About _gnu_source macros

is a functional test macro for feature Control in Features.h

/user/include/features.h

/* If _gnu_source is defined by the user, turn on all the other features. */

#ifdef _gnu_source

# undef _isoc99_source

# define _isoc99_source 1

# undef _posix_source

# define _posix_source 1

# undef _posix_c_source

# define _posix_c_source 200809L

# undef _xopen_source

# define _xopen_source 700

# undef _xopen_source_extended

# define _xopen_source_extended 1

# undef _largefile64_source

# define _largefile64_source 1

# undef _bsd_source

# define _bsd_source 1

# undef _svid_source

# define _svid_source 1

# undef _atfile_source

# define _atfile_source 1

#endif

which _isoc99_source,? _posix_source,huh? _xopen_source are functional test macros,

An attribute that indicates whether a corresponding standard is included.

these different characteristics stem from the various standardized work that has been done over the years (ANSI,? ISO,? POSIX,huh? FIPS , etc.),

Different standard support implements different features,

such as the acquisition of system time,the stat structure is not supported in the ANSI Standard,

The definition of _gnu_source is equivalent to opening up support for all features.

About _gnu_source macros