V10/cmd/gcc/cpp-1
File: cpp, 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 `-T', `-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, 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 `-T' option to enable
trigraph processing. *note Invocation::.
File: cpp, 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, 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'.
File: cpp, 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, 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 tried first because it is presumed to be the location of
the files of the program being compiled. (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, Node: Include Operation, 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, 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, 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, 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 does not
change what you have written. 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, 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, 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.
`__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, 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. However,
the GNU C preprocessor would be useless if it did not predefine the same
names that are normally predefined on the system and machine you are using.
Even system header files check the predefined names and will generate
incorrect declarations if they do not find the names that are expected.
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, the
definition
#define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
is used.
The `-ansi' option which requests complete support for ANSI C inhibits the
definition of these predefined symbols.
File: cpp, 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, 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, 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, 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 reefinition 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, 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, 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, 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, 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 layed 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, 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 way 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').
File: cpp, Node: Self-Reference, Next: Argument Prescan, Prev: Side Effects, Up: Macro Pitfalls
Self-Referential Macros
.......................
A "self-referential" 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)'. Therefore, this macro definition has the
possibly useful effect of causing the program to add 4 to the value of
`foo' wherever `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 a the
identifier `foo' in the program and think its value should be that of the
variable `foo', whereas in fact the value is four greater.
The special rule for self-reference applies also to "indirect"
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 definition 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
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 useful, but it is
specified by the ANSI C standard, so you need to understand it.