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

       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.

       getgroups — get supplementary group IDs

       #include <unistd.h>

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

       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

       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.

       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.

   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);


       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.


       getegid(), setgid()

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

       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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
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IEEE/The Open Group                  2013                        GETGROUPS(3P)