The C Preprocessor: 1. 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 preprocessing directives, you
can include or exclude parts of the program according to various
conditions. -
Line control. If you use a program to combine or rearrange source files into
an intermediate file which is then compiled, you can use line control
to inform the compiler of where each source line originally came from.
C preprocessors vary in some details. This manual discusses the GNU C preprocessor, the C Compatible Compiler Preprocessor. The GNU C preprocessor provides a superset of the features of ANSI Standard C.
ANSI Standard C requires the rejection of many harmless constructs commonly
used by today’s C programs. Such incompatibility would be inconvenient for
users, so the GNU C preprocessor is configured to accept these constructs
by default. Strictly speaking, to get ANSI Standard C, you must use the
options -trigraphs', -undef’ and `-pedantic’, but in
practice the consequences of having strict ANSI Standard C make it
undesirable to do this. See section 1.9 Invoking the C Preprocessor.
The C preprocessor is designed for C-like languages; you may run into
problems if you apply it to other kinds of languages, because it assumes
that it is dealing with C. For example, the C preprocessor sometimes
outputs extra white space to avoid inadvertent C token concatenation,
and this may cause problems with other languages.
1.1 Transformations Made Globally #
Most C preprocessor features are inactive unless you give specific directives
to request their use. (Preprocessing directives are lines starting with
`#’; see section 1.2 Preprocessing Directives). But there are three transformations that the
preprocessor always makes on all the input it receives, even in the absence
of directives.
-
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
(see section 1.4.3 Predefined Macros).
The first two transformations are done before nearly all other parsing
and before preprocessing directives are recognized. Thus, for example, you
can split a line cosmetically with Backslash-Newline anywhere (except
when trigraphs are in use; see below).
is equivalent into #define FOO 1020'. You can split even an escape sequence with Backslash-Newline. For example, you can split “foobar” between the’ and the `b’ to get
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' directive in which the file name is delimited with’ and `'. -
C comments and predefined macro names are never recognized within a character or string constant. (Strictly speaking, this is the rule,
not an exception, but it is worth noting here anyway.) -
Backslash-Newline may not safely be used within an ANSI “trigraph”.
Trigraphs are converted before Backslash-Newline is deleted. If you
write what looks like a trigraph with a Backslash-Newline inside, the
Backslash-Newline is deleted as usual, but it is then too late to
recognize the trigraph.
This exception is relevant only if you use the `-trigraphs’
option to enable trigraph processing. See section 1.9 Invoking the C Preprocessor.
1.2 Preprocessing Directives #
Most preprocessor features are active only if you use preprocessing directives to request their use.Preprocessing directives are lines in your program that start with #'. The #’ is followed by an identifier that is the directive name.
For example, #define' is the directive that defines a macro. Whitespace is also allowed before and after the #'.
The set of valid directive names is fixed. Programs cannot define new
preprocessing directives.
Some directive names require arguments; these make up the rest of the directive
line and must be separated from the directive name by whitespace. For example,
`#define’ must be followed by a macro name and the intended expansion
of the macro. See section 1.4.1 Simple Macros.
A preprocessing directive 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
directive into multiple lines, but the comments are changed to Spaces
before the directive is interpreted. The only way a significant Newline
can occur in a preprocessing directive 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 directive 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 preprocessing directive.
1.3 Header Files #
A header file is a file containing C declarations and macro definitions (see section 1.4 Macros) to be shared between several source files. You request the use of a header file in your program with the C preprocessing directive `#include'.1.3.1 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 and
declarations 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’. Avoid unusual characters in header file names, as they
reduce portability.
1.3.2 The `#include’ Directive #
Both user and system header files are included using the preprocessing directive `#include'. It has three variants:#include <var>file</var>
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' (see section 1.9 Invoking the C Preprocessor). 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 directive 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 "<var>file</var>"
This variant is used for header files of your own program. It
searches for a file named file first in the current directory,
then in the same directories used for system header files. The
current directory is the directory of the current input file. It is
tried first because it is presumed to be the location of the files
that the current input file refers to. (If the `-I-’ option is
used, the special treatment of the current directory is inhibited.)
The argument file may not contain "' characters. If backslashes occur within <var>file</var>, 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 “xn\y”’ specifies a filename containing three
backslashes. It is not clear why this behavior is ever useful, but
the ANSI standard specifies it.
#include <var>anything else</var>
This variant is called a computed #include. Any `#include’
directive 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 (see section 1.4 Macros). When this is done, the result
must fit one of the above two variants–in particular, the expanded
text must in the end be surrounded by either quotes or angle braces.
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-specific 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.
1.3.3 How `#include’ Works #
The #include' directive 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’
directive. For example, given a header file `header.h’ as follows,
and a main program called `program.c’ that uses the header file,
like this,
the output generated by the C preprocessor for `program.c’ as input
would be
Included files are not limited to declarations and macro definitions; those
are merely the typical uses. 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.
It is possible for a header file to begin or end a syntactic unit such
as a function definition, but that would be very confusing, so don’t do
it.
The line following the `#include’ directive is always treated as a
separate line by the C preprocessor even if the included file lacks a final
newline.
1.3.4 Once-Only Include Files #
Very often, one header file includes another. It can easily result that a
certain header file is included more than once. This may lead to errors,
if the header file defines structure types or typedefs, and is certainly
wasteful. Therefore, we often wish to prevent multiple inclusion of a
header file.
The standard way to do this is to enclose the entire real contents of the
file in a conditional, like this:
The macro FILE_FOO_SEEN indicates that the file has been included
once already. In a user header file, the macro name should not begin
with _'. In a system header file, this name should begin with __’ to avoid conflicts with user programs. In any kind of header
file, the macro name should contain the name of the file and some
additional text, to avoid conflicts with other header files.
The GNU C preprocessor is programmed to notice when a header file uses
this particular construct and handle it efficiently. If a header file
is contained entirely in a #ifndef' conditional, then it records that fact. If a subsequent #include’ specifies the same file,
and the macro in the `#ifndef’ is already defined, then the file
is entirely skipped, without even reading it.
#import' is obsolete because it is not a well designed feature. It requires the users of a header file--the applications programmers--to know that a certain header file should only be included once. It is much better for the header file's implementor to write the file so that users don't need to know this. Using #ifndef’
accomplishes this goal.
1.3.5 Inheritance and Header Files #
Inheritance is what happens when one object or file derives some
of its contents by virtual copying from another object or file. In
the case of C header files, inheritance means that one header file
includes another header file and then replaces or adds something.
If the inheriting header file and the base header file have different
names, then inheritance is straightforward: simply write `#include
“base”’ in the inheriting file.
Sometimes it is necessary to give the inheriting file the same name as
the base file. This is less straightforward.
For example, suppose an application program uses the system header
sys/signal.h', but the version of /usr/include/sys/signal.h’
on a particular system doesn’t do what the application program expects.
It might be convenient to define a “local” version, perhaps under the
name `/usr/local/include/sys/signal.h’, to override or add to the
one supplied by the system.
You can do this by compiling with the option -I.', and writing a file sys/signal.h’ that does what the application
program expects. But making this file include the standard
sys/signal.h' is not so easy--writing #include
sys/signal.h’ in that file doesn’t work, because it includes your own
version of the file, not the standard system version. Used in that file
itself, this leads to an infinite recursion and a fatal error in
compilation.
`#include /usr/include/sys/signal.h’ would find the proper file,
but that is not clean, since it makes an assumption about where the
system header file is found. This is bad for maintenance, since it
means that any change in where the system’s header files are kept
requires a change somewhere else.
Suppose you specify -I /usr/local/include', and the list of directories to search also includes /usr/include’; and suppose
both directories contain sys/signal.h'. Ordinary #include sys/signal.h’ finds the file under
/usr/local/include'. If that file contains #include_next
sys/signal.h’, it starts searching after that directory, and finds the
file in `/usr/include'.
1.4 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.
1.4.1 Simple Macros #
A simple macro is a kind of abbreviation. It is a name which
stands for a fragment of code. Some people refer to these as
manifest constants.
Before you can use a macro, you must define it explicitly with the
#define' directive. #define’ is followed by the name of the
macro and then the code it should be an abbreviation for. For example,
defines a macro named BUFFER_SIZE' as an abbreviation for the text 1020’. If somewhere after this `#define’ directive there comes
a C statement of the form
then the C preprocessor will recognize and expand the macro
`BUFFER_SIZE’, resulting in
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
preprocessing directives. (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. This is because 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. (But if it does not, you may 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
produces as output
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
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.
See section 1.4.8.7 Cascaded Use of Macros.
1.4.2 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. A macro that
accepts arguments is called a function-like macro because the
syntax for using it looks like a function call.
For example, here is a macro that computes the minimum of two numeric
values, as it is defined in many C programs:
(This is not the best way to define a “minimum” macro in GNU C.
See section 1.4.8.4 Duplication of 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
where 1' has been substituted for X’ and 2' for Y'.
Likewise, `min (x + 28, *p)’ expands into
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, and they do not prevent a comma from
separating arguments. Thus,
passes two arguments to macro: array[x = y' and x +
1]’. 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 this text:
(Line breaks shown here for clarity would not actually be generated.)
If a macro `foo` takes one argument, and you want to supply an empty argument, you must write at least some whitespace between the parentheses, like this: `foo ( )'. Just `foo ()' is providing no arguments, which is an error if `foo` expects an argument. But `foo0 ()' is the correct way to call a macro defined to take zero arguments, like this: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'.
You may not 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 line 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:
(which defines `FOO’ to take an argument and expand into minus the
reciprocal of that argument) or this:
(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.
1.4.3 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.1.4.3.1 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. The precise name returned is the one that was
specified in `#include’ or as the input file name argument.
__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,
A #include' directive 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’ directive, 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’ directive is used. See section 1.6 Combining Source Files.
__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 `“Feb 1 1996”'.
__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.)
On some hosts, system include files use a different convention, where
`STDC’ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. The preprocessor follows the host convention
when processing system include files, but when processing user files it follows
the usual GNU C convention.
This macro is not defined if the `-traditional’ option is used.
__STDC_VERSION__
This macro expands to the C Standard’s version number,
a long integer constant of the form <var>yyyy</var><var>mm</var>L' where <var>yyyy</var> and <var>mm</var> are the year and month of the Standard version. This signifies which version of the C Standard the preprocessor conforms to. Like STDC’, whether this version number is accurate
for the entire implementation depends on what C compiler
will operate on the output from the preprocessor.
This macro is not defined if the `-traditional’ option is used.
__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. The value identifies the major version number of GNU CC (1’ for GNU CC
version 1, which is now obsolete, and `2’ for version 2).
__GNUC_MINOR__
The macro contains the minor version number of the compiler. This can
be used to work around differences between different releases of the
compiler (for example, if gcc 2.6.3 is known to support a feature, you
can test for __GNUC__ 2 || (__GNUC__ == 2 __GNUC_MINOR__ = 6)).
The last number, `3’ in the
example above, denotes the bugfix level of the compiler; no macro
contains this value.
__GNUG__
The GNU C compiler defines this when the compilation language is
C++; use `GNUG’ to distinguish between GNU C and GNU
C++.
__cplusplus
The draft ANSI standard for C++ used to require predefining this
variable. Though it is no longer required, GNU C++ continues to define
it, as do other popular C++ compilers. You can use `__cplusplus’
to test whether a header is compiled by a C compiler or a C++ compiler.
__STRICT_ANSI__
GNU C defines this macro 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.
__BASE_FILE__
This macro expands to the name of the main input file, in the form
of a C string constant. This is the source file that was specified
as an argument when the C compiler was invoked.
__INCLUDE_LEVEL__
This macro expands to a decimal integer constant that represents the
depth of nesting in include files. The value of this macro is
incremented on every `#include’ directive and decremented at every
end of file. For input files specified by command line arguments,
the nesting level is zero.
__VERSION__
This macro expands to a string constant which describes the version number of
GNU C. The string is normally a sequence of decimal numbers separated
by periods, such as `“2.6.0”'.
__OPTIMIZE__
GNU CC defines this macro in optimizing compilations. It causes certain
GNU header files to define alternative macro definitions for some system
library functions. You should not 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__
GNU C defines this macro if and only if the data type char is
unsigned on the target machine. It exists to cause the standard header
file limits.h' to work correctly. You should not refer to this macro yourself; instead, refer to the standard macros defined in limits.h’. The preprocessor uses this macro to determine whether
or not to sign-extend large character constants written in octal; see
The `#if’ Directive.
__REGISTER_PREFIX__
This macro expands to a string (not a string constant) describing the
prefix applied to CPU registers in assembler code. You can use it to
write assembler code that is usable in multiple environments. For
example, in the m68k-aout' environment it expands to the null string, but in the m68k-coff’ environment it expands to the string
`%'.
__USER_LABEL_PREFIX__
Similar to __REGISTER_PREFIX__, but describes the prefix applied
to user generated labels in assembler code. For example, in the
m68k-aout' environment it expands to the string _’, but in
the m68k-coff' environment it expands to the null string. This does not work with the -mno-underscores’ option that the i386
OSF/rose and m88k targets provide nor with the `-mcall*’ options of
the rs6000 System V Release 4 target.
1.4.3.2 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. You can use `cpp -dM’ to see the values of
predefined macros; see 1.9 Invoking the C Preprocessor.
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’, which
are less specific.
_AM29K
_AM29000
Both _AM29K' and _AM29000’ are predefined for the AMD 29000
CPU family.
ns32000
`ns32000’ is predefined on computers which use the National
Semiconductor 32000 series CPU.
Yet other nonstandard predefined macros describe the manufacturer of
the system. For example,
sun
`sun’ is predefined on all models of Sun computers.
pyr
`pyr’ is predefined on all models of Pyramid computers.
sequent
`sequent’ is predefined on all models of Sequent computers.
These predefined symbols are not only nonstandard, they are contrary to the
ANSI standard because their names do not start with underscores.
Therefore, the option `-ansi’ inhibits the definition of these
symbols.
This tends to make -ansi' useless, since many programs depend on the customary nonstandard predefined symbols. Even system header files check them and will generate incorrect declarations if they do not find the names that are expected. You might think that the header files supplied for the Uglix computer would not need to test what machine they are running on, because they can simply assume it is the Uglix; but often they do, and they do so using the customary names. As a result, very few C programs will compile with -ansi’. We intend to avoid such problems on the GNU
system.
What, then, should you do in an ANSI C program to test the type of machine
it will run on?
GNU C offers a parallel series of symbols for this purpose, whose names
are made from the customary ones by adding __' at the beginning and end. Thus, the symbol vax` would be available on a Vax,
and so on.
The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when cpp is itself compiled) 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:
This macro is usually specified in `tm.h'.
1.4.4 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:
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; see 1.4.8.3 Swallowing the 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 the actual value of `EXP’
into a separate string constant, resulting in text like
but the C compiler then sees three consecutive string constants and
concatenates them into one, producing effectively
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 = "foon";' 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.
1.4.5 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:
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:
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.
1.4.6 Undefining Macros #
To _undefine_ a macro means to cancel its definition. This is done with the `#undef' directive. `#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,
expands into
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' directive will cancel definitions with arguments or definitions that don't expect arguments. The #undef’
directive has no effect when used on a name not currently defined as a macro.
1.4.7 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 (see section 1.3 Header Files), so they are accepted
silently and without effect.
Nontrivial redefinition is considered likely to be an error, so
it provokes a warning message from the preprocessor. However, sometimes it
is useful to change the definition of a macro in mid-compilation. You can
inhibit the warning by undefining the macro with `#undef’ before the
second definition.
In order for a redefinition to be trivial, the new definition must
exactly match the one already in effect, with two possible exceptions:
-
Whitespace may be added or deleted at the beginning or the end.
-
Whitespace may be changed in the middle (but not inside strings).
However, it may not be eliminated entirely, and it may not be added
where there was no whitespace at all.
Recall that a comment counts as whitespace.
1.4.8 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.
1.4.8.1 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,
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,
This bizarre example expands to `fprintf (stderr, “%s %d”, p, 35)'!
1.4.8.2 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,
whose purpose is to divide, rounding up. (One use for this operation is
to compute how many int' objects are needed to hold a certain number of char’ objects.) Then suppose it is used as follows:
This expands into
which does not do what is intended. The operator-precedence rules of
C make it equivalent to this:
But what we want is this:
Defining the macro as
provides the desired result.
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:
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':
1.4.8.3 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:Here Backslash-Newline is used to split the macro definition, which must
be a single line, so that it resembles the way such C code would be
laid out if not part of a macro definition.
A call to this macro might be SKIP_SPACES (p, lim)'. Strictly speaking, the call expands to a compound statement, which is a complete statement with no need for a semicolon to end it. But it looks like a function call. So it minimizes confusion if you can use it like a function call, writing a semicolon afterward, as in SKIP_SPACES (p, lim);'
But this can cause trouble before `else’ statements, because the
semicolon is actually a null statement. Suppose you write
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:
Now `SKIP_SPACES (p, lim);’ expands into
which is one statement.
1.4.8.4 Duplication of Side Effects #
Many C programs define a macro `min', for "minimum", like this:When you use this macro with an argument containing a side effect,
as shown here,
it expands as follows:
where x + y' has been substituted for X’ and foo (z)' for Y'.
The function foo' is used only once in the statement as it appears in the program, but the expression foo (z)’ has been substituted
twice into the macro expansion. As a result, foo' might be called two times when the statement is executed. If it has side effects or if it takes a long time to compute, the results might not be what you intended. We say that min’ is an unsafe macro.
The best solution to this problem is to define min' in a way that computes the value of foo (z)’ only once. The C language offers no
standard way to do this, but it can be done with GNU C extensions as
follows:
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':
(where we assume that foo' returns type int’).
1.4.8.5 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:
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 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 the expansion of y’ 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
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 may need to understand it.
1.4.8.6 Separate Expansion of Macro Arguments #
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 twice 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 1.4.8.5 Self-Referential Macros): 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,
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 actual argument would be used in
prescanned form if it is substituted elsewhere without stringification or
concatenation.
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 nested 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 cancels an undesirable side effect
(in the medical, not computational, 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:
We would like bar(foo)' to turn into (1 + (foo))’, which
would then turn into (1 + (a,b))'. But instead, bar(foo)’
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:
The problem is more serious when the operands of the macro are not
expressions; for example, when they are statements. Then parentheses
are unacceptable because they would make for invalid C code:
In GNU C you can shield the commas using the `({…})’
construct which turns a compound statement into an expression:
Or you can rewrite the macro definition to avoid such commas:
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:
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 forms the actual argument for
str'. str’ uses its argument without prescan because it
performs stringification; but it cannot prevent or undo the prescanning
already done by `xstr'.
1.4.8.7 Cascaded Use of Macros #
A _cascade_ of macros is when one macro's body contains a reference to another macro. This is very common practice. For example,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.
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:
Now TABLESIZE' expands (in two stages) to 37’. (The
#undef' is to prevent any warning about the nontrivial redefinition of BUFSIZE`.)
1.4.9 Newlines in Macro Arguments #
Traditional macro processing carries forward all newlines in macro
arguments into the expansion of the macro. This means that, if some of
the arguments are substituted more than once, or not at all, or out of
order, newlines can be duplicated, lost, or moved around within the
expansion. If the expansion consists of multiple statements, then the
effect is to distort the line numbers of some of these statements. The
result can be incorrect line numbers, in error messages or displayed in
a debugger.
The GNU C preprocessor operating in ANSI C mode adjusts appropriately
for multiple use of an argument–the first use expands all the
newlines, and subsequent uses of the same argument produce no newlines.
But even in this mode, it can produce incorrect line numbering if
arguments are used out of order, or not used at all.
Here is an example illustrating this problem:
The syntax error triggered by the tokens `syntax error’ results
in an error message citing line four, even though the statement text
comes from line five.
1.5 Conditionals #
In a macro processor, a _conditional_ is a directive that allows a part of the program to be ignored during compilation, on some conditions. In the C preprocessor, a conditional 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 understand 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. The condition
in a preprocessing conditional directive 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.
1.5.1 Why Conditionals are Used #
Generally there are three kinds of reason to use a conditional.
-
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 system 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 preprocessing conditional, the
offending code can be effectively excised from the program 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, or prints the values of those data for debugging,
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 preprocessing conditionals.
1.5.2 Syntax of Conditionals #
A conditional in the C preprocessor begins with a _conditional directive_: `#if', `#ifdef' or `#ifndef'. See section 1.5.4 Conditionals and Macros, for information on `#ifdef' and `#ifndef'; only `#if' is explained here.1.5.2.1 The `#if’ Directive #
The `#if’ directive in its simplest form consists of
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.
expression is a C expression of integer type, subject to stringent
restrictions. It may contain
-
Integer constants, which are all regarded as
longor
unsigned long. -
Character constants, which are interpreted according to the character
set and conventions of the machine and operating system on which the
preprocessor is running. The GNU C preprocessor uses the C data type
char' for these character constants; therefore, whether some character codes are negative is determined by the C compiler used to compile the preprocessor. If it treatschar’ as signed, then
character codes large enough to set the sign bit will be considered
negative; otherwise, no character code is considered negative. -
Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
operations (' 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 controlled text inside of a conditional can include
preprocessing directives. Then the directives inside the conditional are
obeyed only if that branch of the conditional succeeds. The text can
also contain other conditional groups. However, the #if' and #endif’ directives must balance.
1.5.2.2 The `#else’ Directive #
The `#else' directive can be added to a conditional to provide alternative text to be used if the condition is false. This is what it looks like:If expression is nonzero, and thus the text-if-true is
active, then #else' acts like a failing conditional and the <var>text-if-false</var> is ignored. Contrariwise, if the #if’
conditional fails, the text-if-false is considered included.
1.5.2.3 The `#elif’ Directive #
One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might haveAnother conditional directive, `#elif’, allows this to be abbreviated
as follows:
#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’ directive includes an expression to be tested.
The text following the #elif' is processed only if the original #if’-condition failed and the #elif' condition succeeds. More than one #elif’ can go in the same #if'-#endif’
group. Then the text after each #elif' is processed only if the #elif’ condition succeeds after the original #if' and any previous #elif’ directives within it have failed. #else' is equivalent to #elif 1’, and #else' is allowed after any number of #elif’ directives, but #elif' may not follow #else'.
1.5.3 Keeping Deleted Code for Future Reference #
If you replace or delete a part of the program but want to keep the old
code around as a comment for future reference, the easy way to do this
is to put #if 0' before it and #endif’ after it. This is
better than using comment delimiters /*' and */’ since those
won’t work if the code already contains comments (C comments do not
nest).
This works even if the code being turned off contains conditionals, but
they must be entire conditionals (balanced #if' and #endif’).
Conversely, do not use #if 0' for comments which are not C code. Use the comment delimiters /’ and */' instead. The interior of #if 0’ must consist of complete tokens; in particular,
singlequote characters must balance. But comments often contain
unbalanced singlequote characters (known in English as apostrophes).
These confuse #if 0'. They do not confuse /'.
1.5.4 Conditionals and Macros #
Conditionals are useful in connection with macros or assertions, because
those are the only ways that an expression’s value can vary from one
compilation to another. A #if' directive whose expression uses no macros or assertions is equivalent to #if 1’ or `#if 0’; you
might as well determine which one, by computing the value of the
expression yourself, and then simplify the program.
For example, here is a conditional that tests the expression
BUFSIZE == 1020', where BUFSIZE’ must be a macro.
(Programmers often wish they could test the size of a variable or data
type in #if', but this does not work. The preprocessor does not understand sizeof, or typedef names, or even the type keywords such as int`.)
would succeed if either of the names vax' and ns16000’ is
defined as a macro. You can test the same condition using assertions
(see section 1.5.5 Assertions), like this:
If a macro is defined and later undefined with #undef', subsequent use of the defined’ operator returns 0, because
the name is no longer defined. If the macro is defined again with
another #define', defined’ will recommence returning 1.
#ifdef <var>name</var>
is equivalent to `#if defined (name)'.
#ifndef <var>name</var>
is equivalent to `#if ! defined (name)'.
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 different 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 customize a program for
different machines or applications. For example, the macro
BUFSIZE' might be defined in a configuration file for your program that is included as a header file in each source file. You would useBUFSIZE’ in a preprocessing 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. 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 1.9 Invoking the C Preprocessor.
Assertions are usually predefined, but can be defined with preprocessor
directives or command-line options.
1.5.5 Assertions #
_Assertions_ are a more systematic alternative to macros in writing conditionals to test what sort of computer or system the compiled program will run on. Assertions are usually predefined, but you can define them with preprocessing directives or command-line options. The macros traditionally used to describe the type of target are not classified in any way according to which question they answer; they may indicate a hardware architecture, a particular hardware model, an operating system, a particular version of an operating system, or specific configuration options. These are jumbled together in a single namespace. In contrast, each assertion consists of a named question and an answer. The question is usually called the _predicate_. An assertion looks like this:You must use a properly formed identifier for predicate. The
value of answer can be any sequence of words; all characters are
significant except for leading and trailing whitespace, and differences
in internal whitespace sequences are ignored. Thus, x + y' is different from x+y’ but equivalent to x + y'. )’ is
not allowed in an answer.
There may be more than one answer asserted for a given predicate. If
you omit the answer, you can test whether any answer is asserted
for predicate:
and perhaps others. The alternatives with
more or less version information let you ask more or less detailed
questions about the type of system software.
On a Unix system, you would find #system (unix) and perhaps one of:
#system (aix), #system (bsd), #system (hpux),
#system (lynx), #system (mach), #system (posix),
#system (svr3), #system (svr4), or #system (xpg4)
with possible version numbers following.
Other values for system are #system (mvs)
and #system (vms).
Portability note: Many Unix C compilers provide only one answer
for the system assertion: #system (unix), if they support
assertions at all. This is less than useful.
An assertion with a multi-word answer is completely different from several
assertions with individual single-word answers. For example, the presence
of system (mach 3.0) does not mean that system (3.0) is true.
It also does not directly imply system (mach), but in GNU C, that
last will normally be asserted as well.
The current list of possible assertion values for cpu is:
#cpu (a29k), #cpu (alpha), #cpu (arm), #cpu (clipper), #cpu (convex), #cpu (elxsi), #cpu (tron), #cpu (h8300), #cpu (i370), #cpu (i386),
#cpu (i860), #cpu (i960), #cpu (m68k), #cpu (m88k), #cpu (mips), #cpu (ns32k), #cpu (hppa),
#cpu (pyr), #cpu (ibm032), #cpu (rs6000),
#cpu (sh), #cpu (sparc), #cpu (spur), #cpu (tahoe), #cpu (vax), #cpu (we32000).
(Note the absence of a `#’ before predicate.)
Each time you do this, you assert a new true answer for predicate. Asserting one answer does not invalidate previously asserted answers; they all remain true. The only way to remove an assertion is with `#unassert'. `#unassert' has the same syntax as `#assert'. You can also remove all assertions about predicate like this:You can also add or cancel assertions using command options
when you run gcc or cpp. See section 1.9 Invoking the C Preprocessor.
1.5.6 The #error' and #warning’ Directives
#
The directive `#error' causes the preprocessor to report a fatal
error. The rest of the line that follows `#error' is used as the
error message. The line must consist of complete tokens.
You would use `#error’ inside of a conditional that detects a
combination of parameters which you know the program does not properly
support. For example, if you know that the program will not run
properly on a Vax, you might write
See section 1.4.3.2 Nonstandard Predefined Macros, 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,
You might use `#warning’ in obsolete header files, with a message
directing the user to the header file which should be used instead.
1.6 Combining Source Files #
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 preprocessor output to be different
from the line’s number in the original source file. You will appreciate
the value of 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 directive 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 input.
#line <var>linenum</var>
Here linenum is a decimal integer constant. This specifies that
the line number of the following line of input, in its original source file,
was linenum.
#line <var>linenum</var> <var>filename</var>
Here linenum is a decimal integer constant and filename
is a string constant. This specifies that the following line of input
came originally from source file filename and its line number there
was linenum. Keep in mind that filename is not just a
file name; it is surrounded by doublequote characters so that it looks
like a string constant.
#line <var>anything else</var>
anything else is checked for macro calls, which are expanded.
The result should be a decimal integer constant followed optionally
by a string constant, as described above.
#line' directives alter the results of the FILE’ and
`LINE’ predefined macros from that point on. See section 1.4.3.1 Standard Predefined Macros.
The output of the preprocessor (which is the input for the rest of the
compiler) contains directives that look much like #line' directives. They start with just #’ instead of #line', but this is followed by a line number and file name as in #line’. See section 1.8 C Preprocessor Output.
1.7 Miscellaneous Preprocessing Directives #
This section describes three additional preprocessing directives. They are not very useful, but are mentioned for completeness.The null directive consists of a #' followed by a Newline, with only whitespace (including comments) in between. A null directive is understood as a preprocessing directive but has no effect on the preprocessor output. The primary significance of the existence of the null directive is that an input line consisting of just a #’ will produce no output,
rather than a line of output containing just a `#’. Supposedly
some old C programs contain such lines.
1.8 C Preprocessor Output #
The output from the C preprocessor looks much like the input, except that all preprocessing directive lines have been replaced with blank lines and all comments with spaces. Whitespace within a line is not altered; however, unless `-traditional' is used, spaces may be inserted into the expansions of macro calls to prevent tokens from being concatenated.Source file name and line number information is conveyed by lines of
the form
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 filename at line linenum.
After the file name comes zero or more flags, which are 1', 2’, 3', or 4’. If there are multiple flags, spaces separate
them. Here is what the flags mean:
1' This indicates the start of a new file. 2’
This indicates returning to a file (after having included another file).
3' This indicates that the following text comes from a system header file, so certain warnings should be suppressed. 4’
This indicates that the following text should be treated as C.
1.9 Invoking the C Preprocessor #
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 useful on its own.
The C preprocessor expects two file names as arguments, infile and
outfile. The preprocessor reads infile together with any other
files it specifies with `#include’. All the output generated by the
combined input files is written in outfile.
Either infile or outfile may be `-’, which as infile
means to read from standard input and as outfile means to write to
standard output. Also, if outfile or both file names are omitted,
the standard output and standard input are used for the omitted file names.
-P' <a name="IDX80" /> Inhibit generation of #’-lines with line-number information in
the output from the preprocessor (see section 1.8 C Preprocessor 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.
`-traditional’
Try to imitate the behavior of old-fashioned C, as opposed to ANSI C.
-
Traditional macro expansion pays no attention to singlequote or
doublequote characters; macro argument symbols are replaced by the
argument values even when they appear within apparent string or
character constants. -
Traditionally, it is permissible for a macro expansion to end in the
middle of a string or character constant. The constant continues into
the text surrounding the macro call. -
However, traditionally the end of the line terminates a string or
character constant, with no error. -
In traditional C, a comment is equivalent to no text at all. (In ANSI
C, a comment counts as whitespace.) -
Traditional C does not have the concept of a “preprocessing number”.
It considers1.0e+4' to be three tokens:1.0e’,+', and4'. -
A macro is not suppressed within its own definition, in traditional C.
Thus, any macro that is used recursively inevitably causes an error. -
The character `#’ has no special meaning within a macro definition
in traditional C. -
In traditional C, the text at the end of a macro expansion can run
together with the text after the macro call, to produce a single token.
(This is impossible in ANSI C.) -
Traditionally, `’ inside a macro argument suppresses the syntactic
significance of the following character.
However, this option does not prevent diagnostics
about unterminated comments
when a C-style comment appears to start, but not end,
within Fortran-style commentary.
So, the following Fortran comment lines are accepted with
`-traditional':
However, this type of comment line will likely produce a diagnostic,
or at least unexpected output from the preprocessor,
due to the unterminated comment:
-trigraphs' <a name="IDX86" /> Process ANSI standard trigraph sequences. These are three-character sequences, all starting with ??’, that are defined by ANSI C to
stand for single characters. For example, ??/' stands for ’, so '??/n'' is a character constant for a newline. Strictly speaking, the GNU C preprocessor does not support all programs in ANSI Standard C unless -trigraphs’ is used, but if
you ever notice the difference it will be with relief.
You don’t want to know any more about trigraphs.
-pedantic' <a name="IDX87" /> Issue warnings required by the ANSI C standard in certain cases such as when text other than a comment follows #else’ or `#endif'.
-pedantic-errors' <a name="IDX88" /> Like -pedantic’, except that errors are produced rather than
warnings.
-Wtrigraphs' <a name="IDX89" /> Warn if any trigraphs are encountered. Currently this only works if you have turned trigraphs on with -trigraphs’ or `-ansi’; in the
future this restriction will be removed.
-Wcomment' <a name="IDX90" /> Warn whenever a comment-start sequence /*’ appears in a /*' comment, or whenever a Backslash-Newline appears in a //’ comment.
-Wall' <a name="IDX91" /> Requests both -Wtrigraphs’ and -Wcomment' (but not -Wtraditional’ or `-Wundef’).
`-Wtraditional’
Warn about certain constructs that behave differently in traditional and
ANSI C.
-Wundef' <a name="IDX93" /> Warn if an undefined identifier is evaluated in an #if’ directive.
-I <var>directory</var>' <a name="IDX94" /> Add the directory <var>directory</var> to the head of the list of directories to be searched for header files (see section 1.3.2 The #include’ Directive).
This can be used to override a system header file, substituting your
own version, since these directories are searched before the system
header file directories. 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 “file”’;
they are not searched for `#include file'.
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 directory for #include “file”’.
Therefore, the current directory is searched only if it is requested
explicitly with -I.'. Specifying both -I-’ and `-I.’
allows you to control precisely which directories are searched before
the current one and which are searched after.
-nostdinc' <a name="IDX95" /> 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.
`-nostdinc++’
Do not search for header files in the C++-specific standard directories,
but do still search the other standard directories.
(This option is used when building the C++ library.)
-remap' <a name="IDX97" /> When searching for a header file in a directory, remap file names if a file named header.gcc’ exists in that directory. This can be used
to work around limitations of file systems with file name restrictions.
The `header.gcc’ file should contain a series of lines with two
tokens on each line: the first token is the name to map, and the second
token is the actual name to use.
-D <var>name</var>' <a name="IDX98" /> Predefine <var>name</var> as a macro, with definition 1'.
-D <var>name</var>=<var>definition</var>' Predefine <var>name</var> as a macro, with definition <var>definition</var>. There are no restrictions on the contents of <var>definition</var>, but if you are invoking the preprocessor 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. If you use more than one -D’ for the same name, the rightmost definition takes
effect.
-U <var>name</var>' <a name="IDX99" /> Do not predefine <var>name</var>. If both -U’ and -D' are specified for one name, the -U’ beats the `-D’ and the name
is not predefined.
`-undef’
Do not predefine any nonstandard macros.
-gcc' <a name="IDX101" /> Define the macros <var>__GNUC__</var> and <var>__GNUC_MINOR__</var>. These are defined automatically when you use gcc -E’; you can turn them off
in that case with `-no-gcc'.
`-A predicate(answer)’
Make an assertion with the predicate predicate and answer
answer. See section 1.5.5 Assertions.
You can use `-A-’ to disable all predefined assertions; it also
undefines all predefined macros and all macros that preceded it on the
command line.
-dM' <a name="IDX103" /> Instead of outputting the result of preprocessing, output a list of #define’ directives for all the macros defined during the
execution of the preprocessor, including predefined macros. This gives
you a way of finding out what is predefined in your version of the
preprocessor; assuming you have no file `foo.h’, the command
will show the values of any predefined macros.
-dD' <a name="IDX104" /> Like -dM’ except in two respects: it does not include the
predefined macros, and it outputs both the `#define’
directives and the result of preprocessing. Both kinds of output go to
the standard output file.
-dI' <a name="IDX105" /> Output #include’ directives in addition to the result of preprocessing.
-M [-MG]' <a name="IDX106" /> Instead of outputting the result of preprocessing, output a rule suitable for makedescribing the dependencies of the main source file. The preprocessor outputs onemakerule 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.
-MG' says to treat missing header files as generated files and assume they live in the same directory as the source file. It must be specified in addition to -M'.
This feature is used in automatic updating of makefiles.
-MM [-MG]' <a name="IDX107" /> Like -M’ but mention only the files included with #include "<var>file</var>"'. System header files included with #include
file’ are omitted.
-MD <var>file</var>' <a name="IDX108" /> Like -M’ but the dependency information is written to file.
This is in addition to compiling the file as specified—-MD' does not inhibit ordinary compilation the way -M’ does.
When invoking gcc, do not specify the file argument.
gcc will create file names made by replacing “.c” with “.d” at
the end of the input file names.
In Mach, you can use the utility md to merge multiple dependency
files into a single dependency file suitable for using with the `make’
command.
-MMD <var>file</var>' <a name="IDX109" /> Like -MD’ except mention only user header files, not system
header files.
`-H’
Print the name of each header file used, in addition to other normal
activities.
-imacros <var>file</var>' <a name="IDX111" /> Process <var>file</var> as input, discarding the resulting output, before processing the regular input file. Because the output generated from <var>file</var> is discarded, the only effect of -imacros file’
is to make the macros defined in file available for use in the
main input.
`-include file’
Process file as input, and include all the resulting output,
before processing the regular input file.
-idirafter <var>dir</var>' <a name="IDX113" /> <a name="IDX114" /> Add the directory <var>dir</var> to the second include path. The directories on the second include path are searched when a header file is not found in any of the directories in the main include path (the one that -I’ adds to).
-iprefix <var>prefix</var>' <a name="IDX115" /> Specify <var>prefix</var> as the prefix for subsequent -iwithprefix’
options.
-iwithprefix <var>dir</var>' <a name="IDX116" /> Add a directory to the second include path. The directory's name is made by concatenating <var>prefix</var> and <var>dir</var>, where <var>prefix</var> was specified previously with -iprefix'.
`-isystem dir’
Add a directory to the beginning of the second include path, marking it
as a system directory, so that it gets the same special treatment as
is applied to the standard system directories.
-x c' -x c++’
-x objective-c' -x assembler-with-cpp’
Specify the source language: C, C++, Objective-C, or assembly. This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file:
.c', .cc’, .m', or .S’. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang' option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l’
option.
-std=<var>standard</var>' -ansi’
Specify the standard to which the code should conform. Currently cpp
only knows about the standards for C; other language standards will be
added in the future.
standard
may be one of:
iso9899:1990
The ISO C standard from 1990.
iso9899:199409
c89
The 1990 C standard, as amended in 1994. `c89’ is the customary
shorthand for this version of the standard.
The -ansi' option is equivalent to -std=c89'.
iso9899:199x
c9x
The revised ISO C standard, which is expected to be promulgated some
time in 1999. It has not been approved yet, hence the `x'.
gnu89
The 1990 C standard plus GNU extensions. This is the default.
gnu9x
The 199x C standard plus GNU extensions.
-Wp,-lint' <a name="IDX123" /> Look for commands to the program checker lintembedded in comments, and emit them preceded by#pragma lint’. For example,
the comment /* NOTREACHED */' becomes #pragma lint
NOTREACHED'.
Because of the clash with -l', you must use the awkward syntax above. In a future release, this option will be replaced by -flint’ or `-Wlint’; we are not sure which yet.
-$' <a name="IDX124" /> Forbid the use of $’ in identifiers. The C standard does not
permit this, but it is a common extension.
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