Conditional Syntax#
A conditional in the C preprocessor begins with a conditional
directive: #if
, #ifdef
or #ifndef
.
Ifdef#
The simplest sort of conditional is
#ifdef MACRO
controlled text
#endif /* MACRO */
This block is called a conditional group. controlled text
will be included in the output of the preprocessor if and only if
MACRO
is defined. We say that the conditional succeeds if
MACRO
is defined, fails if it is not.
The controlled text
inside of a conditional can include
preprocessing directives. They are executed only if the conditional
succeeds. You can nest conditional groups inside other conditional
groups, but they must be completely nested. In other words,
#endif
always matches the nearest #ifdef
(or
#ifndef
, or #if
). Also, you cannot start a conditional
group in one file and end it in another.
Even if a conditional fails, the controlled text
inside it is
still run through initial transformations and tokenization. Therefore,
it must all be lexically valid C. Normally the only way this matters is
that all comments and string literals inside a failing conditional group
must still be properly ended.
The comment following the #endif
is not required, but it is a
good practice if there is a lot of controlled text
, because it
helps people match the #endif
to the corresponding #ifdef
.
Older programs sometimes put MACRO
directly after the
#endif
without enclosing it in a comment. This is invalid code
according to the C standard. CPP accepts it with a warning. It
never affects which #ifndef
the #endif
matches.
Sometimes you wish to use some code if a macro is not defined.
You can do this by writing #ifndef
instead of #ifdef
.
One common use of #ifndef
is to include code only the first
time a header file is included. See Once-Only Headers.
Macro definitions can vary between compilations for several reasons. Here are some samples.
Some macros are predefined on each kind of machine (see System-specific Predefined Macros). This allows you to provide code specially tuned for a particular machine.
System header files define more macros, associated with the features they implement. You can test these macros with conditionals to avoid using a system feature on a machine where it is not implemented.
Macros can be defined or undefined with the
-D
and-U
command-line 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 command-line options, perhaps set in the Makefile. See Invocation.Your program might have a special header file (often called
config.h
) that is adjusted when the program is compiled. It can define or not define macros depending on the features of the system and the desired capabilities of the program. The adjustment can be automated by a tool such as autoconf, or done by hand.
If#
The #if
directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is
#if expression
controlled text
#endif /* expression */
expression
is a C expression of integer type, subject to stringent
restrictions. It may contain
Integer constants.
Character constants, which are interpreted as they would be in normal code.
Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and logical operations (
&&
and||
). The latter two obey the usual short-circuiting rules of standard C.Macros. All macros in the expression are expanded before actual computation of the expression’s value begins.
Uses of the
defined
operator, which lets you check whether macros are defined in the middle of an#if
.Identifiers that are not macros, which are all considered to be the number zero. This allows you to write
#if MACRO
instead of#ifdef MACRO
, if you know that MACRO, when defined, will always have a nonzero value. Function-like macros used without their function call parentheses are also treated as zero.In some contexts this shortcut is undesirable. The
-Wundef
option causes GCC to warn whenever it encounters an identifier which is not a macro in an#if
.
The preprocessor does not know anything about types in the language.
Therefore, sizeof
operators are not recognized in #if
, and
neither are enum
constants. They will be taken as identifiers
which are not macros, and replaced by zero. In the case of
sizeof
, this is likely to cause the expression to be invalid.
The preprocessor calculates the value of expression
. It carries
out all calculations in the widest integer type known to the compiler;
on most machines supported by GCC this is 64 bits. This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases. If the value
comes out to be nonzero, the #if
succeeds and the controlled
text
is included; otherwise it is skipped.
Defined#
The special operator defined
is used in #if
and
#elif
expressions to test whether a certain name is defined as a
macro. defined name
and defined (name)
are
both expressions whose value is 1 if name
is defined as a macro at
the current point in the program, and 0 otherwise. Thus, #if
defined MACRO
is precisely equivalent to #ifdef MACRO
.
defined
is useful when you wish to test more than one macro for
existence at once. For example,
#if defined (__vax__) || defined (__ns16000__)
would succeed if either of the names __vax__
or
__ns16000__
is defined as a macro.
Conditionals written like this:
#if defined BUFSIZE && BUFSIZE >= 1024
can generally be simplified to just #if BUFSIZE >= 1024
,
since if BUFSIZE
is not defined, it will be interpreted as having
the value zero.
If the defined
operator appears as a result of a macro expansion,
the C standard says the behavior is undefined. GNU cpp treats it as a
genuine defined
operator and evaluates it normally. It will warn
wherever your code uses this feature if you use the command-line option
-Wpedantic
, since other compilers may handle it differently. The
warning is also enabled by -Wextra
, and can also be enabled
individually with -Wexpansion-to-defined
.
Else#
The #else
directive can be added to a conditional to provide
alternative text to be used if the condition fails. This is what it
looks like:
#if expression
text-if-true
#else /* Not expression */
text-if-false
#endif /* Not expression */
If expression
is nonzero, the text-if-true
is included and
the text-if-false
is skipped. If expression
is zero, the
opposite happens.
You can use #else
with #ifdef
and #ifndef
, too.
Elif#
One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have
#if X == 1
...
#else /* X != 1 */
#if X == 2
...
#else /* X != 2 */
...
#endif /* X != 2 */
#endif /* X != 1 */
Another conditional directive, #elif
, allows this to be
abbreviated as follows:
#if X == 1
...
#elif X == 2
...
#else /* X != 2 and X != 1*/
...
#endif /* X != 2 and X != 1*/
#elif
stands for ‘else if’. Like #else
, it goes in the
middle of a conditional group 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 conditional group. Then
the text after each #elif
is processed only if the #elif
condition succeeds after the original #if
and all previous
#elif
directives within it have failed.
#else
is allowed after any number of #elif
directives, but
#elif
may not follow #else
.
__has_attribute#
The special operator __has_attribute (operand)
may be used
in #if
and #elif
expressions to test whether the attribute
referenced by its operand
is recognized by GCC. Using the operator
in other contexts is not valid. In C code, if compiling for strict
conformance to standards before C2x, operand
must be
a valid identifier. Otherwise, operand
may be optionally
introduced by the attribute-scope::
prefix.
The attribute-scope
prefix identifies the ‘namespace’ within
which the attribute is recognized. The scope of GCC attributes is
gnu
or __gnu__
. The __has_attribute
operator by
itself, without any operand
or parentheses, acts as a predefined
macro so that support for it can be tested in portable code. Thus,
the recommended use of the operator is as follows:
#if defined __has_attribute
# if __has_attribute (nonnull)
# define ATTR_NONNULL __attribute__ ((nonnull))
# endif
#endif
The first #if
test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used. Only when that
test succeeds is it valid to use __has_attribute
as a preprocessor
operator. As a result, combining the two tests into a single expression as
shown below would only be valid with a compiler that supports the operator
but not with others that don’t.
#if defined __has_attribute && __has_attribute (nonnull) /* not portable */
...
#endif
__has_cpp_attribute#
The special operator __has_cpp_attribute (operand)
may be used
in #if
and #elif
expressions in C++ code to test whether
the attribute referenced by its operand
is recognized by GCC.
__has_cpp_attribute (operand)
is equivalent to
__has_attribute (operand)
except that when operand
designates a supported standard attribute it evaluates to an integer
constant of the form YYYYMM
indicating the year and month when
the attribute was first introduced into the C++ standard. For additional
information including the dates of the introduction of current standard
attributes, see SD-6: SG10 Feature Test Recommendations.
__has_c_attribute#
The special operator __has_c_attribute (operand)
may be
used in #if
and #elif
expressions in C code to test
whether the attribute referenced by its operand
is recognized by
GCC in attributes using the [[]]
syntax. GNU attributes must
be specified with the scope gnu
or __gnu__
with
__has_c_attribute
. When operand
designates a supported
standard attribute it evaluates to an integer constant of the form
YYYYMM
indicating the year and month when the attribute was
first introduced into the C standard, or when the syntax of operands
to the attribute was extended in the C standard.
__has_builtin#
The special operator __has_builtin (operand)
may be used in
constant integer contexts and in preprocessor #if
and #elif
expressions to test whether the symbol named by its operand
is
recognized as a built-in function by GCC in the current language and
conformance mode. It evaluates to a constant integer with a nonzero
value if the argument refers to such a function, and to zero otherwise.
The operator may also be used in preprocessor #if
and #elif
expressions. The __has_builtin
operator by itself, without any
operand
or parentheses, acts as a predefined macro so that support
for it can be tested in portable code. Thus, the recommended use of
the operator is as follows:
#if defined __has_builtin
# if __has_builtin (__builtin_object_size)
# define builtin_object_size(ptr) __builtin_object_size (ptr, 2)
# endif
#endif
#ifndef builtin_object_size
# define builtin_object_size(ptr) ((size_t)-1)
#endif
__has_include#
The special operator __has_include (operand)
may be used in
#if
and #elif
expressions to test whether the header referenced
by its operand
can be included using the #include
directive. Using
the operator in other contexts is not valid. The operand
takes
the same form as the file in the #include
directive (see Include Syntax) and evaluates to a nonzero value if the header can be included and
to zero otherwise. Note that that the ability to include a header doesn’t
imply that the header doesn’t contain invalid constructs or #error
directives that would cause the preprocessor to fail.
The __has_include
operator by itself, without any operand
or
parentheses, acts as a predefined macro so that support for it can be tested
in portable code. Thus, the recommended use of the operator is as follows:
#if defined __has_include
# if __has_include (<stdatomic.h>)
# include <stdatomic.h>
# endif
#endif
The first #if
test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used. Only when that
test succeeds is it valid to use __has_include
as a preprocessor
operator. As a result, combining the two tests into a single expression
as shown below would only be valid with a compiler that supports the operator
but not with others that don’t.
#if defined __has_include && __has_include ("header.h") /* not portable */
...
#endif