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

       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.

       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.1‐2017.

       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.


       getegid(), setgid()

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

       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 .

       Any typographical or formatting errors that appear in this page are most
       likely to have been introduced during the conversion of the source files
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IEEE/The Open Group                   2017                         GETGROUPS(3P)