NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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 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).

STANDARDS

       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 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)