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GETGROUPS(2)               Linux Programmer's Manual              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  supple-
       mentary  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
       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  addi-
       tion 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 supplemen-
       tary 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 4.15 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/.
Linux                             2017-09-15                      GETGROUPS(2)