Unlike the older "SysV IPC" semaphores (semctl, semop, etc), the POSIX semaphore API is not implemented directly by the kernel. Instead, the C library implements it on top of the futex generic synchronization primitive.

The code you are looking for is in the nptl subdirectory of the GNU libc source tree. You may also have to go digging around in sysdeps to find all of it.

It allows you to use functions that are not part of the standard C library but are part of the POSIX.1 (IEEE Standard 1003.1) standard. Using the macros described in feature_test_macros allows you to control the definitions exposed by the system header files.

As far as I know _POSIX_SOURCE is obsolete and you should use _POSIX_C_SOURCE instead.

For example, if you want to use strndup, you have to use

#define _POSIX_C_SOURCE 200809L

The numbers to use are defined here:

# All numbers >= to the below

1       : then the 1990 edition of the POSIX.1  standard (IEEE Standard 1003.1-1990)  is made available.
2       : then the 1992 edition of the POSIX.2  standard (IEEE Standard 1003.2-1992)  is made available.
199309L : then the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.
199506L : then the 1995 edition of the POSIX.1c standard (IEEE Standard 1003.1c-1995) is made available.
200112L : then the 2001 edition of the POSIX    standard (IEEE Standard 1003.1-2001)  is made available.
200809L : then the 2008 edition of the POSIX    standard (IEEE Standard 1003.1-2008)  is made available
202405L : then the 2024 edition of the POSIX    standard (IEEE Standard 1003.1-2024) is made available. 

See also

• 1.3.4 Feature Test Macros
• 5.12 Posix Variants
• man 7 feature_test_macros

It's there under "dot".

NAME

dot - execute commands in the current environment

SYNOPSIS

. file

[etc.]