The common predefined macros are GNU C extensions. They are available with the same meanings regardless of the machine or operating system on which you are using GNU C or GNU Fortran. Their names all start with double underscores.
This macro expands to sequential integral values starting from 0. In
conjunction with the
## operator, this provides a convenient means to
generate unique identifiers. Care must be taken to ensure that
__COUNTER__ is not expanded prior to inclusion of precompiled headers
which use it. Otherwise, the precompiled headers will not be used.
The GNU Fortran compiler defines this.
These macros are defined by all GNU compilers that use the C
preprocessor: C, C++, Objective-C and Fortran. Their values are the major
version, minor version, and patch level of the compiler, as integer
constants. For example, GCC version x.y.z
__GNUC__ to x,
__GNUC_MINOR__ to y,
__GNUC_PATCHLEVEL__ to z. These
macros are also defined if you invoke the preprocessor directly.
If all you need to know is whether or not your program is being compiled
by GCC, or a non-GCC compiler that claims to accept the GNU C dialects,
you can simply test
__GNUC__. If you need to write code
which depends on a specific version, you must be more careful. Each
time the minor version is increased, the patch level is reset to zero;
each time the major version is increased, the
minor version and patch level are reset. If you wish to use the
predefined macros directly in the conditional, you will need to write it
/* Test for GCC > 3.2.0 */ #if __GNUC__ > 3 || \ (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ (__GNUC_MINOR__ == 2 && \ __GNUC_PATCHLEVEL__ > 0))
Another approach is to use the predefined macros to calculate a single number, then compare that against a threshold:
#define GCC_VERSION (__GNUC__ * 10000 \ + __GNUC_MINOR__ * 100 \ + __GNUC_PATCHLEVEL__) … /* Test for GCC > 3.2.0 */ #if GCC_VERSION > 30200
Many people find this form easier to understand.
The GNU C++ compiler defines this. Testing it is equivalent to
(__GNUC__ && __cplusplus).
GCC defines this macro if and only if the -ansi switch, or a -std switch specifying strict conformance to some version of ISO C or ISO C++, was specified when GCC was invoked. It is defined to ‘1’. This macro exists primarily to direct GNU libc’s header files to use only definitions found in standard C.
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 on the command line of the preprocessor or C compiler.
This macro expands to the basename of the current input file, in the
form of a C string constant. This is the last path component by which
the preprocessor opened the file. For example, processing
"/usr/local/include/myheader.h" would set this
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 the end of every included file. It starts out at 0, its value within the base file specified on the command line.
This macro is defined if the target uses the ELF object format.
This macro expands to a string constant which describes the version of the compiler in use. You should not rely on its contents having any particular form, but it can be counted on to contain at least the release number.
These macros describe the compilation mode.
defined in all optimizing compilations.
defined if the compiler is optimizing for size, not speed.
__NO_INLINE__ is defined if no functions will be inlined into
their callers (when not optimizing, or when inlining has been
specifically disabled by -fno-inline).
These macros cause certain GNU header files to provide optimized definitions, using macros or inline functions, of system library functions. You should not use these macros in any way unless you make sure that programs will execute with the same effect whether or not they are defined. If they are defined, their value is 1.
GCC defines this macro if functions declared
inline will be
handled in GCC’s traditional gnu90 mode. Object files will contain
externally visible definitions of all functions declared
static. They will not contain any
definitions of any functions declared
GCC defines this macro if functions declared
inline will be
handled according to the ISO C99 or later standards. Object files will contain
externally visible definitions of all functions declared
inline. They will not contain definitions of any functions declared
If this macro is defined, GCC supports the
attribute as a way to always get the gnu90 behavior.
GCC defines this macro if and only if the data type
unsigned on the target machine. It exists to cause the standard header
file limits.h to work correctly. You should not use this macro
yourself; instead, refer to the standard macros defined in limits.h.
__CHAR_UNSIGNED__, this macro is defined if and only if the
wchar_t is unsigned and the front-end is in C++ mode.
This macro expands to a single token (not a string constant) which is
the prefix applied to CPU register names in assembly language for this
target. You can use it to write assembly that is usable in multiple
environments. For example, in the
m68k-aout environment it
expands to nothing, but in the
m68k-coff environment it expands
to a single ‘%’.
This macro expands to a single token which is the prefix applied to
user labels (symbols visible to C code) in assembly. For example, in
m68k-aout environment it expands to an ‘_’, but in the
m68k-coff environment it expands to nothing.
This macro will have the correct definition even if -f(no-)underscores is in use, but it will not be correct if target-specific options that adjust this prefix are used (e.g. the OSF/rose -mno-underscores option).
These macros are defined to the correct underlying types for the
respectively. They exist to make the standard header files
stddef.h, stdint.h, and wchar.h work correctly.
You should not use these macros directly; instead, include the
appropriate headers and use the typedefs. Some of these macros may
not be defined on particular systems if GCC does not provide a
stdint.h header on those systems.
Defined to the number of bits used in the representation of the
char data type. It exists to make the standard header given
numerical limits work correctly. You should not use
this macro directly; instead, include the appropriate headers.
Defined to the maximum value of the
signed long long,
uintptr_t types and
to the minimum value of the
sig_atomic_t types respectively. They exist to make the
standard header given numerical limits work correctly. You should not
use these macros directly; instead, include the appropriate headers.
Some of these macros may not be defined on particular systems if GCC
does not provide a stdint.h header on those systems.
Defined to implementations of the standard stdint.h macros with
the same names without the leading
__. They exist the make the
implementation of that header work correctly. You should not use
these macros directly; instead, include the appropriate headers. Some
of these macros may not be defined on particular systems if GCC does
not provide a stdint.h header on those systems.
Defined to the bit widths of the corresponding types. They exist to make the implementations of limits.h and stdint.h behave correctly. You should not use these macros directly; instead, include the appropriate headers. Some of these macros may not be defined on particular systems if GCC does not provide a stdint.h header on those systems.
Defined to the number of bytes of the C standard data types:
__BYTE_ORDER__ is defined to one of the values
__ORDER_PDP_ENDIAN__ to reflect the layout of multi-byte and
multi-word quantities in memory. If
__BYTE_ORDER__ is equal to
multi-byte and multi-word quantities are laid out identically: the
byte (word) at the lowest address is the least significant or most
significant byte (word) of the quantity, respectively. If
__BYTE_ORDER__ is equal to
bytes in 16-bit words are laid out in a little-endian fashion, whereas
the 16-bit subwords of a 32-bit quantity are laid out in big-endian
You should use these macros for testing like this:
/* Test for a little-endian machine */ #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
__FLOAT_WORD_ORDER__ is defined to one of the values
__ORDER_BIG_ENDIAN__ to reflect
the layout of the words of multi-word floating-point quantities.
This macro is defined, with value 1, when compiling a C++ source file with warnings about deprecated constructs enabled. These warnings are enabled by default, but can be disabled with -Wno-deprecated.
This macro is defined, with value 1, when compiling a C++ source file with exceptions enabled. If -fno-exceptions is used when compiling the file, then this macro is not defined.
This macro is defined, with value 1, when compiling a C++ source file with runtime type identification enabled. If -fno-rtti is used when compiling the file, then this macro is not defined.
This macro is defined, with value 1, if the compiler uses the old
mechanism based on
longjmp for exception
This macro is defined when compiling a C++ source file with C++11 features
enabled, i.e., for all C++ language dialects except -std=c++98
and -std=gnu++98. This macro is obsolete, but can be used to
detect experimental C++0x features in very old versions of GCC. Since
GCC 4.7.0 the
__cplusplus macro is defined correctly, so most
code should test
__cplusplus >= 201103L instead of using this
This macro is defined when compiling a C++ source file. It has the value 1 if the compiler will use weak symbols, COMDAT sections, or other similar techniques to collapse symbols with “vague linkage” that are defined in multiple translation units. If the compiler will not collapse such symbols, this macro is defined with value 0. In general, user code should not need to make use of this macro; the purpose of this macro is to ease implementation of the C++ runtime library provided with G++.
This macro is defined, with value 1, if (and only if) the NeXT runtime (as in -fnext-runtime) is in use for Objective-C. If the GNU runtime is used, this macro is not defined, so that you can use this macro to determine which runtime (NeXT or GNU) is being used.
These macros are defined, with value 1, if (and only if) the compilation
is for a target where
long int and pointer both use 64-bits and
int uses 32-bit.
This macro is defined, with value 1, when -fstack-protector is in use.
This macro is defined, with value 2, when -fstack-protector-all is in use.
This macro is defined, with value 3, when -fstack-protector-strong is in use.
This macro is defined, with value 4, when -fstack-protector-explicit is in use.
This macro is defined, with value 1, when -fsanitize=address or -fsanitize=kernel-address are in use.
This macro is defined, with value 1, when -fsanitize=thread is in use.
This macro expands to a string constant that describes the date and time
of the last modification of the current source file. The string constant
contains abbreviated day of the week, month, day of the month, time in
hh:mm:ss form, year and looks like
"Sun Sep 16 01:03:52 1973".
If the day of the month is less than 10, it is padded with a space on the left.
If GCC cannot determine the current date, it will emit a warning message
(once per compilation) and
__TIMESTAMP__ will expand to
"??? ??? ?? ??:??:?? ????".
These macros are defined when the target processor supports atomic compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively.
This macro is defined with the value 1 to show that this version of GCC
This macro is defined when the compiler is emitting DWARF CFI directives to the assembler. When this is defined, it is possible to emit those same directives in inline assembly.
These macros are defined with value 1 if the backend supports the
fmal builtin functions, so that
the include file math.h can define the macros
for compatibility with the 1999 C standard.
These macros are defined with the value 1 if the backend supports the
fma functions using the additional
_Floatnx types that are defined in ISO/IEC TS
18661-3:2015. The include file math.h can define the
FP_FAST_FMAFnx macros if
the user defined
This macro is defined to indicate the intended level of support for
IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a
nonnegative integer value. If 0, it indicates that the combination of
the compiler configuration and the command-line options is not
intended to support IEEE 754 arithmetic for
double as defined in C99 and C11 Annex F (for example, that the
standard rounding modes and exceptions are not supported, or that
optimizations are enabled that conflict with IEEE 754 semantics). If
1, it indicates that IEEE 754 arithmetic is intended to be supported;
this does not mean that all relevant language features are supported
by GCC. If 2 or more, it additionally indicates support for IEEE
754-2008 (in particular, that the binary encodings for quiet and
signaling NaNs are as specified in IEEE 754-2008).
This macro does not indicate the default state of command-line options
that control optimizations that C99 and C11 permit to be controlled by
standard pragmas, where those standards do not require a particular
default state. It does not indicate whether optimizations respect
signaling NaN semantics (the macro for that is
__SUPPORT_SNAN__). It does not indicate support for decimal
floating point or the IEEE 754 binary16 and binary128 types.
This macro is defined to indicate the intended level of support for IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as defined in C99 and C11 Annex G. It expands to a nonnegative integer value. If 0, it indicates that the combination of the compiler configuration and the command-line options is not intended to support Annex G requirements (for example, because -fcx-limited-range was used). If 1 or more, it indicates that it is intended to support those requirements; this does not mean that all relevant language features are supported by GCC.
This macro is defined if -fno-math-errno is used, or enabled by another option such as -ffast-math or by default.
These macros are defined to expand to a narrow string literal of
the name of the narrow and wide compile-time execution character
set used. It directly reflects the name passed to the options
-fexec-charset and -fwide-exec-charset, or the defaults
documented for those options (that is, it can expand to something like
"UTF-8"). See Invocation.