Untitled Document

The C Preprocessor

First Edition

April 1989

Richard M. Stallman

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 transla- tions of this manual into another language, under the above conditions for modified versions.

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The C preprocessor is a _m_a_c_r_o _p_r_o_c_e_s_s_o_r 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 _m_a_c_r_o_s, 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 de- clarations that can be substituted into your pro- gram.

Macro expansion. You can define _m_a_c_r_o_s, which are abbreviations for arbitrary fragments of C code, and then the C preprocessor will replace the mac- ros with their definitions throughout the program.

Conditional compilation. Using special preproces- sor 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 incom- patibility 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 Stan- dard C make it undesirable to do this. See section Invoca- tion.

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Most C preprocessor features are inactive unless you give specific commands to request their use. (Preprocessor commands are lines starting with ‘#’; see section Com- mands). But there are three transformations that the preprocessor always makes on all the input it receives, even in the absence of commands.

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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 ex- pansions (see section Predefined).

The first two transformations are done _b_e_f_o_r_e nearly all other parsing and before preprocessor commands are recognized. Thus, for example, you can split a line cosmet- ically with Backslash-Newline anywhere (except when tri- graphs 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 charac- ter 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 transforma- tions.

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 ex-

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ception, 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 in- side, 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. See section Invocation.

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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 _c_o_m_m_a_n_d _n_a_m_e. 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 can- not define new preprocessor commands.

Some command names require arguments; these make up the rest of the command line and must be separated from the com- mand 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 signifi- cant 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.

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A header file is a file containing C declarations and macro definitions (see section 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’.

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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 in- terfaces 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’.

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Both user and system header files are included using the preprocessor command ‘#include’. It has three variants:

#include <_f_i_l_e> This variant is used for system header files. It searches for a file named _f_i_l_e in a list of direc- tories specified by you, then in a standard list of system directories. You specify directories to search for header files with the command option ‘-I’ (see section Invocation). The option ‘- nostdinc’ inhibits searching the standard system

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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 un- likely to exist on Unix, where shell wildcard features would make it hard to manipulate.

The argument _f_i_l_e may not contain a ‘>’ character. It may, however, contain a ‘<’ character.

#include "_f_i_l_e" This variant is used for header files of your own program. It searches for a file named _f_i_l_e first in the current directory, then in the same direc- tories 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 in- put file refers to. (If the ‘-I-’ option is used, the special treatment of the current directory is inhibited.)

The argument _f_i_l_e may not contain ‘"’ characters. If backslashes occur within _f_i_l_e, they are con- sidered ordinary text characters, not escape char- acters. None of the character escape sequences appropriate to string constants in C are pro- cessed. 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 _a_n_y_t_h_i_n_g _e_l_s_e This variant is called a _c_o_m_p_u_t_e_d #_i_n_c_l_u_d_e. Any ‘#include’ command whose argument does not fit the above two forms is a computed include. The text _a_n_y_t_h_i_n_g _e_l_s_e is checked for macro calls, which are expanded (see section 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 al- low 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.

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The ‘#include’ command works by directing the C prepro- cessor 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 con- stant 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

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included file lacks a final newline.

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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 waste- ful. 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__

_t_h_e _e_n_t_i_r_e _f_i_l_e

#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 _i_n _a_d_d_i_t_i_o_n _t_o _t_h_e _c_o_n_d_i_- _t_i_o_n_a_l_s _d_e_s_c_r_i_b_e_d _a_b_o_v_e:

#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 _r_e_l_y on ‘#pragma once’ to prevent multi- ple inclusion of the file. It is just a hint, and a non- standard 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

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name, even in GNU CC, despite ‘#pragma once’.

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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.

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A _s_i_m_p_l_e _m_a_c_r_o is a kind of abbreviation. It is a name which stands for a fragment of code.

Before you can use a macro, you must _d_e_f_i_n_e it expli- citly 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 _e_x_p_a_n_d 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 charac- ter 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

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to contain an unbalanced quote character; the definition automatically extends to include the matching quote charac- ter 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 _u_s_e ‘TABLESIZE’ that the result of its

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expansion is checked for more macro names. See section Cas- caded Macros.

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A simple macro always stands for exactly the same text, each time it is used. Macros can be more flexible when they accept _a_r_g_u_m_e_n_t_s. 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 _a_r_g_u_m_e_n_t _n_a_m_e_s 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. See section Side Effects, for more information.)

To use a macro that expects arguments, you write the name of the macro followed by a list of _a_c_t_u_a_l _a_r_g_u_m_e_n_t_s 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 argu- ments 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))

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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 contin- ues. 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 func- tion 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’.

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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 _u_s_e_s of a macro with arguments can have spaces before the left parenthesis; it’s the _d_e_f_i_n_i_t_i_o_n where it matters whether there is a space.

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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.

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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 exten- sions.

__FILE__ This macro expands to the name of the current in- put file, in the form of a C string constant.

__BASE_FILE__ This macro expands to the name of the main input

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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 con- tained 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 pro- cessing moves to the line after the ‘#include’).

The expansions of both ‘__FILE__’ and ‘__LINE__’ are altered if a ‘#line’ command is used. See section 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 sig-

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nify that this is ANSI Standard C. (Whether that is actually true depends on what C com- piler 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 unde- fined.

__STRICT_ANSI__ This macro is defined if and only if the ‘- ansi’ switch was specified when GNU C was in- voked. Its definition is the null string. This macro exists primarily to direct certain GNU header files not to define certain tradi- tional 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 incor- porate it into a string constant.

__OPTIMIZE__ This macro is defined in optimizing compila- tions. 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 prac- tice to refer to this macro yourself; instead, refer to the standard macros defined in ‘limit.h’.

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The C preprocessor normally has several predefined mac- ros 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 operat- ing system in use, with more or less specificity. For exam- ple,

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 mak- ers 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 comput- ers.

pyr ‘pyr’ is predefined on all models of Pyramid com- puters.

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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’ inhi- bits 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 sup- plied 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 com- pile 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 pur- pose, 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"

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_S_t_r_i_n_g_i_f_i_c_a_t_i_o_n 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 argu- ment when it is substituted at that point in the definition.

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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 (_a_r_g);’, which the resemblance of ‘WARN_IF’ to a function would make C programmers want to do; see section 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 con- stants 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 double- quote 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"’.

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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 sin- gle space in the stringified result.

11..44..55.. CCoonnccaatteennaattiioonn

_C_o_n_c_a_t_e_n_a_t_i_o_n means joining two strings into one. In the context of macro expansion, concatenation refers to joining two lexical units into one longer one. Specifi- cally, 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 func- tion, 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:

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#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 concaten- ate 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 lexi- cal 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 charac- ter. Also, you can freely use comments next to a ‘##’ in a macro definition, or in actual arguments that will be con- catenated, because the comments will be converted to spaces at first sight, and concatenation will later discard the spaces.

11..44..66.. UUnnddeeffiinniinngg MMaaccrrooss

To _u_n_d_e_f_i_n_e a macro means to cancel its definition. This is done with the ‘#undef’ command. ‘#undef’ is fol- lowed 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,

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#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 defini- tions with arguments or definitions that don’t expect argu- ments. The ‘#undef’ command has no effect when used on a name not currently defined as a macro.

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_R_e_d_e_f_i_n_i_n_g 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 (see section 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 preproces- sor. However, sometimes it is useful to change the defini- tion 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 begin- ning or the end.

Whitespace may be changed in the middle (but not inside strings). However, it may not be eliminat-

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ed entirely, and it may not be added where there was no whitespace at all.

Recall that a comment counts as whitespace.

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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.

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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)’!

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11..44..88..22.. UUnniinntteennddeedd GGrroouuppiinngg ooff AArriitthhmmeettiicc

You may have noticed that in most of the macro defini- tion examples shown above, each occurrence of a macro argu- ment name had parentheses around it. In addition, another pair of parentheses usually surround the entire macro defin- ition. 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)

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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 dif- ferent. 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))

11..44..88..33.. SSwwaalllloowwiinngg tthhee SSeemmiiccoolloonn

Often it is desirable to define a macro that expands into a compound statement. Consider, for example, the fol- lowing 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 defini- tion, 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 minim- izes confusion if you can use it like a function call, writ- ing a semicolon afterward, as in ‘SKIP_SPACES (p, lim);’

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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’ state- ment. 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.

11..44..88..44.. DDuupplliiccaattiioonn ooff SSiiddee EEffffeeccttss

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));

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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 _u_n_s_a_f_e 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 _u_s_i_n_g 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’).

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11..44..88..55.. SSeellff--RReeffeerreennttiiaall MMaaccrrooss

A _s_e_l_f-_r_e_f_e_r_e_n_t_i_a_l macro is one whose name appears in its definition. A special feature of ANSI Standard C is that the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged.

Let’s consider an example:

#define foo (4 + foo)

where ‘foo’ is also a variable in your program.

Following the ordinary rules, each reference to ‘foo’ will expand into ‘(4 + foo)’; then this will be rescanned and will expand into ‘(4 + (4 + foo))’; and so on until it causes a fatal error (memory full) in the preprocessor.

However, the special rule about self-reference cuts this process short after one step, at ‘(4 + foo)’. There- fore, this macro definition has the possibly useful effect of causing the program to add 4 to the value of ‘foo’ wher- ever ‘foo’ is referred to.

In most cases, it is a bad idea to take advantage of this feature. A person reading the program who sees that ‘foo’ is a variable will not expect that it is a macro as well. The reader will come across the identifier ‘foo’ in the program and think its value should be that of the vari- able ‘foo’, whereas in fact the value is four greater.

The special rule for self-reference applies also to _i_n_d_i_r_e_c_t self-reference. This is the case where a macro _x expands to use a macro ‘y’, and ‘y”s expansion refers to the macro ‘x’. The resulting reference to ‘x’ comes indirectly from the expansion of ‘x’, so it is a self- reference and is not further expanded. Thus, after

#define x (4 + y) #define y (2 * x)

‘x’ would expand into ‘(4 + (2 * x))’. Clear?

But suppose ‘y’ is used elsewhere, not from the defini- tion of ‘x’. Then the use of ‘x’ in the expansion of ‘y’ is not a self-reference because ‘x’ is not “in progress”. So it does expand. However, the expansion of ‘x’ contains a reference to ‘y’, and that is an indirect self-reference now

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because ‘y’ is “in progress”. The result is that ‘y’ expands to ‘(2 * (4 + y))’.

It is not clear that this behavior would ever be use- ful, but it is specified by the ANSI C standard, so you need to understand it.

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We have explained that the expansion of a macro, including the substituted actual arguments, is scanned over again for macro calls to be expanded.

What really happens is more subtle: first each actual argument text is scanned separately for macro calls. Then the results of this are substituted into the macro body to produce the macro expansion, and the macro expansion is scanned again for macros to expand.

The result is that the actual arguments are scanned _t_w_i_c_e to expand macro calls in them.

Most of the time, this has no effect. If the actual argument contained any macro calls, they are expanded during the first scan. The result therefore contains no macro calls, so the second scan does not change it. If the actual argument were substituted as given, with no prescan, the single remaining scan would find the same macro calls and produce the same results.

You might expect the double scan to change the results when a self-referential macro is used in an actual argument of another macro (see section Self-Reference): the self- referential macro would be expanded once in the first scan, and a second time in the second scan. But this is not what happens. The self-references that do not expand in the first scan are marked so that they will not expand in the second scan either.

The prescan is not done when an argument is stringified or concatenated. Thus,

#define str(s) #s #define foo 4 str (foo)

expands to ‘"foo"’. Once more, prescan has been prevented from having any noticeable effect.

More precisely, stringification and concatenation use the argument as written, in un-prescanned form. The same

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actual argument would be used in prescanned form if it is substituted elsewhere without stringification or concatena- tion.

#define str(s) #s lose(s) #define foo 4 str (foo)

expands to ‘"foo" lose(4)’.

You might now ask, “Why mention the prescan, if it makes no difference? And why not skip it and make the preprocessor faster?” The answer is that the prescan does make a difference in three special cases:

Nested calls to a macro.

Macros that call other macros that stringify or concatenate.

Macros whose expansions contain unshielded commas.

We say that _n_e_s_t_e_d calls to a macro occur when a macro’s actual argument contains a call to that very macro. For example, if ‘f’ is a macro that expects one argument, ‘f (f (1))’ is a nested pair of calls to ‘f’. The desired expansion is made by expanding ‘f (1)’ and substituting that into the definition of ‘f’. The prescan causes the expected result to happen. Without the prescan, ‘f (1)’ itself would be substituted as an actual argument, and the inner use of ‘f’ would appear during the main scan as an indirect self- reference and would not be expanded. Here, the prescan can- cels an undesirable side effect (in the medical, not compu- tational, sense of the term) of the special rule for self- referential macros.

But prescan causes trouble in certain other cases of nested macro calls. Here is an example:

#define foo a,b #define bar(x) lose(x) #define lose(x) (1 + (x))

bar(foo)

We would like ‘bar(foo)’ to turn into ‘(1 + (foo))’, which would then turn into ‘(1 + (a,b))’. But instead, ‘bar(foo)’

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expands into ‘lose(a,b)’, and you get an error because lose requires a single argument. In this case, the problem is easily solved by the same parentheses that ought to be used to prevent misnesting of arithmetic operations:

#define foo (a,b) #define bar(x) lose((x))

The problem is more serious when the operands of the macro are not expressions; for example, when they are state- ments. Then parentheses are unacceptable because they would make for invalid C code:

#define foo { int a, b; ...

In GNU C you can shield the commas using the ‘({...)’ con- struct which turns a compound statement into an expression:

#define foo ({ int a, b; ... )

Or you can rewrite the macro definition to avoid such commas:

#define foo { int a; int b; ...

There is also one case where prescan is useful. It is possible to use prescan to expand an argument and then stringify it—if you use two levels of macros. Let’s add a new macro ‘xstr’ to the example shown above:

#define xstr(s) str(s) #define str(s) #s #define foo 4 xstr (foo)

This expands into ‘"4"’, not ‘"foo"’. The reason for the difference is that the argument of ‘xstr’ is expanded at prescan (because ‘xstr’ does not specify stringification or concatenation of the argument). The result of prescan then

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forms the actual argument for ‘str’. ‘str’ uses its argu- ment without prescan because it performs stringification; but it cannot prevent or undo the prescanning already done by ‘xstr’.

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A _c_a_s_c_a_d_e of macros is when one macro’s body contains a reference to another macro. This is very common practice. For example,

#define BUFSIZE 1020 #define TABLESIZE BUFSIZE

It’s only when you _u_s_e ‘TABLESIZE’ that the result of its expansion is checked for more macro names.

This makes a difference if you change the definition of ‘BUFSIZE’ at some point in the source file. ‘TABLESIZE’, defined as shown, will always expand using the definition of ‘BUFSIZE’ that is currently in effect:

#define BUFSIZE 1020 #define TABLESIZE BUFSIZE #undef BUFSIZE #define BUFSIZE 37

Now ‘TABLESIZE’ expands (in two stages) to ‘37’.

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In a macro processor, a _c_o_n_d_i_t_i_o_n_a_l is a command that allows a part of the program to be ignored during compila- tion, on some conditions. In the C preprocessor, a condi- tional can test either an arithmetic expression or whether a name is defined as a macro.

A conditional in the C preprocessor resembles in some ways an ‘if’ statement in C, but it is important to under- stand the difference between them. The condition in an ‘if’ statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on.

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The condition in a preprocessor conditional command is tested when your program is compiled. Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation.

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Generally there are three kinds of reason to use a con- ditional.

A program may need to use different code depending on the machine or operating system it is to run on. In some cases the code for one operating sys- tem may be erroneous on another operating system; for example, it might refer to library routines that do not exist on the other system. When this happens, it is not enough to avoid executing the invalid code: merely having it in the program makes it impossible to link the program and run it. With a preprocessor conditional, the offend- ing code can be effectively excised from the pro- gram when it is not valid.

You may want to be able to compile the same source file into two different programs. Sometimes the difference between the programs is that one makes frequent time-consuming consistency checks on its intermediate data while the other does not.

A conditional whose condition is always false is a good way to exclude code from the program but keep it as a sort of comment for future reference.

Most simple programs that are intended to run on only one machine will not need to use preprocessor conditionals.

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A conditional in the C preprocessor begins with a _c_o_n_- _d_i_t_i_o_n_a_l _c_o_m_m_a_n_d: ‘#if’, ‘#ifdef’ or ‘#ifndef’. See section Conditionals-Macros, for info on ‘#ifdef’ and ‘#ifndef’; only ‘#if’ is explained here.

11..55..22..11.. TThhee ‘#if’ CCoommmmaanndd

The ‘#if’ command in its simplest form consists of

#if _e_x_p_r_e_s_s_i_o_n _c_o_n_t_r_o_l_l_e_d _t_e_x_t #endif /* _e_x_p_r_e_s_s_i_o_n */

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The comment following the ‘#endif’ is not required, but it is a good practice because it helps people match the ‘#endif’ to the corresponding ‘#if’. Such comments should always be used, except in short conditionals that are not nested. In fact, you can put anything at all after the ‘#endif’ and it will be ignored by the GNU C preprocessor, but only comments are acceptable in ANSI Standard C.

_e_x_p_r_e_s_s_i_o_n is a C expression of integer type, subject to stringent restrictions. It may contain

Integer constants, which are all regarded as long or unsigned long.

Character constants, which are interpreted accord- ing to the character set and conventions of the machine and operating system on which the prepro- cessor is running. The GNU C preprocessor uses the C data type ‘char’ for these character con- stants; therefore, whether some character codes are negative is determined by the C compiler used to compile the preprocessor. If it treats ‘char’ as signed, then character codes large enough to set the sign bit will be considered negative; oth- erwise, no character code is considered negative.

Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and ‘&&’ and ‘||’.

Identifiers that are not macros, which are all treated as zero(!).

Macro calls. All macro calls in the expression are expanded before actual computation of the expression’s value begins.

Note that ‘sizeof’ operators and enum-type values are not allowed. enum-type values, like all other identifiers that are not taken as macro calls and expanded, are treated as zero.

The text inside of a conditional can include preproces- sor commands. Then the commands inside the conditional are obeyed only if that branch of the conditional succeeds. The text can also contain other conditional groups. However, the ‘#if”s and ‘#endif”s must balance.

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11..55..22..22.. TThhee ‘#else’ CCoommmmaanndd

The ‘#else’ command can be added to a conditional to provide alternative text to be used if the condition is false. This looks like

#if _e_x_p_r_e_s_s_i_o_n _t_e_x_t-_i_f-_t_r_u_e #else /* Not _e_x_p_r_e_s_s_i_o_n */ _t_e_x_t-_i_f-_f_a_l_s_e #endif /* Not _e_x_p_r_e_s_s_i_o_n */

If _e_x_p_r_e_s_s_i_o_n is nonzero, and the _t_e_x_t-_i_f-_t_r_u_e is con- sidered included, then ‘#else’ acts like a failing condi- tional and the _t_e_x_t-_i_f-_f_a_l_s_e is ignored. Contrariwise, if the ‘#if’ conditional fails, the _t_e_x_t-_i_f-_f_a_l_s_e is considered included.

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One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have

#if X == 1 ... #else /* X != 1 */ #if X == 2 ... #else /* X != 2 */ ... #endif /* X != 2 */ #endif /* X != 1 */

Another conditional command, ‘#elif’, allows this to be abbreviated as follows:

#if X == 1 ... #elif X == 2 ... #else /* X != 2 and X != 1*/ ... #endif /* X != 2 and X != 1*/

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‘#elif’ stands for “else if”. Like ‘#else’, it goes in the middle of a ‘#if’-‘#endif’ pair and subdivides it; it does not require a matching ‘#endif’ of its own. Like ‘#if’, the ‘#elif’ command includes an expression to be tested.

The text following the ‘#elif’ is processed only if the original ‘#if’-condition failed and the ‘#elif’ condition succeeeds. More than one ‘#elif’ can go in the same ‘#if’- ‘#endif’ group. Then the text after each ‘#elif’ is pro- cessed only if the ‘#elif’ condition succeeds after the ori- ginal ‘#if’ and any previous ‘#elif”s within it have failed. ‘#else’ is equivalent to ‘#elif 1’, and ‘#else’ is allowed after any number of ‘#elif”s, but ‘#elif’ may not follow a ‘#else’.

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If you replace or delete a part of the program but want to keep the old code around as a comment for future refer- ence, the easy way to do this is to put ‘#if 0’ before it and ‘#endif’ after it.

This works even if the code being turned off contains conditionals, but they must be entire conditionals (balanced ‘#if’ and ‘#endif’).

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Conditionals are rarely useful except in connection with macros. A ‘#if’ command whose expression uses no mac- ros is equivalent to ‘#if 1’ or ‘#if 0’; you might as well determine which one, by computing the value of the expres- sion yourself, and then simplify the program. But when the expression uses macros, its value can vary from compilation to compilation.

For example, here is a conditional that tests the expression ‘BUFSIZE == 1020’, where ‘BUFSIZE’ must be a macro.

#if BUFSIZE == 1020 printf ("Large buffers!\n"); #endif /* BUFSIZE is large */

The special operator ‘defined’ may be used in ‘#if’ expressions to test whether a certain name is defined as a macro. Either ‘defined _n_a_m_e’ or ‘defined (_n_a_m_e)’ is an expression whose value is 1 if _n_a_m_e is defined as macro at the current point in the program, and 0 otherwise. For the ‘defined’ operator it makes no difference what the

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definition of the macro is; all that matters is whether there is a definition. Thus, for example,

#if defined (vax) || defined (ns16000)

would include the following code if either of the names ‘vax’ and ‘ns16000’ is defined as a macro.

If a macro is defined and later undefined with ‘#undef’, subsequent use of the ‘defined’ operator will return 0, because the name is no longer defined. If the macro is defined again with another ‘#define’, ‘defined’ will recommence returning 1.

Conditionals that test just the definedness of one name are very common, so there are two special short conditional commands for this case. They are

#ifdef _n_a_m_e is equivalent to ‘#if defined (_n_a_m_e)’.

#ifndef _n_a_m_e is equivalent to ‘#if ! defined (_n_a_m_e)’.

Macro definitions can vary between compilations for several reasons.

Some macros are predefined on each kind of machine. For example, on a Vax, the name ‘vax’ is a predefined macro. On other machines, it would not be defined.

Many more macros are defined by system header files. Different systems and machines define dif- ferent macros, or give them different values. It is useful to test these macros with conditionals to avoid using a system feature on a machine where it is not implemented.

Macros are a common way of allowing users to cus- tomize a program for different machines or appli- cations. For example, the macro ‘BUFSIZE’ might be defined in a configuration file for your pro- gram that is included as a header file in each source file. You would use ‘BUFSIZE’ in a prepro- cessor conditional in order to generate different code depending on the chosen configuration.

Macros can be defined or undefined with ‘-D’ and ‘-U’ command options when you compile the program.

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You can arrange to compile the same source file into two different programs by choosing a macro name to specify which program you want, writing conditionals to test whether or how this macro is defined, and then controlling the state of the macro with compiler command options. See section Invocation.

11..55..55.. TThhee ‘#error’ CCoommmmaanndd

The command ‘#error’ causes the preprocessor to report a fatal error. The rest of the line that follows ‘#error’ is used as the error message.

You would use ‘#error’ inside of a conditional that detects a combination of parameters which you know the pro- gram does not properly support. For example, if you know that the program will not run properly on a Vax, you might write

#ifdef vax #error Won’t work on Vaxen. See comments at get_last_object. #endif

See section Nonstandard Predefined, for why this works.

If you have several configuration parameters that must be set up by the installation in a consistent way, you can use conditionals to detect an inconsistency and report it with ‘#error’. For example,

#if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \ || HASH_TABLE_SIZE % 5 == 0 #error HASH_TABLE_SIZE should not be divisible by a small prime #endif

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One of the jobs of the C preprocessor is to inform the C compiler of where each line of C code came from: which source file and which line number.

C code can come from multiple source files if you use ‘#include’; both ‘#include’ and the use of conditionals and macros can cause the line number of a line in the preproces- sor output to be different from the line’s number in the original source file. You will appreciate the value of

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making both the C compiler (in error messages) and symbolic debuggers such as GDB use the line numbers in your source file.

The C preprocessor builds on this feature by offering a command by which you can control the feature explicitly. This is useful when a file for input to the C preprocessor is the output from another program such as the bison parser generator, which operates on another file that is the true source file. Parts of the output from bison are generated from scratch, other parts come from a standard parser file. The rest are copied nearly verbatim from the source file, but their line numbers in the bison output are not the same as their original line numbers. Naturally you would like compiler error messages and symbolic debuggers to know the original source file and line number of each line in the bison output.

bison arranges this by writing ‘#line’ commands into the output file. ‘#line’ is a command that specifies the original line number and source file name for subsequent input in the current preprocessor input file. ‘#line’ has three variants:

#line _l_i_n_e_n_u_m Here _l_i_n_e_n_u_m is a decimal integer constant. This specifies that the line number of the following line of input, in its original source file, was _l_i_n_e_n_u_m.

#line _l_i_n_e_n_u_m _f_i_l_e_n_a_m_e Here _l_i_n_e_n_u_m is a decimal integer constant and _f_i_l_e_n_a_m_e is a string constant. This specifies that the following line of input came originally from source file _f_i_l_e_n_a_m_e and its line number there was _l_i_n_e_n_u_m. Keep in mind that _f_i_l_e_n_a_m_e is not just a file name; it is surrounded by double- quote characters so that it looks like a string constant.

#line _a_n_y_t_h_i_n_g _e_l_s_e _a_n_y_t_h_i_n_g _e_l_s_e is checked for macro calls, which are expanded. The result should be a decimal in- teger constant followed optionally by a string constant, as described above.

‘#line’ commands alter the results of the ‘__FILE__’ and ‘__LINE__’ predefined macros from that point on. See section Standard Predefined.

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This section describes three additional preprocessor commands. They are not very useful, but are mentioned for completeness.

The _n_u_l_l _c_o_m_m_a_n_d consists of a ‘#’ followed by a New- line, with only whitespace (including comments) in between. A null command is understood as a preprocessor command but has no effect on the preprocessor output. The primary sig- nificance of the existence of the null command is that an input line consisting of just a ‘#’ will produce no output, rather than a line of output containing just a ‘#’. Sup- posedly some old C programs contain such lines.

The ‘#pragma’ command is specified in the ANSI standard to have an arbitrary implementation-defined effect. In the GNU C preprocessor, ‘#pragma’ commands are ignored, except for ‘#pragma once’ (see section Once-Only).

The ‘#ident’ command is supported for compatibility with certain other systems. It is followed by a line of text. On certain systems, the text is copied into a special place in the object file; on most systems, the text is ignored and this directive has no effect.

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The output from the C preprocessor looks much like the input, except that all preprocessor command lines have been replaced with blank lines and all comments with spaces. Whitespace within a line is not altered; however, a space is inserted after the expansions of most macro calls.

Source file name and line number information is con- veyed by lines of the form

# _l_i_n_e_n_u_m _f_i_l_e_n_a_m_e _f_l_a_g

which are inserted as needed into the middle of the input (but never within a string or character constant). Such a line means that the following line originated in file _f_i_l_e_n_a_m_e at line _l_i_n_e_n_u_m.

The third field, _f_l_a_g, may be a number, or may be absent. It is ‘1’ for the beginning of a new source file, and ‘2’ for return to an old source file at the end of an included file. It is absent otherwise.

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Most often when you use the C preprocessor you will not have to invoke it explicitly: the C compiler will do so automatically. However, the preprocessor is sometimes use- ful individually.

The C preprocessor expects two file names as arguments, _i_n_f_i_l_e and _o_u_t_f_i_l_e. The preprocessor reads _i_n_f_i_l_e together with any other files it specifies with ‘#include’. All the output generated by the combined input files is written in _o_u_t_f_i_l_e.

Either _i_n_f_i_l_e or _o_u_t_f_i_l_e may be ‘-’, which as _i_n_f_i_l_e means to read from standard input and as _o_u_t_f_i_l_e means to write to standard output. Also, if _o_u_t_f_i_l_e or both file names are omitted, the standard output and standard input are used for the omitted file names.

Here is a table of command options accepted by the C preprocessor. Most of them can also be given when compiling a C program; they are passed along automatically to the preprocessor when it is invoked by the compiler.

‘-P’ Inhibit generation of ‘#’-lines with line-number information in the output from the preprocessor (see section Output). This might be useful when running the preprocessor on something that is not C code and will be sent to a program which might be confused by the ‘#’-lines

‘-C’ Do not discard comments: pass them through to the output file. Comments appearing in arguments of a macro call will be copied to the output before the expansion of the macro call.

‘-trigraphs’ Process ANSI standard trigraph sequences. These are three-character sequences, all starting with ‘??’, that are defined by ANSI C to stand for sin- gle characters. For example, ‘??/’ stands for ‘\’, so ‘’??/n” is a character constant for a newline. Strictly speaking, the GNU C preproces- sor does not support all programs in ANSI Standard C unless ‘-trigraphs’ is used, but if you ever no- tice the difference it will be with relief.

You don’t want to know any more about trigraphs.

‘-pedantic’ Issue warnings required by the ANSI C standard in certain cases such as when text other than a com-

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ment follows ‘#else’ or ‘#endif’.

‘-I _d_i_r_e_c_t_o_r_y’ Add the directory _d_i_r_e_c_t_o_r_y to the end of the list of directories to be searched for header files (see section Include Syntax). This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file direc- tories. If you use more than one ‘-I’ option, the directories are scanned in left-to-right order; the standard system directories come after.

‘-I-’ Any directories specified with ‘-I’ options before the ‘-I-’ option are searched only for the case of ‘#include "_f_i_l_e"’; they are not searched for ‘#include <_f_i_l_e>’.

If additional directories are specified with ‘-I’ options after the ‘-I-’, these directories are searched for all ‘#include’ directives.

In addition, the ‘-I-’ option inhibits the use of the current directory as the first search directo- ry for ‘#include "_f_i_l_e"’. Therefore, the current directory is searched only if it is requested ex- plicitly with ‘-I.’. Specifying both ‘-I-’ and ‘-I.’ allows you to control precisely which direc- tories are searched before the current one and which are searched after.

‘-nostdinc’ Do not search the standard system directories for header files. Only the directories you have specified with ‘-I’ options (and the current directory, if appropriate) are searched.

‘-D _n_a_m_e’ Predefine _n_a_m_e as a macro, with definition ‘1’.

‘-D _n_a_m_e=_d_e_f_i_n_i_t_i_o_n’ Predefine _n_a_m_e as a macro, with definition _d_e_f_i_n_i_- _t_i_o_n. There are no restrictions on the contents of _d_e_f_i_n_i_t_i_o_n, but if you are invoking the prepro- cessor from a shell or shell-like program you may need to use the shell’s quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

‘-U _n_a_m_e’ Do not predefine _n_a_m_e. If both ‘-U’ and ‘-D’ are specified for one name, the ‘-U’ beats the ‘-D’ and the name is not predefined.

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‘-undef’ Do not predefine any nonstandard macros.

‘-d’ Instead of outputting the result of preprocessing, output a list of ‘#define’ commands for all the macros defined during the execution of the prepro- cessor.

‘-M’ Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The prepro- cessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files. If there are many included files then the rule is split into several lines using ‘\’-newline.

This feature is used in automatic updating of makefiles.

‘-MM’ Like ‘-M’ but mention only the files included with ‘#include "_f_i_l_e"’. System header files included with ‘#include <_f_i_l_e>’ are omitted.

‘-i _f_i_l_e’ Process _f_i_l_e as input, discarding the resulting output, before processing the regular input file. Because the output generated from _f_i_l_e is discard- ed, the only effect of ‘-i _f_i_l_e’ is to make the macros defined in _f_i_l_e available for use in the main input.

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1 The C Preprocessor ............................ 2 1.1 Transformations Made Globally ................ 2 1.2 Preprocessor Commands ........................ 4 1.3 Header Files ................................. 5 1.3.1 Uses of Header Files ......................... 5 1.3.2 The ‘#include’ CCoommmmaanndd ....................... 55 11..33..33 HHooww ‘#include’ WWoorrkkss ......................... 77 11..33..44 OOnnccee--OOnnllyy IInncclluuddee FFiilleess ...................... 88 11..44 MMaaccrrooss ....................................... 99 11..44..11 SSiimmppllee MMaaccrrooss ................................ 99 11..44..22 MMaaccrrooss wwiitthh AArrgguummeennttss ........................ 1111 11..44..33 PPrreeddeeffiinneedd MMaaccrrooss ............................ 1133 11..44..33..11 SSttaannddaarrdd PPrreeddeeffiinneedd MMaaccrrooss ................... 1133 11..44..33..22 NNoonnssttaannddaarrdd PPrreeddeeffiinneedd MMaaccrrooss ................ 1166 11..44..44 SSttrriinnggiiffiiccaattiioonn .............................. 1177 11..44..55 CCoonnccaatteennaattiioonn ................................ 1199 11..44..66 UUnnddeeffiinniinngg MMaaccrrooss ............................ 2200 11..44..77 RReeddeeffiinniinngg MMaaccrrooss ............................ 2211 11..44..88 PPiittffaallllss aanndd SSuubbttlleettiieess ooff MMaaccrrooss ............ 2222 11..44..88..11 IImmpprrooppeerrllyy NNeesstteedd CCoonnssttrruuccttss ................. 2222 11..44..88..22 UUnniinntteennddeedd GGrroouuppiinngg ooff AArriitthhmmeettiicc ............ 2233 11..44..88..33 SSwwaalllloowwiinngg tthhee SSeemmiiccoolloonn ..................... 2244 11..44..88..44 DDuupplliiccaattiioonn ooff SSiiddee EEffffeeccttss .................. 2255 11..44..88..55 SSeellff--RReeffeerreennttiiaall MMaaccrrooss ...................... 2277 11..44..88..66 SSeeppaarraattee EExxppaannssiioonn ooff MMaaccrroo AArrgguummeennttss ........ 2288 11..44..88..77 CCaassccaaddeedd UUssee ooff MMaaccrrooss ....................... 3311 11..55 CCoonnddiittiioonnaallss ................................. 3311 11..55..11 WWhhyy CCoonnddiittiioonnaallss aarree UUsseedd .................... 3322 11..55..22 SSyynnttaaxx ooff CCoonnddiittiioonnaallss ....................... 3322 11..55..22..11 TThhee ‘#if’ CCoommmmaanndd ............................ 3322 11..55..22..22 TThhee ‘#else’ CCoommmmaanndd .......................... 3344 11..55..22..33 TThhee ‘#elif’ CCoommmmaanndd .......................... 3344 11..55..33 KKeeeeppiinngg DDeelleetteedd CCooddee ffoorr FFuuttuurree RReeffeerreennccee .................................................. 3355 11..55..44 CCoonnddiittiioonnaallss aanndd MMaaccrrooss ...................... 3355 11..55..55 TThhee ‘#error’ CCoommmmaanndd ......................... 3377 11..66 CCoommbbiinniinngg SSoouurrccee FFiilleess ....................... 3377 11..77 MMiisscceellllaanneeoouuss PPrreepprroocceessssoorr CCoommmmaannddss .......... 3399 11..88 CC PPrreepprroocceessssoorr OOuuttppuutt ........................ 3399 11..99 IInnvvookkiinngg tthhee CC PPrreepprroocceessssoorr .................. 4400 CCoonncceepptt IInnddeexx ......................................... 4422 IInnddeexx ooff CCoommmmaannddss,, MMaaccrrooss aanndd OOppttiioonnss ................. 4422

c preprocessor commands 4 c commands 4 c header file 5 f system header files 5 f #include 5 c repeated inclu- sion 8 c predefined macros 13 f __FILE__ 13 f __BASE_FILE__ 13 f __LINE__ 14 f __DATE__ 14 f __TIME__ 14 f __STDC__ 14 f unix 16 f BSD 16 f vax 16 f mc68000 16 f m68k 16 f M68020 16 f ns32000 16 f sun 16 f pyr 16 f sequent 17 c stringification 17 c concatenation 19 c undefining macros 20 c redefining macros 21 c semicolons (after macro calls) 24 c side effects (in macro arguments) 25 c unsafe macros 25 c self-reference 27 c cascaded macros 31 c macro body uses macro 31 c conditionals 31 f #if 32 f #else 34 f #elif 34 f defined 35 f #ifdef 36 f #ifndef 36 f #error 37 c line control 37 f #line 37 f #pragma 39 f #ident 39 c null command 39 c output format 39 c options 40 f -P 40 f -C 40 f -trigraphs 40 f -pedan- tic 40 f -I 41 f -D 41 f -U 41 f -undef 42 f -d 42 f -M 42 f -MM 42 f -i 42

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