osx man page for getgroups

GETGROUPS(2)						      BSD System Calls Manual						      GETGROUPS(2)

NAME
     getgroups -- get group access list

SYNOPSIS
     #include <unistd.h>

     int
     getgroups(int gidsetsize, gid_t grouplist[]);

DESCRIPTION
     getgroups() gets the current group access list of the current user process and stores it in the array grouplist[].  The parameter gidsetsize
     indicates the number of entries that may be placed in grouplist[].  getgroups() returns the actual number of groups returned in grouplist[].
     However, no more than {NGROUPS_MAX} will be returned. If gidsetsize is 0, getgroups() returns the number of groups without modifying the
     grouplist[] array.

     Calling initgroups(3) to opt-in for supplementary groups will cause getgroups() to return a single entry, the GID that was passed to
     initgroups(3).

     To provide compatibility with applications that use getgroups() in environments where users may be in more than {NGROUPS_MAX} groups, a vari-
     ant of getgroups(), obtained when compiling with either the macros _DARWIN_UNLIMITED_GETGROUPS or _DARWIN_C_SOURCE defined, can be used that
     is not limited to {NGROUPS_MAX} groups.  However, this variant only returns the user's default group access list and not the group list modi-
     fied by a call to setgroups(2) (either in the current process or an ancestor process).  Use of setgroups(2) is highly discouraged, and there
     is no foolproof way to determine if it has been previously called.

RETURN VALUES
     A successful call returns the number of groups in the group set.  Otherwise, a value of -1 is returned and the global integer variable errno
     is set to indicate the error.

ERRORS
     The possible errors for getgroups() are:

     [EFAULT]		The argument grouplist specifies an invalid address.

     [EINVAL]		The argument gidsetsize, although non-zero, is smaller than the number of groups in the group set.

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

     The include files <sys/param.h> and <sys/types.h> are necessary.

SEE ALSO
     setgroups(2), initgroups(3), compat(5)

HISTORY
     The getgroups() function call appeared in 4.2BSD.

4.2 Berkeley Distribution					 October 28, 2011					 4.2 Berkeley Distribution