PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       getgroups — get supplementary group IDs

SYNOPSIS

       #include <unistd.h>

       int getgroups(int gidsetsize, gid_t grouplist[]);

DESCRIPTION

       The  getgroups()  function  shall fill in the array grouplist with the current supplementary group IDs of
       the calling process. It is implementation-defined whether getgroups() also returns the effective group ID
       in the grouplist array.

       The  gidsetsize  argument  specifies the number of elements in the array grouplist.  The actual number of
       group IDs stored in the array shall be returned.  The values of array entries with indices  greater  than
       or equal to the value returned are undefined.

       If  gidsetsize  is  0,  getgroups()  shall  return the number of group IDs that it would otherwise return
       without modifying the array pointed to by grouplist.

       If the effective group ID of the process is returned with the supplementary group IDs, the value returned
       shall always be greater than or equal to one and less than or equal to the value of {NGROUPS_MAX}+1.

RETURN VALUE

       Upon successful completion, the number of supplementary group IDs shall be returned. A return value of −1
       indicates failure and errno shall be set to indicate the error.

ERRORS

       The getgroups() function shall fail if:

       EINVAL The gidsetsize argument is non-zero and less than the number of group IDs  that  would  have  been
              returned.

       The following sections are informative.

EXAMPLES

   Getting the Supplementary Group IDs of the Calling Process
       The  following  example  places the current supplementary group IDs of the calling process into the group
       array.

           #include <sys/types.h>
           #include <unistd.h>
           ...
           gid_t *group;
           int nogroups;
           long ngroups_max;

           ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1;
           group = (gid_t *)malloc(ngroups_max *sizeof(gid_t));

           ngroups = getgroups(ngroups_max, group);

APPLICATION USAGE

       None.

RATIONALE

       The related function setgroups() is a privileged operation and therefore is not covered by this volume of
       POSIX.1‐2008.

       As  implied  by  the  definition  of supplementary groups, the effective group ID may appear in the array
       returned by getgroups() or it may be  returned  only  by  getegid().   Duplication  may  exist,  but  the
       application  needs to call getegid() to be sure of getting all of the information. Various implementation
       variations and administrative sequences cause the set of groups appearing in the result of getgroups() to
       vary  in  order  and as to whether the effective group ID is included, even when the set of groups is the
       same (in the mathematical sense of ``set''). (The history of a process and its parents could  affect  the
       details of the result.)

       Application   developers   should   note  that  {NGROUPS_MAX}  is  not  necessarily  a  constant  on  all
       implementations.

FUTURE DIRECTIONS

       None.

SEE ALSO

       getegid(), setgid()

       The Base Definitions volume of POSIX.1‐2008, <sys_types.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event
       of any discrepancy between this version and the original IEEE and The Open Group Standard,  the  original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .