getgroups(2) — Linux manual page


getgroups(2)               System Calls Manual              getgroups(2)

NAME         top

       getgroups, setgroups - get/set list of supplementary group IDs

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

       int getgroups(int size, gid_t list[]);

       #include <grp.h>

       int setgroups(size_t size, const gid_t *_Nullable list);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.19:
           glibc 2.19 and earlier:

DESCRIPTION         top

       getgroups() returns the supplementary group IDs of the calling
       process in list.  The argument size should be set to the maximum
       number of items that can be stored in the buffer pointed to by
       list.  If the calling process is a member of more than size
       supplementary groups, then an error results.

       It is unspecified whether the effective group ID of the calling
       process is included in the returned list.  (Thus, an application
       should also call getegid(2) and add or remove the resulting

       If size is zero, list is not modified, but the total number of
       supplementary group IDs for the process is returned.  This allows
       the caller to determine the size of a dynamically allocated list
       to be used in a further call to getgroups().

       setgroups() sets the supplementary group IDs for the calling
       process.  Appropriate privileges are required (see the
       description of the EPERM error, below).  The size argument
       specifies the number of supplementary group IDs in the buffer
       pointed to by list.  A process can drop all of its supplementary
       groups with the call:

           setgroups(0, NULL);

RETURN VALUE         top

       On success, getgroups() returns the number of supplementary group
       IDs.  On error, -1 is returned, and errno is set to indicate the

       On success, setgroups() returns 0.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       EFAULT list has an invalid address.

       getgroups() can additionally fail with the following error:

       EINVAL size is less than the number of supplementary group IDs,
              but is not zero.

       setgroups() can additionally fail with the following errors:

       EINVAL size is greater than NGROUPS_MAX (32 before Linux 2.6.4;
              65536 since Linux 2.6.4).

       ENOMEM Out of memory.

       EPERM  The calling process has insufficient privilege (the caller
              does not have the CAP_SETGID capability in the user
              namespace in which it resides).

       EPERM (since Linux 3.19)
              The use of setgroups() is denied in this user namespace.
              See the description of /proc/pid/setgroups in

VERSIONS         top

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread
       attribute.  However, POSIX requires that all threads in a process
       share the same credentials.  The NPTL threading implementation
       handles the POSIX requirements by providing wrapper functions for
       the various system calls that change process UIDs and GIDs.
       These wrapper functions (including the one for setgroups())
       employ a signal-based technique to ensure that when one thread
       changes credentials, all of the other threads in the process also
       change their credentials.  For details, see nptl(7).

STANDARDS         top



HISTORY         top

              SVr4, 4.3BSD, POSIX.1-2001.

              SVr4, 4.3BSD.  Since setgroups() requires privilege, it is
              not covered by POSIX.1.

       The original Linux getgroups() system call supported only 16-bit
       group IDs.  Subsequently, Linux 2.4 added getgroups32(),
       supporting 32-bit IDs.  The glibc getgroups() wrapper function
       transparently deals with the variation across kernel versions.

NOTES         top

       A process can have up to NGROUPS_MAX supplementary group IDs in
       addition to the effective group ID.  The constant NGROUPS_MAX is
       defined in <limits.h>.  The set of supplementary group IDs is
       inherited from the parent process, and preserved across an

       The maximum number of supplementary group IDs can be found at run
       time using sysconf(3):

           long ngroups_max;
           ngroups_max = sysconf(_SC_NGROUPS_MAX);

       The maximum return value of getgroups() cannot be larger than one
       more than this value.  Since Linux 2.6.4, the maximum number of
       supplementary group IDs is also exposed via the Linux-specific
       read-only file, /proc/sys/kernel/ngroups_max.

SEE ALSO         top

       getgid(2), setgid(2), getgrouplist(3), group_member(3),
       initgroups(3), capabilities(7), credentials(7)

Linux man-pages (unreleased)     (date)                     getgroups(2)

Pages that refer to this page: capsh(1)groups(1@@shadow-utils)ps(1)unshare(1)syscalls(2)cap_get_proc(3)getgrouplist(3)group_member(3)id_t(3type)initgroups(3)credentials(7)nptl(7)path_resolution(7)signal-safety(7)user_namespaces(7)