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

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

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

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

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 namespace.  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 privilege, 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 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 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  get‐
       groups() 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)

Linux                                                                          2021-03-22                                                                   GETGROUPS(2)