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Info file cpp.info, produced by Makeinfo, -*- Text -*- from input file cpp.texinfo. This file documents the GNU C Preprocessor. Copyright (C) 1987, 1989 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. File: cpp.info, Node: Top, Next: Global Actions, Up: (DIR) The C Preprocessor ****************** The C preprocessor is a "macro processor" that is used automatically by the C compiler to transform your program before actual compilation. It is called a macro processor because it allows you to define "macros", which are brief abbreviations for longer constructs. The C preprocessor provides four separate facilities that you can use as you see fit: * Inclusion of header files. These are files of declarations that can be substituted into your program. * Macro expansion. You can define "macros", which are abbreviations for arbitrary fragments of C code, and then the C preprocessor will replace the macros with their definitions throughout the program. * Conditional compilation. Using special preprocessor commands, you can include or exclude parts of the program according to various conditions. * Line control. If you use a program to combine or rearrange source files into an intermediate file which is then compiled, you can use line control to inform the compiler of where each source line originally came from. C preprocessors vary in some details. This manual discusses the GNU C preprocessor, the C Compatible Compiler Preprocessor. The GNU C preprocessor provides a superset of the features of ANSI Standard C. ANSI Standard C requires the rejection of many harmless constructs commonly used by today's C programs. Such incompatibility would be inconvenient for users, so the GNU C preprocessor is configured to accept these constructs by default. Strictly speaking, to get ANSI Standard C, you must use the options `-trigraphs', `-undef' and `-pedantic', but in practice the consequences of having strict ANSI Standard C make it undesirable to do this. *Note Invocation::. * Menu: * Global Actions:: Actions made uniformly on all input files. * Commands:: General syntax of preprocessor commands. * Header Files:: How and why to use header files. * Macros:: How and why to use macros. * Conditionals:: How and why to use conditionals. * Combining Sources:: Use of line control when you combine source files. * Other Commands:: Miscellaneous preprocessor commands. * Output:: Format of output from the C preprocessor. * Invocation:: How to invoke the preprocessor; command options. * Concept Index:: Index of concepts and terms. * Index:: Index of commands, predefined macros and options. File: cpp.info, Node: Global Actions, Next: Commands, Prev: Top, Up: Top Transformations Made Globally ============================= Most C preprocessor features are inactive unless you give specific commands to request their use. (Preprocessor commands are lines starting with `#'; *note Commands::.). But there are three transformations that the preprocessor always makes on all the input it receives, even in the absence of commands. * All C comments are replaced with single spaces. * Backslash-Newline sequences are deleted, no matter where. This feature allows you to break long lines for cosmetic purposes without changing their meaning. * Predefined macro names are replaced with their expansions (*note Predefined::.). The first two transformations are done *before* nearly all other parsing and before preprocessor commands are recognized. Thus, for example, you can split a line cosmetically with Backslash-Newline anywhere (except when trigraphs are in use; see below). /* */ # /* */ defi\ ne FO\ O 10\ 20 is equivalent into `#define FOO 1020'. You can split even an escape sequence with Backslash-Newline. For example, you can split `"foo\bar"' between the `\' and the `b' to get "foo\\ bar" This behavior is unclean: in all other contexts, a Backslash can be inserted in a string constant as an ordinary character by writing a double Backslash, and this creates an exception. But the ANSI C standard requires it. (Strict ANSI C does not allow Newlines in string constants, so they do not consider this a problem.) But there are a few exceptions to all three transformations. * C comments and predefined macro names are not recognized inside a `#include' command in which the file name is delimited with `<' and `>'. * C comments and predefined macro names are never recognized within a character or string constant. (Strictly speaking, this is the rule, not an exception, but it is worth noting here anyway.) * Backslash-Newline may not safely be used within an ANSI ``trigraph''. Trigraphs are converted before Backslash-Newline is deleted. If you write what looks like a trigraph with a Backslash-Newline inside, the Backslash-Newline is deleted as usual, but it is then too late to recognize the trigraph. This exception is relevant only if you use the `-trigraphs' option to enable trigraph processing. *Note Invocation::. File: cpp.info, Node: Commands, Next: Header Files, Prev: Global Actions, Up: Top Preprocessor Commands ===================== Most preprocessor features are active only if you use preprocessor commands to request their use. Preprocessor commands are lines in your program that start with `#'. The `#' is followed by an identifier that is the "command name". For example, `#define' is the command that defines a macro. Whitespace is also allowed before and after the `#'. The set of valid command names is fixed. Programs cannot define new preprocessor commands. Some command names require arguments; these make up the rest of the command line and must be separated from the command name by whitespace. For example, `#define' must be followed by a macro name and the intended expansion of the macro. A preprocessor command cannot be more than one line in normal circumstances. It may be split cosmetically with Backslash-Newline, but that has no effect on its meaning. Comments containing Newlines can also divide the command into multiple lines, but the comments are changed to Spaces before the command is interpreted. The only way a significant Newline can occur in a preprocessor command is within a string constant or character constant. Note that most C compilers that might be applied to the output from the preprocessor do not accept string or character constants containing Newlines. The `#' and the command name cannot come from a macro expansion. For example, if `foo' is defined as a macro expanding to `define', that does not make `#foo' a valid preprocessor command. File: cpp.info, Node: Header Files, Next: Macros, Prev: Commands, Up: Top Header Files ============ A header file is a file containing C declarations and macro definitions (*note Macros::.) to be shared between several source files. You request the use of a header file in your program with the C preprocessor command `#include'. * Menu: * Header Uses:: What header files are used for. * Include Syntax:: How to write `#include' commands. * Include Operation:: What `#include' does. * Once-Only:: Preventing multiple inclusion of one header file. File: cpp.info, Node: Header Uses, Next: Include Syntax, Prev: Header Files, Up: Header Files Uses of Header Files -------------------- Header files serve two kinds of purposes. * System header files declare the interfaces to parts of the operating system. You include them in your program to supply the definitions you need to invoke system calls and libraries. * Your own header files contain declarations for interfaces between the source files of your program. Each time you have a group of related declarations and macro definitions all or most of which are needed in several different source files, it is a good idea to create a header file for them. Including a header file produces the same results in C compilation as copying the header file into each source file that needs it. But such copying would be time-consuming and error-prone. With a header file, the related declarations appear in only one place. If they need to be changed, they can be changed in one place, and programs that include the header file will automatically use the new version when next recompiled. The header file eliminates the labor of finding and changing all the copies as well as the risk that a failure to find one copy will result in inconsistencies within a program. The usual convention is to give header files names that end with `.h'. File: cpp.info, Node: Include Syntax, Next: Include Operation, Prev: Header Uses, Up: Header Files The `#include' Command ---------------------- Both user and system header files are included using the preprocessor command `#include'. It has three variants: `#include <FILE>' This variant is used for system header files. It searches for a file named FILE in a list of directories specified by you, then in a standard list of system directories. You specify directories to search for header files with the command option `-I' (*note Invocation::.). The option `-nostdinc' inhibits searching the standard system directories; in this case only the directories you specify are searched. The parsing of this form of `#include' is slightly special because comments are not recognized within the `<...>'. Thus, in `#include <x/*y>' the `/*' does not start a comment and the command specifies inclusion of a system header file named `x/*y'. Of course, a header file with such a name is unlikely to exist on Unix, where shell wildcard features would make it hard to manipulate. The argument FILE may not contain a `>' character. It may, however, contain a `<' character. `#include "FILE"' This variant is used for header files of your own program. It searches for a file named FILE first in the current directory, then in the same directories used for system header files. The current directory is the directory of the current input file. It is tried first because it is presumed to be the location of the files that the current input file refers to. (If the `-I-' option is used, the special treatment of the current directory is inhibited.) The argument FILE may not contain `"' characters. If backslashes occur within FILE, they are considered ordinary text characters, not escape characters. None of the character escape sequences appropriate to string constants in C are processed. Thus, `#include "x\n\\y"' specifies a filename containing three backslashes. It is not clear why this behavior is ever useful, but the ANSI standard specifies it. `#include ANYTHING ELSE' This variant is called a "computed #include". Any `#include' command whose argument does not fit the above two forms is a computed include. The text ANYTHING ELSE is checked for macro calls, which are expanded (*note Macros::.). When this is done, the result must fit one of the above two variants. This feature allows you to define a macro which controls the file name to be used at a later point in the program. One application of this is to allow a site-configuration file for your program to specify the names of the system include files to be used. This can help in porting the program to various operating systems in which the necessary system header files are found in different places. File: cpp.info, Node: Include Operation, Next: Once-Only, Prev: Include Syntax, Up: Header Files How `#include' Works -------------------- The `#include' command works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the `#include' command. For example, given two files as follows: /* File program.c */ int x; #include "header.h" main () { printf (test ()); } /* File header.h */ char *test (); the output generated by the C preprocessor for `program.c' as input would be int x; char *test (); main () { printf (test ()); } Included files are not limited to declarations and macro definitions; they are merely the typical use. Any fragment of a C program can be included from another file. The include file could even contain the beginning of a statement that is concluded in the containing file, or the end of a statement that was started in the including file. However, a comment or a string or character constant may not start in the included file and finish in the including file. An unterminated comment, string constant or character constant in an included file is considered to end (with an error message) at the end of the file. The line following the `#include' command is always treated as a separate line by the C preprocessor even if the included file lacks a final newline. File: cpp.info, Node: Once-Only, Prev: Include Operation, Up: Header Files Once-Only Include Files ----------------------- Very often, one header file includes another. It can easily result that a certain header file is included more than once. This may lead to errors, if the header file defines structure types or typedefs, and is certainly wasteful. Therefore, we often wish to prevent multiple inclusion of a header file. The standard way to do this is to enclose the entire real contents of the file in a conditional, like this: #ifndef __FILE_FOO_SEEN__ #define __FILE_FOO_SEEN__ THE ENTIRE FILE #endif /* __FILE_FOO_SEEN__ */ The macro `__FILE_FOO_SEEN__' indicates that the file has been included once already; its name should begin with `__', and should contain the name of the file to avoid accidental conflicts. One drawback of this method is that the preprocessor must scan the input file completely in order to determine that all of it is to be ignored. This makes compilation slower. You can avoid the delay by inserting the following command near the beginning of file *in addition to the conditionals described above*: #pragma once This command tells the GNU C preprocessor to ignore any future commands to include the same file (whichever file the `#pragma' appears in). You should not *rely* on `#pragma once' to prevent multiple inclusion of the file. It is just a hint, and a nonstandard one at that. Most C compilers will ignore it entirely. For this reason, you still need the conditionals if you want to make certain that the file's contents are not included twice. Note that `#pragma once' works by file name; if a file has more than one name, it can be included once under each name, even in GNU CC, despite `#pragma once'. File: cpp.info, Node: Macros, Next: Conditionals, Prev: Header Files, Up: Top Macros ====== A macro is a sort of abbreviation which you can define once and then use later. There are many complicated features associated with macros in the C preprocessor. * Menu: * Simple Macros:: Macros that always expand the same way. * Argument Macros:: Macros that accept arguments that are substituted into the macro expansion. * Predefined:: Predefined macros that are always available. * Stringification:: Macro arguments converted into string constants. * Concatenation:: Building tokens from parts taken from macro arguments. * Undefining:: Cancelling a macro's definition. * Redefining:: Changing a macro's definition. * Macro Pitfalls:: Macros can confuse the unwary. Here we explain several common problems and strange features. File: cpp.info, Node: Simple Macros, Next: Argument Macros, Prev: Macros, Up: Macros Simple Macros ------------- A "simple macro" is a kind of abbreviation. It is a name which stands for a fragment of code. Before you can use a macro, you must "define" it explicitly with the `#define' command. `#define' is followed by the name of the macro and then the code it should be an abbreviation for. For example, #define BUFFER_SIZE 1020 defines a macro named `BUFFER_SIZE' as an abbreviation for the text `1020'. Therefore, if somewhere after this `#define' command there comes a C statement of the form foo = (char *) xmalloc (BUFFER_SIZE); then the C preprocessor will recognize and "expand" the macro `BUFFER_SIZE', resulting in foo = (char *) xmalloc (1020); the definition must be a single line; however, it may not end in the middle of a multi-line string constant or character constant. The use of all upper case for macro names is a standard convention. Programs are easier to read when it is possible to tell at a glance which names are macros. Normally, a macro definition must be a single line, like all C preprocessor commands. (You can split a long macro definition cosmetically with Backslash-Newline.) There is one exception: Newlines can be included in the macro definition if within a string or character constant. By the same token, it is not possible for a macro definition to contain an unbalanced quote character; the definition automatically extends to include the matching quote character that ends the string or character constant. Comments within a macro definition may contain Newlines, which make no difference since the comments are entirely replaced with Spaces regardless of their contents. Aside from the above, there is no restriction on what can go in a macro body. Parentheses need not balance. The body need not resemble valid C code. (Of course, you might get error messages from the C compiler when you use the macro.) The C preprocessor scans your program sequentially, so macro definitions take effect at the place you write them. Therefore, the following input to the C preprocessor foo = X; #define X 4 bar = X; produces as output foo = X; bar = 4; After the preprocessor expands a macro name, the macro's definition body is appended to the front of the remaining input, and the check for macro calls continues. Therefore, the macro body can contain calls to other macros. For example, after #define BUFSIZE 1020 #define TABLESIZE BUFSIZE the name `TABLESIZE' when used in the program would go through two stages of expansion, resulting ultimately in `1020'. This is not at all the same as defining `TABLESIZE' to be `1020'. The `#define' for `TABLESIZE' uses exactly the body you specify--in this case, `BUFSIZE'--and does not check to see whether it too is the name of a macro. It's only when you *use* `TABLESIZE' that the result of its expansion is checked for more macro names. *Note Cascaded Macros::. File: cpp.info, Node: Argument Macros, Next: Predefined, Prev: Simple Macros, Up: Macros Macros with Arguments --------------------- A simple macro always stands for exactly the same text, each time it is used. Macros can be more flexible when they accept "arguments". Arguments are fragments of code that you supply each time the macro is used. These fragments are included in the expansion of the macro according to the directions in the macro definition. To define a macro that uses arguments, you write a `#define' command with a list of "argument names" in parentheses after the name of the macro. The argument names may be any valid C identifiers, separated by commas and optionally whitespace. The open-parenthesis must follow the macro name immediately, with no space in between. For example, here is a macro that computes the minimum of two numeric values, as it is defined in many C programs: #define min(X, Y) ((X) < (Y) ? (X) : (Y)) (This is not the best way to define a ``minimum'' macro in GNU C. *Note Side Effects::, for more information.) To use a macro that expects arguments, you write the name of the macro followed by a list of "actual arguments" in parentheses. separated by commas. The number of actual arguments you give must match the number of arguments the macro expects. Examples of use of the macro `min' include `min (1, 2)' and `min (x + 28, *p)'. The expansion text of the macro depends on the arguments you use. Each of the argument names of the macro is replaced, throughout the macro definition, with the corresponding actual argument. Using the same macro `min' defined above, `min (1, 2)' expands into ((1) < (2) ? (1) : (2)) where `1' has been substituted for `X' and `2' for `Y'. Likewise, `min (x + 28, *p)' expands into ((x + 28) < (*p) ? (x + 28) : (*p)) Parentheses in the actual arguments must balance; a comma within parentheses does not end an argument. However, there is no requirement for brackets or braces to balance; thus, if you want to supply `array[x = y, x + 1]' as an argument, you must write it as `array[(x = y, x + 1)]', which is equivalent C code. After the actual arguments are substituted into the macro body, the entire result is appended to the front of the remaining input, and the check for macro calls continues. Therefore, the actual arguments can contain calls to other macros, either with or without arguments, or even to the same macro. The macro body can also contain calls to other macros. For example, `min (min (a, b), c)' expands into ((((a) < (b) ? (a) : (b))) < (c) ? (((a) < (b) ? (a) : (b))) : (c)) (Line breaks shown here for clarity would not actually be generated.) If you use the macro name followed by something other than an open-parenthesis (after ignoring any spaces, tabs and comments that follow), it is not a call to the macro, and the preprocessor leaves the name unaltered. Therefore, it is possible for the same name to be a variable or function in your program as well as a macro, and you can choose in each instance whether to refer to the macro (if an actual argument list follows) or the variable or function (if an argument list does not follow). Such dual use of one name could be confusing and should be avoided except when the two meanings are effectively synonymous: that is, when the name is both a macro and a function and the two have similar effects. You can think of the name simply as a function; use of the name for purposes other than calling it (such as, to take the address) will refer to the function, while calls will expand the macro and generate better but equivalent code. For example, you can use a function named `min' in the same source file that defines the macro. If you write `&min' with no argument list, you refer to the function. If you write `min (x, bb)', with an argument list, the macro is expanded. If you write `(min) (a, bb)', where the name `min' is not followed by an open-parenthesis, the macro is not expanded, so you wind up with a call to the function `min'. It is not allowed to define the same name as both a simple macro and a macro with arguments. In the definition of a macro with arguments, the list of argument names must follow the macro name immediately with no space in between. If there is a space after the macro name, the macro is defined as taking no arguments, and all the rest of the name is taken to be the expansion. The reason for this is that it is often useful to define a macro that takes no arguments and whose definition begins with an identifier in parentheses. This rule about spaces makes it possible for you to do either this: #define FOO(x) - 1 / (x) (which defines `FOO' to take an argument and expand into minus the reciprocal of that argument) or this: #define BAR (x) - 1 / (x) (which defines `BAR' to take no argument and always expand into `(x) - 1 / (x)'). Note that the *uses* of a macro with arguments can have spaces before the left parenthesis; it's the *definition* where it matters whether there is a space. File: cpp.info, Node: Predefined, Next: Stringification, Prev: Argument Macros, Up: Macros Predefined Macros ----------------- Several simple macros are predefined. You can use them without giving definitions for them. They fall into two classes: standard macros and system-specific macros. * Menu: * Standard Predefined:: Standard predefined macros. * Nonstandard Predefined:: Nonstandard predefined macros. File: cpp.info, Node: Standard Predefined, Next: Nonstandard Predefined, Prev: Predefined, Up: Predefined Standard Predefined Macros .......................... The standard predefined macros are available with the same meanings regardless of the machine or operating system on which you are using GNU C. Their names all start and end with double underscores. Those preceding `__GNUC__' in this table are standardized by ANSI C; the rest are GNU C extensions. `__FILE__' This macro expands to the name of the current input file, in the form of a C string constant. `__BASE_FILE__' This macro expands to the name of the main input file, in the form of a C string constant. This is the source file that was specified as an argument when the C compiler was invoked. `__LINE__' This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it's a pretty strange macro, since its ``definition'' changes with each new line of source code. This and `__FILE__' are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example, fprintf (stderr, "Internal error: negative string length " "%d at %s, line %d.", length, __FILE__, __LINE__); A `#include' command changes the expansions of `__FILE__' and `__LINE__' to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the `#include' command, the expansions of `__FILE__' and `__LINE__' revert to the values they had before the `#include' (but `__LINE__' is then incremented by one as processing moves to the line after the `#include'). The expansions of both `__FILE__' and `__LINE__' are altered if a `#line' command is used. *Note Combining Sources::. `__DATE__' This macro expands to a string constant that describes the date on which the preprocessor is being run. The string constant contains eleven characters and looks like `"Jan 29 1987"' or `"Apr 1 1905"'. `__TIME__' This macro expands to a string constant that describes the time at which the preprocessor is being run. The string constant contains eight characters and looks like `"23:59:01"'. `__STDC__' This macro expands to the constant 1, to signify that this is ANSI Standard C. (Whether that is actually true depends on what C compiler will operate on the output from the preprocessor.) `__GNUC__' This macro is defined if and only if this is GNU C. This macro is defined only when the entire GNU C compiler is in use; if you invoke the preprocessor directly, `__GNUC__' is undefined. `__STRICT_ANSI__' This macro is defined if and only if the `-ansi' switch was specified when GNU C was invoked. Its definition is the null string. This macro exists primarily to direct certain GNU header files not to define certain traditional Unix constructs which are incompatible with ANSI C. `__VERSION__' This macro expands to a string which describes the version number of GNU C. The string is normally a sequence of decimal numbers separated by periods, such as `"1.18"'. The only reasonable use of this macro is to incorporate it into a string constant. `__OPTIMIZE__' This macro is defined in optimizing compilations. It causes certain GNU header files to define alternative macro definitions for some system library functions. It is unwise to refer to or test the definition of this macro unless you make very sure that programs will execute with the same effect regardless. `__CHAR_UNSIGNED__' This macro is defined if and only if the data type `char' is unsigned on the target machine. It exists to cause the standard header file `limit.h' to work correctly. It is bad practice to refer to this macro yourself; instead, refer to the standard macros defined in `limit.h'. File: cpp.info, Node: Nonstandard Predefined, Prev: Standard Predefined, Up: Predefined Nonstandard Predefined Macros ............................. The C preprocessor normally has several predefined macros that vary between machines because their purpose is to indicate what type of system and machine is in use. This manual, being for all systems and machines, cannot tell you exactly what their names are; instead, we offer a list of some typical ones. Some nonstandard predefined macros describe the operating system in use, with more or less specificity. For example, `unix' `unix' is normally predefined on all Unix systems. `BSD' `BSD' is predefined on recent versions of Berkeley Unix (perhaps only in version 4.3). Other nonstandard predefined macros describe the kind of CPU, with more or less specificity. For example, `vax' `vax' is predefined on Vax computers. `mc68000' `mc68000' is predefined on most computers whose CPU is a Motorola 68000, 68010 or 68020. `m68k' `m68k' is also predefined on most computers whose CPU is a 68000, 68010 or 68020; however, some makers use `mc68000' and some use `m68k'. Some predefine both names. What happens in GNU C depends on the system you are using it on. `M68020' `M68020' has been observed to be predefined on some systems that use 68020 CPUs--in addition to `mc68000' and `m68k' that are less specific. `ns32000' `ns32000' is predefined on computers which use the National Semiconductor 32000 series CPU. Yet other nonstandard predefined macros describe the manufacturer of the system. For example, `sun' `sun' is predefined on all models of Sun computers. `pyr' `pyr' is predefined on all models of Pyramid computers. `sequent' `sequent' is predefined on all models of Sequent computers. These predefined symbols are not only nonstandard, they are contrary to the ANSI standard because their names do not start with underscores. Therefore, the option `-ansi' inhibits the definition of these symbols. This tends to make `-ansi' useless, since many programs depend on the customary nonstandard predefined symbols. Even system header files check them and will generate incorrect declarations if they do not find the names that are expected. You might think that the header files supplied for the Uglix computer would not need to test what machine they are running on, because they can simply assume it is the Uglix; but often they do, and they do so using the customary names. As a result, very few C programs will compile with `-ansi'. We intend to avoid such problems on the GNU system. What, then, should you do in an ANSI C program to test the type of machine it is to run on? GNU C offers a parallel series of symbols for this purpose, whose names are made from the customary ones by adding `__' at the beginning and end. Thus, the symbol `__vax__' would be available on a vax, and so on. The set of nonstandard predefined names in the GNU C preprocessor is controlled by the macro `CPP_PREDEFINES', which should be a string containing `-D' options, separated by spaces. For example, on the Sun 3, we use the following definition: #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k" File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Predefined, Up: Macros Stringification --------------- "Stringification" means turning a code fragment into a string constant whose contents are the text for the code fragment. For example, stringifying `foo (z)' results in `"foo (z)"'. In the C preprocessor, stringification is an option available when macro arguments are substituted into the macro definition. In the body of the definition, when an argument name appears, the character `#' before the name specifies stringification of the corresponding actual argument when it is substituted at that point in the definition. The same argument may be substituted in other places in the definition without stringification if the argument name appears in those places with no `#'. Here is an example of a macro definition that uses stringification: #define WARN_IF(EXP) \ do { if (EXP) fprintf (stderr, "Warning: " #EXP "\n"); } while (0) Here the actual argument for `EXP' is substituted once as given, into the `if' statement, and once as stringified, into the argument to `fprintf'. The `do' and `while (0)' are a kludge to make it possible to write `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function would make C programmers want to do; *note Swallow Semicolon::.). The stringification feature is limited to transforming one macro argument into one string constant: there is no way to combine the argument with other text and then stringify it all together. But the example above shows how an equivalent result can be obtained in ANSI Standard C using the feature that adjacent string constants are concatenated as one string constant. The preprocessor stringifies `EXP''s actual argument into a separate string constant, resulting in text like do { if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0) but the C compiler then sees three consecutive string constants and concatenates them into one, producing effectively do { if (x == 0) fprintf (stderr, "Warning: x == 0\n"); } while (0) Stringification in C involves more than putting doublequote characters around the fragment; it is necessary to put backslashes in front of all doublequote characters, and all backslashes in string and character constants, in order to get a valid C string constant with the proper contents. Thus, stringifying `p = "foo\n";' results in `"p = \"foo\\n\";"'. However, backslashes that are not inside of string or character constants are not duplicated: `\n' by itself stringifies to `"\n"'. Whitespace (including comments) in the text being stringified is handled according to precise rules. All leading and trailing whitespace is ignored. Any sequence of whitespace in the middle of the text is converted to a single space in the stringified result. File: cpp.info, Node: Concatenation, Next: Undefining, Prev: Stringification, Up: Macros Concatenation ------------- "Concatenation" means joining two strings into one. In the context of macro expansion, concatenation refers to joining two lexical units into one longer one. Specifically, an actual argument to the macro can be concatenated with another actual argument or with fixed text to produce a longer name. The longer name might be the name of a function, variable or type, or a C keyword; it might even be the name of another macro, in which case it will be expanded. When you define a macro, you request concatenation with the special operator `##' in the macro body. When the macro is called, after actual arguments are substituted, all `##' operators are deleted, and so is any whitespace next to them (including whitespace that was part of an actual argument). The result is to concatenate the syntactic tokens on either side of the `##'. Consider a C program that interprets named commands. There probably needs to be a table of commands, perhaps an array of structures declared as follows: struct command { char *name; void (*function) (); }; struct command commands[] = { { "quit", quit_command}, { "help", help_command}, ... }; It would be cleaner not to have to give each command name twice, once in the string constant and once in the function name. A macro which takes the name of a command as an argument can make this unnecessary. The string constant can be created with stringification, and the function name by concatenating the argument with `_command'. Here is how it is done: #define COMMAND(NAME) { #NAME, NAME ## _command } struct command commands[] = { COMMAND (quit), COMMAND (help), ... }; The usual case of concatenation is concatenating two names (or a name and a number) into a longer name. But this isn't the only valid case. It is also possible to concatenate two numbers (or a number and a name, such as `1.5' and `e3') into a number. Also, multi-character operators such as `+=' can be formed by concatenation. In some cases it is even possible to piece together a string constant. However, two pieces of text that don't together form a valid lexical unit cannot be concatenated. For example, concatenation with `x' on one side and `+' on the other is not meaningful because those two characters can't fit together in any lexical unit of C. The ANSI standard says that such attempts at concatenation are undefined, but in the GNU C preprocessor it is well defined: it puts the `x' and `+' side by side with no particular special results. Keep in mind that the C preprocessor converts comments to whitespace before macros are even considered. Therefore, you cannot create a comment by concatenating `/' and `*': the `/*' sequence that starts a comment is not a lexical unit, but rather the beginning of a ``long'' space character. Also, you can freely use comments next to a `##' in a macro definition, or in actual arguments that will be concatenated, because the comments will be converted to spaces at first sight, and concatenation will later discard the spaces. File: cpp.info, Node: Undefining, Next: Redefining, Prev: Concatenation, Up: Macros Undefining Macros ----------------- To "undefine" a macro means to cancel its definition. This is done with the `#undef' command. `#undef' is followed by the macro name to be undefined. Like definition, undefinition occurs at a specific point in the source file, and it applies starting from that point. The name ceases to be a macro name, and from that point on it is treated by the preprocessor as if it had never been a macro name. For example, #define FOO 4 x = FOO; #undef FOO x = FOO; expands into x = 4; x = FOO; In this example, `FOO' had better be a variable or function as well as (temporarily) a macro, in order for the result of the expansion to be valid C code. The same form of `#undef' command will cancel definitions with arguments or definitions that don't expect arguments. The `#undef' command has no effect when used on a name not currently defined as a macro. File: cpp.info, Node: Redefining, Next: Macro Pitfalls, Prev: Undefining, Up: Macros Redefining Macros ----------------- "Redefining" a macro means defining (with `#define') a name that is already defined as a macro. A redefinition is trivial if the new definition is transparently identical to the old one. You probably wouldn't deliberately write a trivial redefinition, but they can happen automatically when a header file is included more than once (*note Header Files::.), so they are accepted silently and without effect. Nontrivial redefinition is considered likely to be an error, so it provokes a warning message from the preprocessor. However, sometimes it is useful to change the definition of a macro in mid-compilation. You can inhibit the warning by undefining the macro with `#undef' before the second definition. In order for a redefinition to be trivial, the new definition must exactly match the one already in effect, with two possible exceptions: * Whitespace may be added or deleted at the beginning or the end. * Whitespace may be changed in the middle (but not inside strings). However, it may not be eliminated entirely, and it may not be added where there was no whitespace at all. Recall that a comment counts as whitespace. File: cpp.info, Node: Macro Pitfalls, Prev: Redefining, Up: Macros Pitfalls and Subtleties of Macros --------------------------------- In this section we describe some special rules that apply to macros and macro expansion, and point out certain cases in which the rules have counterintuitive consequences that you must watch out for. * Menu: * Misnesting:: Macros can contain unmatched parentheses. * Macro Parentheses:: Why apparently superfluous parentheses may be necessary to avoid incorrect grouping. * Swallow Semicolon:: Macros that look like functions but expand into compound statements. * Side Effects:: Unsafe macros that cause trouble when arguments contain side effects. * Self-Reference:: Macros whose definitions use the macros' own names. * Argument Prescan:: Actual arguments are checked for macro calls before they are substituted. * Cascaded Macros:: Macros whose definitions use other macros. File: cpp.info, Node: Misnesting, Next: Macro Parentheses, Prev: Macro Pitfalls, Up: Macro Pitfalls Improperly Nested Constructs ............................ Recall that when a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls. It is possible to piece together a macro call coming partially from the macro body and partially from the actual arguments. For example, #define double(x) (2*(x)) #define call_with_1(x) x(1) would expand `call_with_1 (double)' into `(2*(1))'. Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example, #define strange(file) fprintf (file, "%s %d", ... strange(stderr) p, 35) This bizarre example expands to `fprintf (stderr, "%s %d", p, 35)'! File: cpp.info, Node: Macro Parentheses, Next: Swallow Semicolon, Prev: Misnesting, Up: Macro Pitfalls Unintended Grouping of Arithmetic ................................. You may have noticed that in most of the macro definition examples shown above, each occurrence of a macro argument name had parentheses around it. In addition, another pair of parentheses usually surround the entire macro definition. Here is why it is best to write macros that way. Suppose you define a macro as follows, #define ceil_div(x, y) (x + y - 1) / y whose purpose is to divide, rounding up. (One use for this operation is to compute how many `int''s are needed to hold a certain number of `char''s.) Then suppose it is used as follows: a = ceil_div (b & c, sizeof (int)); This expands into a = (b & c + sizeof (int) - 1) / sizeof (int); which does not do what is intended. The operator-precedence rules of C make it equivalent to this: a = (b & (c + sizeof (int) - 1)) / sizeof (int); But what we want is this: a = ((b & c) + sizeof (int) - 1)) / sizeof (int); Defining the macro as #define ceil_div(x, y) ((x) + (y) - 1) / (y) provides the desired result. However, unintended grouping can result in another way. Consider `sizeof ceil_div(1, 2)'. That has the appearance of a C expression that would compute the size of the type of `ceil_div (1, 2)', but in fact it means something very different. Here is what it expands to: sizeof ((1) + (2) - 1) / (2) This would take the size of an integer and divide it by two. The precedence rules have put the division outside the `sizeof' when it was intended to be inside. Parentheses around the entire macro definition can prevent such problems. Here, then, is the recommended way to define `ceil_div': #define ceil_div(x, y) (((x) + (y) - 1) / (y)) File: cpp.info, Node: Swallow Semicolon, Next: Side Effects, Prev: Macro Parentheses, Up: Macro Pitfalls Swallowing the Semicolon ........................ Often it is desirable to define a macro that expands into a compound statement. Consider, for example, the following macro, that advances a pointer (the argument `p' says where to find it) across whitespace characters: #define SKIP_SPACES (p, limit) \ { register char *lim = (limit); \ while (p != lim) { \ if (*p++ != ' ') { \ p--; break; }}} Here Backslash-Newline is used to split the macro definition, which must be a single line, so that it resembles the way such C code would be laid out if not part of a macro definition. A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly speaking, the call expands to a compound statement, which is a complete statement with no need for a semicolon to end it. But it looks like a function call. So it minimizes confusion if you can use it like a function call, writing a semicolon afterward, as in `SKIP_SPACES (p, lim);' But this can cause trouble before `else' statements, because the semicolon is actually a null statement. Suppose you write if (*p != 0) SKIP_SPACES (p, lim); else ... The presence of two statements--the compound statement and a null statement--in between the `if' condition and the `else' makes invalid C code. The definition of the macro `SKIP_SPACES' can be altered to solve this problem, using a `do ... while' statement. Here is how: #define SKIP_SPACES (p, limit) \ do { register char *lim = (limit); \ while (p != lim) { \ if (*p++ != ' ') { \ p--; break; }}} \ while (0) Now `SKIP_SPACES (p, lim);' expands into do {...} while (0); which is one statement. File: cpp.info, Node: Side Effects, Next: Self-Reference, Prev: Swallow Semicolon, Up: Macro Pitfalls Duplication of Side Effects ........................... Many C programs define a macro `min', for ``minimum'', like this: #define min(X, Y) ((X) < (Y) ? (X) : (Y)) When you use this macro with an argument containing a side effect, as shown here, next = min (x + y, foo (z)); it expands as follows: next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); where `x + y' has been substituted for `X' and `foo (z)' for `Y'. The function `foo' is used only once in the statement as it appears in the program, but the expression `foo (z)' has been substituted twice into the macro expansion. As a result, `foo' might be called two times when the statement is executed. If it has side effects or if it takes a long time to compute, the results might not be what you intended. We say that `min' is an "unsafe" macro. The best solution to this problem is to define `min' in a way that computes the value of `foo (z)' only once. The C language offers no standard way to do this, but it can be done with GNU C extensions as follows: #define min(X, Y) \ ({ typeof (X) __x = (X), __y = (Y); \ (__x < __y) ? __x : __y; }) If you do not wish to use GNU C extensions, the only solution is to be careful when *using* the macro `min'. For example, you can calculate the value of `foo (z)', save it in a variable, and use that variable in `min': #define min(X, Y) ((X) < (Y) ? (X) : (Y)) ... { int tem = foo (z); next = min (x + y, tem); } (where I assume that `foo' returns type `int').