getgroups(3p) — Linux manual page

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

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(3p), setgid(3p)

       The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),
       unistd.h(0p)

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(3P)

Pages that refer to this page: unistd.h(0p), id(1p), open(3p)

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