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 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.

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  namespace.   See
          the description of /proc/[pid]/setgroups in user_namespaces(7).

CONFORMING TO

   SVr4,   4.3BSD.   The  getgroups()  function  is  in  POSIX.1-2001  and
   POSIX.1-2008.  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)

COLOPHON

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   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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