The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


getgroups - get supplementary group IDs


#include <unistd.h>

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


The getgroups() function shall fill in the array grouplist with the current supplementary group IDs of the calling process. It is implementation-defined whether getgroups() also returns the effective group ID in the grouplist array.

The gidsetsize argument specifies the number of elements in the array grouplist. The actual number of group IDs stored in the array shall be returned. The values of array entries with indices greater than or equal to the value returned are undefined.

If gidsetsize is 0, getgroups() shall return the number of group IDs that it would otherwise return without modifying the array pointed to by grouplist.

If the effective group ID of the process is returned with the supplementary group IDs, the value returned shall always be greater than or equal to one and less than or equal to the value of {NGROUPS_MAX}+1.


Upon successful completion, the number of supplementary group IDs shall be returned. A return value of -1 indicates failure and errno shall be set to indicate the error.


The getgroups() function shall fail if:

The gidsetsize argument is non-zero and less than the number of group IDs that would have been returned.

The following sections are informative.


Getting the Supplementary Group IDs of the Calling Process

The following example places the current supplementary group IDs of the calling process into the group array.

#include <sys/types.h>
#include <unistd.h>
gid_t *group;
int nogroups;
long ngroups_max;

ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1; group = (gid_t *)malloc(ngroups_max *sizeof(gid_t));
ngroups = getgroups(ngroups_max, group);




The related function setgroups() is a privileged operation and therefore is not covered by this volume of POSIX.1-2008.

As implied by the definition of supplementary groups, the effective group ID may appear in the array returned by getgroups() or it may be returned only by getegid(). Duplication may exist, but the application needs to call getegid() to be sure of getting all of the information. Various implementation variations and administrative sequences cause the set of groups appearing in the result of getgroups() to vary in order and as to whether the effective group ID is included, even when the set of groups is the same (in the mathematical sense of "set"). (The history of a process and its parents could affect the details of the result.)

Application developers should note that {NGROUPS_MAX} is not necessarily a constant on all implementations.




getegid, setgid

XBD <sys/types.h>, <unistd.h>


First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.

Issue 6

In the SYNOPSIS, the optional include of the <sys/types.h> header is removed.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes were made to align with the IEEE P1003.1a draft standard:

End of informative text.


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