__GNUC__
__GNUC_MINOR__
__GNUC_PATCHLEVEL__

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 3.2.1 will define

__GNUC__

to 3,

__GNUC_MINOR__

to 2, and

__GNUC_PATCHLEVEL__

to 1. These macros are also defined if you invoke the preprocessor directly.

__GNUC_PATCHLEVEL__

is new to GCC 3.0; it is also present in the widely-used development snapshots leading up to 3.0 (which identify themselves as GCC 2.96 or 2.97, depending on which snapshot you have).

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 (which happens rarely), 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 like this:

/* 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.

Reference: https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html