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(): _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 (Linux: the
       CAP_SETGID  capability)  are required.  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 (it does not have the CAP_SETGID capability).

       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

       This  page  is  part  of  release  4.04  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
       http://www.kernel.org/doc/man-pages/.

Linux                                              2015-07-23                                       GETGROUPS(2)