osx man page for getuid

GETUID(2)						      BSD System Calls Manual							 GETUID(2)

NAME
     geteuid, getuid -- get user identification

SYNOPSIS
     #include <unistd.h>

     uid_t
     geteuid(void);

     uid_t
     getuid(void);

DESCRIPTION
     The getuid() function returns the real user ID of the calling process.  The geteuid() function returns the effective user ID of the calling
     process.

     The real user ID is that of the user who has invoked the program.	As the effective user ID gives the process additional permissions during
     execution of ``set-user-ID'' mode processes, getuid() is used to determine the real-user-id of the calling process.

ERRORS
     The getuid() and geteuid() functions are always successful, and no return value is reserved to indicate an error.

LEGACY SYNOPSIS
     #include <sys/types.h>
     #include <unistd.h>

     The include file <sys/types.h> is necessary for both functions.

SEE ALSO
     getgid(2), setreuid(2), compat(5)

STANDARDS
     Geteuid() and getuid() functions conform to IEEE Std 1003.1-1988 (``POSIX.1'').

4th Berkeley Distribution					   June 4, 1993 					 4th Berkeley Distribution