GETGROUPS(3)               POSIX Programmer's Manual              GETGROUPS(3)



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

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 .



IEEE/The Open Group                  2017                         GETGROUPS(3)
