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SETGROUPS(2) System Calls Manual SETGROUPS(2)


setgroupsset group access list


Standard C Library (libc, -lc)


#include <sys/param.h>
#include <unistd.h>

setgroups(int ngroups, const gid_t *gidset);


The () system call sets the group access list of the current user process according to the array gidset. The ngroups argument indicates the number of entries in the array and must be no more than {NGROUPS_MAX}+1.

Only the super-user may set a new group list.

The first entry of the group array (gidset[0]) is used as the effective group-ID for the process. This entry is over-written when a setgid program is run. To avoid losing access to the privileges of the gidset[0] entry, it should be duplicated later in the group array. By convention, this happens because the group value indicated in the password file also appears in /etc/group. The group value in the password file is placed in gidset[0] and that value then gets added a second time when the /etc/group file is scanned to create the group set.


The setgroups() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The setgroups() system call will fail if:

The caller is not the super-user.
The number specified in the ngroups argument is larger than the {NGROUPS_MAX}+1 limit.
The address specified for gidset is outside the process address space.


getgroups(2), initgroups(3)


The setgroups() system call appeared in 4.2BSD.

January 19, 2018 Debian