GETGROUPS(2)               Linux Programmer's Manual              GETGROUPS(2)

       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)):

           Since glibc 2.19:
           Glibc 2.19 and earlier:

       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.  A process can drop all of
       its supplementary groups with the call:

           setgroups(0, NULL);

       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 (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).

       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.

       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,

       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  re-
       quirements  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).

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

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       description  of  the project, information about reporting bugs, and the
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Linux                             2019-03-06                      GETGROUPS(2)
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