setgroups32(2) — Linux manual page


GETGROUPS(2)              Linux Programmer's Manual             GETGROUPS(2)

NAME         top

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

SYNOPSIS         top

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

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

       #include <grp.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           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 value.)

       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 appropriately.

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

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

CONFORMING TO         top

       getgroups(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

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

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 execve(2).

       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 sup‐
       plementary group IDs is also exposed via the Linux-specific read-only
       file, /proc/sys/kernel/ngroups_max.

       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.

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread attri‐
       bute.  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 func‐
       tions (including the one for setgroups()) employ a signal-based tech‐
       nique to ensure that when one thread changes credentials, all of the
       other threads in the process also change their credentials.  For de‐
       tails, see nptl(7).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                     GETGROUPS(2)

Pages that refer to this page: syscalls(2)