This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  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.opengroup.org/unix/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 .

IEEE/The Open Group                                   2017                                     GETGROUPS(3POSIX)

       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.

       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.

       Thefollowingsectionsareinformative.

GettingtheSupplementaryGroupIDsoftheCallingProcess
       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);

       None.

       getgroups — get supplementary group IDs

       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.

       The related function setgroups() is a privileged operation and therefore is not covered by this volume of
       POSIX.1‐2017.

       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.

       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.

getegid(), setgid()

       The Base Definitions volume of POSIX.1‐2017, <sys_types.h>, <unistd.h>

       #include <unistd.h>

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