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


       #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


       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

       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.


       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.


       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).


       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.


       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


       getgid(2), setgid(2), getgrouplist(3), group_member(3),  initgroups(3),
       capabilities(7), credentials(7)


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