GETGROUPS(2)              (2019-03-06)               GETGROUPS(2)

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

     SYNOPSIS
          #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)):

          setgroups():
              Since glibc 2.19:
                  _DEFAULT_SOURCE
              Glibc 2.19 and earlier:
                  _BSD_SOURCE

     DESCRIPTION
          getgroups() returns the supplementary group IDs of the call-
          ing 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 dynami-
          cally 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);

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     RETURN VALUE
          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
          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 names-
               pace.  See the description of /proc/[pid]/setgroups in
               user_namespaces(7).

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

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

     NOTES
          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 supplemen-
          tary 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):

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              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 maxi-
          mum number of supplementary 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
          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).

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

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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