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.1‐2017.

       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.

FUTURE DIRECTIONS

       None.

SEE ALSO

       getegid(), setgid()

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

COPYRIGHT

       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 .