 555: name of a macro. It's only when you *use* `TABLESIZE' that the result 1.1.1.3 root 556: of its expansion is checked for more macro names. *Note Cascaded 1.1 root 557: Macros::. 558: 559: 560: File: cpp.info, Node: Argument Macros, Next: Predefined, Prev: Simple Macros, Up: Macros 561: 562: Macros with Arguments 563: --------------------- 564: 1.1.1.2 root 565: A simple macro always stands for exactly the same text, each time it 566: is used. Macros can be more flexible when they accept "arguments". 1.1 root 567: Arguments are fragments of code that you supply each time the macro is 568: used. These fragments are included in the expansion of the macro 1.1.1.4 root 569: according to the directions in the macro definition. A macro that 570: accepts arguments is called a "function-like macro" because the syntax 571: for using it looks like a function call. 1.1 root 572: 1.1.1.5 root 573: To define a macro that uses arguments, you write a `#define' 574: directive with a list of "argument names" in parentheses after the name 575: of the macro. The argument names may be any valid C identifiers, 576: separated by commas and optionally whitespace. The open-parenthesis 577: must follow the macro name immediately, with no space in between. 1.1 root 578: 1.1.1.2 root 579: For example, here is a macro that computes the minimum of two numeric 580: values, as it is defined in many C programs: 1.1 root 581: 582: #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 583: 1.1.1.3 root 584: (This is not the best way to define a "minimum" macro in GNU C. *Note 1.1 root 585: Side Effects::, for more information.) 586: 587: To use a macro that expects arguments, you write the name of the 1.1.1.3 root 588: macro followed by a list of "actual arguments" in parentheses, 1.1 root 589: separated by commas. The number of actual arguments you give must 590: match the number of arguments the macro expects. Examples of use of 591: the macro `min' include `min (1, 2)' and `min (x + 28, *p)'. 592: 1.1.1.2 root 593: The expansion text of the macro depends on the arguments you use. 1.1 root 594: Each of the argument names of the macro is replaced, throughout the 595: macro definition, with the corresponding actual argument. Using the 596: same macro `min' defined above, `min (1, 2)' expands into 597: 598: ((1) < (2) ? (1) : (2)) 599: 600: where `1' has been substituted for `X' and `2' for `Y'. 601: 602: Likewise, `min (x + 28, *p)' expands into 603: 604: ((x + 28) < (*p) ? (x + 28) : (*p)) 605: 606: Parentheses in the actual arguments must balance; a comma within 607: parentheses does not end an argument. However, there is no requirement 608: for brackets or braces to balance, and they do not prevent a comma from 609: separating arguments. Thus, 610: 611: macro (array[x = y, x + 1]) 612: 613: passes two arguments to `macro': `array[x = y' and `x + 1]'. If you 614: want to supply `array[x = y, x + 1]' as an argument, you must write it 615: as `array[(x = y, x + 1)]', which is equivalent C code. 616: 617: After the actual arguments are substituted into the macro body, the 618: entire result is appended to the front of the remaining input, and the 619: check for macro calls continues. Therefore, the actual arguments can 620: contain calls to other macros, either with or without arguments, or 1.1.1.2 root 621: even to the same macro. The macro body can also contain calls to other 622: macros. For example, `min (min (a, b), c)' expands into this text: 1.1 root 623: 624: ((((a) < (b) ? (a) : (b))) < (c) 625: ? (((a) < (b) ? (a) : (b))) 626: : (c)) 627: 628: (Line breaks shown here for clarity would not actually be generated.) 629: 1.1.1.2 root 630: If a macro `foo' takes one argument, and you want to supply an empty 631: argument, you must write at least some whitespace between the 632: parentheses, like this: `foo ( )'. Just `foo ()' is providing no 633: arguments, which is an error if `foo' expects an argument. But `foo0 634: ()' is the correct way to call a macro defined to take zero arguments, 635: like this: 636: 637: #define foo0() ... 638: 1.1 root 639: If you use the macro name followed by something other than an 640: open-parenthesis (after ignoring any spaces, tabs and comments that 641: follow), it is not a call to the macro, and the preprocessor does not 642: change what you have written. Therefore, it is possible for the same 643: name to be a variable or function in your program as well as a macro, 644: and you can choose in each instance whether to refer to the macro (if 645: an actual argument list follows) or the variable or function (if an 646: argument list does not follow). 647: 648: Such dual use of one name could be confusing and should be avoided 649: except when the two meanings are effectively synonymous: that is, when 650: the name is both a macro and a function and the two have similar 651: effects. You can think of the name simply as a function; use of the 652: name for purposes other than calling it (such as, to take the address) 653: will refer to the function, while calls will expand the macro and 654: generate better but equivalent code. For example, you can use a 1.1.1.3 root 655: function named `min' in the same source file that defines the macro. 656: If you write `&min' with no argument list, you refer to the function. 1.1 root 657: If you write `min (x, bb)', with an argument list, the macro is 658: expanded. If you write `(min) (a, bb)', where the name `min' is not 1.1.1.2 root 659: followed by an open-parenthesis, the macro is not expanded, so you wind 660: up with a call to the function `min'. 1.1 root 661: 662: You may not define the same name as both a simple macro and a macro 663: with arguments. 664: 665: In the definition of a macro with arguments, the list of argument 1.1.1.3 root 666: names must follow the macro name immediately with no space in between. 1.1.1.2 root 667: If there is a space after the macro name, the macro is defined as 668: taking no arguments, and all the rest of the line is taken to be the 1.1 root 669: expansion. The reason for this is that it is often useful to define a 670: macro that takes no arguments and whose definition begins with an 1.1.1.3 root 671: identifier in parentheses. This rule about spaces makes it possible 672: for you to do either this: 1.1 root 673: 674: #define FOO(x) - 1 / (x) 675: 676: (which defines `FOO' to take an argument and expand into minus the 677: reciprocal of that argument) or this: 678: 679: #define BAR (x) - 1 / (x) 680: 681: (which defines `BAR' to take no argument and always expand into `(x) - 682: 1 / (x)'). 683: 1.1.1.2 root 684: Note that the *uses* of a macro with arguments can have spaces before 685: the left parenthesis; it's the *definition* where it matters whether 686: there is a space. 1.1 root 687: 688: 689: File: cpp.info, Node: Predefined, Next: Stringification, Prev: Argument Macros, Up: Macros 690: 691: Predefined Macros 692: ----------------- 693: 694: Several simple macros are predefined. You can use them without 695: giving definitions for them. They fall into two classes: standard 696: macros and system-specific macros. 697: 698: * Menu: 699: 700: * Standard Predefined:: Standard predefined macros. 701: * Nonstandard Predefined:: Nonstandard predefined macros. 702: 703: 704: File: cpp.info, Node: Standard Predefined, Next: Nonstandard Predefined, Prev: Predefined, Up: Predefined 705: 706: Standard Predefined Macros 707: .......................... 708: 709: The standard predefined macros are available with the same meanings 710: regardless of the machine or operating system on which you are using 1.1.1.3 root 711: GNU C. Their names all start and end with double underscores. Those 1.1.1.2 root 712: preceding `__GNUC__' in this table are standardized by ANSI C; the rest 713: are GNU C extensions. 1.1 root 714: 715: `__FILE__' 716: This macro expands to the name of the current input file, in the 1.1.1.2 root 717: form of a C string constant. The precise name returned is the one 718: that was specified in `#include' or as the input file name 1.1 root 719: argument. 720: 721: `__LINE__' 722: This macro expands to the current input line number, in the form 723: of a decimal integer constant. While we call it a predefined 1.1.1.2 root 724: macro, it's a pretty strange macro, since its "definition" changes 725: with each new line of source code. 1.1 root 726: 727: This and `__FILE__' are useful in generating an error message to 728: report an inconsistency detected by the program; the message can 1.1.1.3 root 729: state the source line at which the inconsistency was detected. 1.1 root 730: For example, 731: 732: fprintf (stderr, "Internal error: " 1.1.1.4 root 733: "negative string length " 1.1 root 734: "%d at %s, line %d.", 735: length, __FILE__, __LINE__); 736: 1.1.1.5 root 737: A `#include' directive changes the expansions of `__FILE__' and 1.1.1.2 root 738: `__LINE__' to correspond to the included file. At the end of that 739: file, when processing resumes on the input file that contained the 1.1.1.5 root 740: `#include' directive, the expansions of `__FILE__' and `__LINE__' 1.1.1.2 root 741: revert to the values they had before the `#include' (but 742: `__LINE__' is then incremented by one as processing moves to the 743: line after the `#include'). 1.1 root 744: 745: The expansions of both `__FILE__' and `__LINE__' are altered if a 1.1.1.5 root 746: `#line' directive is used. *Note Combining Sources::. 1.1 root 747: 748: `__DATE__' 749: This macro expands to a string constant that describes the date on 750: which the preprocessor is being run. The string constant contains 1.1.1.2 root 751: eleven characters and looks like `"Jan 29 1987"' or `"Apr 1 1905"'. 1.1 root 752: 753: `__TIME__' 754: This macro expands to a string constant that describes the time at 755: which the preprocessor is being run. The string constant contains 756: eight characters and looks like `"23:59:01"'. 757: 758: `__STDC__' 759: This macro expands to the constant 1, to signify that this is ANSI 760: Standard C. (Whether that is actually true depends on what C 761: compiler will operate on the output from the preprocessor.) 762: 1.1.1.5 root 763: `__STDC_VERSION__' 764: This macro expands to the C Standard's version number, a long 765: integer constant of the form `YYYYMML' where YYYY and MM are the 766: year and month of the Standard version. This signifies which 767: version of the C Standard the preprocessor conforms to. Like 768: `__STDC__', whether this version number is accurate for the entire 769: implementation depends on what C compiler will operate on the 770: output from the preprocessor. 771: 1.1 root 772: `__GNUC__' 773: This macro is defined if and only if this is GNU C. This macro is 774: defined only when the entire GNU C compiler is in use; if you 1.1.1.4 root 775: invoke the preprocessor directly, `__GNUC__' is undefined. The 776: value identifies the major version number of GNU CC (`1' for GNU CC 777: version 1, which is now obsolete, and `2' for version 2). 1.1 root 778: 1.1.1.5 root 779: `__GNUC_MINOR__' 780: The macro contains the minor version number of the compiler. This 781: can be used to work around differences between different releases 782: of the compiler (for example, if gcc 2.6.3 is known to support a 783: feature, you can test for `__GNUC__ > 2 || (__GNUC__ == 2 && 784: __GNUC_MINOR__ >= 6)'). The last number, `3' in the example 785: above, denotes the bugfix level of the compiler; no macro contains 786: this value. 787: 1.1.1.3 root 788: `__GNUG__' 789: The GNU C compiler defines this when the compilation language is 790: C++; use `__GNUG__' to distinguish between GNU C and GNU C++. 791: 792: `__cplusplus' 793: The draft ANSI standard for C++ used to require predefining this 794: variable. Though it is no longer required, GNU C++ continues to 795: define it, as do other popular C++ compilers. You can use 796: `__cplusplus' to test whether a header is compiled by a C compiler 797: or a C++ compiler. 798: 1.1 root 799: `__STRICT_ANSI__' 800: This macro is defined if and only if the `-ansi' switch was 801: specified when GNU C was invoked. Its definition is the null 1.1.1.3 root 802: string. This macro exists primarily to direct certain GNU header 1.1 root 803: files not to define certain traditional Unix constructs which are 804: incompatible with ANSI C. 805: 806: `__BASE_FILE__' 807: This macro expands to the name of the main input file, in the form 1.1.1.2 root 808: of a C string constant. This is the source file that was specified 809: as an argument when the C compiler was invoked. 1.1 root 810: 1.1.1.5 root 811: `__INCLUDE_LEVEL__' 812: This macro expands to a decimal integer constant that represents 813: the depth of nesting in include files. The value of this macro is 814: incremented on every `#include' directive and decremented at every 815: end of file. For input files specified by command line arguments, 816: the nesting level is zero. 817: 1.1 root 818: `__VERSION__' 819: This macro expands to a string which describes the version number 820: of GNU C. The string is normally a sequence of decimal numbers 1.1.1.4 root 821: separated by periods, such as `"2.6.0"'. The only reasonable use 1.1 root 822: of this macro is to incorporate it into a string constant. 823: 824: `__OPTIMIZE__' 825: This macro is defined in optimizing compilations. It causes 826: certain GNU header files to define alternative macro definitions 827: for some system library functions. It is unwise to refer to or 828: test the definition of this macro unless you make very sure that 829: programs will execute with the same effect regardless. 830: 831: `__CHAR_UNSIGNED__' 832: This macro is defined if and only if the data type `char' is 833: unsigned on the target machine. It exists to cause the standard 834: header file `limit.h' to work correctly. It is bad practice to 835: refer to this macro yourself; instead, refer to the standard 836: macros defined in `limit.h'. The preprocessor uses this macro to 837: determine whether or not to sign-extend large character constants 1.1.1.5 root 838: written in octal; see *Note The `#if' Directive: #if Directive. 839: 840: `__REGISTER_PREFIX__' 841: This macro expands to a string describing the prefix applied to cpu 842: registers in assembler code. It can be used to write assembler 843: code that is usable in multiple environments. For example, in the 844: `m68k-aout' environment it expands to the string `""', but in the 845: `m68k-coff' environment it expands to the string `"%"'. 846: 847: `__USER_LABEL_PREFIX__' 848: This macro expands to a string describing the prefix applied to 849: user generated labels in assembler code. It can be used to write 850: assembler code that is usable in multiple environments. For 851: example, in the `m68k-aout' environment it expands to the string 852: `"_"', but in the `m68k-coff' environment it expands to the string 853: `""'. 1.1 root 854: 855: 856: File: cpp.info, Node: Nonstandard Predefined, Prev: Standard Predefined, Up: Predefined 857: 858: Nonstandard Predefined Macros 859: ............................. 860: 861: The C preprocessor normally has several predefined macros that vary 862: between machines because their purpose is to indicate what type of 863: system and machine is in use. This manual, being for all systems and 864: machines, cannot tell you exactly what their names are; instead, we 865: offer a list of some typical ones. You can use `cpp -dM' to see the 1.1.1.4 root 866: values of predefined macros; see *Note Invocation::. 1.1 root 867: 868: Some nonstandard predefined macros describe the operating system in 869: use, with more or less specificity. For example, 870: 871: `unix' 872: `unix' is normally predefined on all Unix systems. 873: 874: `BSD' 875: `BSD' is predefined on recent versions of Berkeley Unix (perhaps 876: only in version 4.3). 877: 878: Other nonstandard predefined macros describe the kind of CPU, with 879: more or less specificity. For example, 880: 881: `vax' 882: `vax' is predefined on Vax computers. 883: 884: `mc68000' 885: `mc68000' is predefined on most computers whose CPU is a Motorola 886: 68000, 68010 or 68020. 887: 888: `m68k' 889: `m68k' is also predefined on most computers whose CPU is a 68000, 890: 68010 or 68020; however, some makers use `mc68000' and some use 1.1.1.2 root 891: `m68k'. Some predefine both names. What happens in GNU C depends 892: on the system you are using it on. 1.1 root 893: 894: `M68020' 895: `M68020' has been observed to be predefined on some systems that 896: use 68020 CPUs--in addition to `mc68000' and `m68k', which are 897: less specific. 898: 899: `_AM29K' 900: `_AM29000' 901: Both `_AM29K' and `_AM29000' are predefined for the AMD 29000 CPU 902: family. 903: 904: `ns32000' 905: `ns32000' is predefined on computers which use the National 906: Semiconductor 32000 series CPU. 907: 908: Yet other nonstandard predefined macros describe the manufacturer of 909: the system. For example, 910: 911: `sun' 912: `sun' is predefined on all models of Sun computers. 913: 914: `pyr' 915: `pyr' is predefined on all models of Pyramid computers. 916: 917: `sequent' 918: `sequent' is predefined on all models of Sequent computers. 919: 1.1.1.2 root 920: These predefined symbols are not only nonstandard, they are contrary 921: to the ANSI standard because their names do not start with underscores. 922: Therefore, the option `-ansi' inhibits the definition of these symbols. 923: 924: This tends to make `-ansi' useless, since many programs depend on the 925: customary nonstandard predefined symbols. Even system header files 926: check them and will generate incorrect declarations if they do not find 927: the names that are expected. You might think that the header files 928: supplied for the Uglix computer would not need to test what machine 929: they are running on, because they can simply assume it is the Uglix; 930: but often they do, and they do so using the customary names. As a 931: result, very few C programs will compile with `-ansi'. We intend to 932: avoid such problems on the GNU system. 1.1 root 933: 934: What, then, should you do in an ANSI C program to test the type of 935: machine it will run on? 936: 937: GNU C offers a parallel series of symbols for this purpose, whose 938: names are made from the customary ones by adding `__' at the beginning 939: and end. Thus, the symbol `__vax__' would be available on a Vax, and 940: so on. 941: 942: The set of nonstandard predefined names in the GNU C preprocessor is 943: controlled (when `cpp' is itself compiled) by the macro 944: `CPP_PREDEFINES', which should be a string containing `-D' options, 945: separated by spaces. For example, on the Sun 3, we use the following 946: definition: 947: 948: #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k" 949: 950: This macro is usually specified in `tm.h'. 951: 952: 953: File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Predefined, Up: Macros 954: 955: Stringification 956: --------------- 957: 958: "Stringification" means turning a code fragment into a string 959: constant whose contents are the text for the code fragment. For 960: example, stringifying `foo (z)' results in `"foo (z)"'. 961: 962: In the C preprocessor, stringification is an option available when 1.1.1.2 root 963: macro arguments are substituted into the macro definition. In the body 964: of the definition, when an argument name appears, the character `#' 965: before the name specifies stringification of the corresponding actual 966: argument when it is substituted at that point in the definition. The 967: same argument may be substituted in other places in the definition 968: without stringification if the argument name appears in those places 969: with no `#'. 1.1 root 970: 971: Here is an example of a macro definition that uses stringification: 972: 973: #define WARN_IF(EXP) \ 974: do { if (EXP) \ 975: fprintf (stderr, "Warning: " #EXP "\n"); } \ 976: while (0) 977: 978: Here the actual argument for `EXP' is substituted once as given, into 979: the `if' statement, and once as stringified, into the argument to 980: `fprintf'. The `do' and `while (0)' are a kludge to make it possible 981: to write `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a 1.1.1.4 root 982: function would make C programmers want to do; see *Note Swallow 983: Semicolon::. 1.1 root 984: 985: The stringification feature is limited to transforming one macro 986: argument into one string constant: there is no way to combine the 987: argument with other text and then stringify it all together. But the 988: example above shows how an equivalent result can be obtained in ANSI 989: Standard C using the feature that adjacent string constants are 1.1.1.2 root 990: concatenated as one string constant. The preprocessor stringifies the 991: actual value of `EXP' into a separate string constant, resulting in 1.1 root 992: text like 993: 994: do { if (x == 0) \ 995: fprintf (stderr, "Warning: " "x == 0" "\n"); } \ 996: while (0) 997: 998: but the C compiler then sees three consecutive string constants and 999: concatenates them into one, producing effectively 1000: 1001: do { if (x == 0) \ 1002: fprintf (stderr, "Warning: x == 0\n"); } \ 1003: while (0) 1004: 1005: Stringification in C involves more than putting doublequote 1006: characters around the fragment; it is necessary to put backslashes in 1007: front of all doublequote characters, and all backslashes in string and 1.1.1.2 root 1008: character constants, in order to get a valid C string constant with the 1009: proper contents. Thus, stringifying `p = "foo\n";' results in `"p = 1010: \"foo\\n\";"'. However, backslashes that are not inside of string or 1.1 root 1011: character constants are not duplicated: `\n' by itself stringifies to 1012: `"\n"'. 1013: 1014: Whitespace (including comments) in the text being stringified is 1015: handled according to precise rules. All leading and trailing 1.1.1.3 root 1016: whitespace is ignored. Any sequence of whitespace in the middle of the 1.1.1.2 root 1017: text is converted to a single space in the stringified result. 1.1 root 1018: 1019: 1020: File: cpp.info, Node: Concatenation, Next: Undefining, Prev: Stringification, Up: Macros 1021: 1022: Concatenation 1023: ------------- 1024: 1025: "Concatenation" means joining two strings into one. In the context 1026: of macro expansion, concatenation refers to joining two lexical units 1.1.1.2 root 1027: into one longer one. Specifically, an actual argument to the macro can 1028: be concatenated with another actual argument or with fixed text to 1029: produce a longer name. The longer name might be the name of a function, 1030: variable or type, or a C keyword; it might even be the name of another 1031: macro, in which case it will be expanded. 1.1 root 1032: 1033: When you define a macro, you request concatenation with the special 1034: operator `##' in the macro body. When the macro is called, after 1035: actual arguments are substituted, all `##' operators are deleted, and 1036: so is any whitespace next to them (including whitespace that was part 1037: of an actual argument). The result is to concatenate the syntactic 1038: tokens on either side of the `##'. 1039: 1.1.1.2 root 1040: Consider a C program that interprets named commands. There probably 1041: needs to be a table of commands, perhaps an array of structures 1042: declared as follows: 1.1 root 1043: 1044: struct command 1045: { 1046: char *name; 1047: void (*function) (); 1048: }; 1049: 1050: struct command commands[] = 1051: { 1052: { "quit", quit_command}, 1053: { "help", help_command}, 1054: ... 1055: }; 1056: 1057: It would be cleaner not to have to give each command name twice, 1058: once in the string constant and once in the function name. A macro 1059: which takes the name of a command as an argument can make this 1060: unnecessary. The string constant can be created with stringification, 1.1.1.3 root 1061: and the function name by concatenating the argument with `_command'. 1.1 root 1062: Here is how it is done: 1063: 1064: #define COMMAND(NAME) { #NAME, NAME ## _command } 1065: 1066: struct command commands[] = 1067: { 1068: COMMAND (quit), 1069: COMMAND (help), 1070: ... 1071: }; 1072: 1073: The usual case of concatenation is concatenating two names (or a 1074: name and a number) into a longer name. But this isn't the only valid 1075: case. It is also possible to concatenate two numbers (or a number and 1076: a name, such as `1.5' and `e3') into a number. Also, multi-character 1077: operators such as `+=' can be formed by concatenation. In some cases 1078: it is even possible to piece together a string constant. However, two 1079: pieces of text that don't together form a valid lexical unit cannot be 1080: concatenated. For example, concatenation with `x' on one side and `+' 1081: on the other is not meaningful because those two characters can't fit 1082: together in any lexical unit of C. The ANSI standard says that such 1083: attempts at concatenation are undefined, but in the GNU C preprocessor 1084: it is well defined: it puts the `x' and `+' side by side with no 1085: particular special results. 1086: 1.1.1.2 root 1087: Keep in mind that the C preprocessor converts comments to whitespace 1088: before macros are even considered. Therefore, you cannot create a 1089: comment by concatenating `/' and `*': the `/*' sequence that starts a 1090: comment is not a lexical unit, but rather the beginning of a "long" 1091: space character. Also, you can freely use comments next to a `##' in a 1092: macro definition, or in actual arguments that will be concatenated, 1093: because the comments will be converted to spaces at first sight, and 1094: concatenation will later discard the spaces. 1.1 root 1095: 1096: 1097: File: cpp.info, Node: Undefining, Next: Redefining, Prev: Concatenation, Up: Macros 1098: 1099: Undefining Macros 1100: ----------------- 1101: 1102: To "undefine" a macro means to cancel its definition. This is done 1.1.1.5 root 1103: with the `#undef' directive. `#undef' is followed by the macro name to 1.1 root 1104: be undefined. 1105: 1106: Like definition, undefinition occurs at a specific point in the 1107: source file, and it applies starting from that point. The name ceases 1108: to be a macro name, and from that point on it is treated by the 1109: preprocessor as if it had never been a macro name. 1110: 1111: For example, 1112: 1113: #define FOO 4 1114: x = FOO; 1115: #undef FOO 1116: x = FOO; 1117: 1118: expands into 1119: 1120: x = 4; 1121: 1122: x = FOO; 1123: 1124: In this example, `FOO' had better be a variable or function as well as 1125: (temporarily) a macro, in order for the result of the expansion to be 1126: valid C code. 1127: 1.1.1.5 root 1128: The same form of `#undef' directive will cancel definitions with 1.1 root 1129: arguments or definitions that don't expect arguments. The `#undef' 1.1.1.5 root 1130: directive has no effect when used on a name not currently defined as a 1.1 root 1131: macro. 1132: 1133: 1134: File: cpp.info, Node: Redefining, Next: Macro Pitfalls, Prev: Undefining, Up: Macros 1135: 1136: Redefining Macros 1137: ----------------- 1138: 1139: "Redefining" a macro means defining (with `#define') a name that is 1140: already defined as a macro. 1141: 1142: A redefinition is trivial if the new definition is transparently 1143: identical to the old one. You probably wouldn't deliberately write a 1144: trivial redefinition, but they can happen automatically when a header 1145: file is included more than once (*note Header Files::.), so they are 1146: accepted silently and without effect. 1147: 1148: Nontrivial redefinition is considered likely to be an error, so it 1.1.1.2 root 1149: provokes a warning message from the preprocessor. However, sometimes it 1150: is useful to change the definition of a macro in mid-compilation. You 1151: can inhibit the warning by undefining the macro with `#undef' before the 1152: second definition. 1.1 root 1153: 1154: In order for a redefinition to be trivial, the new definition must 1155: exactly match the one already in effect, with two possible exceptions: 1156: 1157: * Whitespace may be added or deleted at the beginning or the end. 1158: 1.1.1.2 root 1159: * Whitespace may be changed in the middle (but not inside strings). 1160: However, it may not be eliminated entirely, and it may not be added 1161: where there was no whitespace at all. 1.1 root 1162: 1163: Recall that a comment counts as whitespace. 1164: 1165: 1166: File: cpp.info, Node: Macro Pitfalls, Prev: Redefining, Up: Macros 1167: 1168: Pitfalls and Subtleties of Macros 1169: --------------------------------- 1170: 1171: In this section we describe some special rules that apply to macros 1.1.1.2 root 1172: and macro expansion, and point out certain cases in which the rules have 1173: counterintuitive consequences that you must watch out for. 1.1 root 1174: 1175: * Menu: 1176: 1177: * Misnesting:: Macros can contain unmatched parentheses. 1178: * Macro Parentheses:: Why apparently superfluous parentheses 1179: may be necessary to avoid incorrect grouping. 1180: * Swallow Semicolon:: Macros that look like functions 1181: but expand into compound statements. 1182: * Side Effects:: Unsafe macros that cause trouble when 1183: arguments contain side effects. 1184: * Self-Reference:: Macros whose definitions use the macros' own names. 1185: * Argument Prescan:: Actual arguments are checked for macro calls 1186: before they are substituted. 1187: * Cascaded Macros:: Macros whose definitions use other macros. 1.1.1.2 root 1188: * Newlines in Args:: Sometimes line numbers get confused. 1.1 root 1189:
This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.