# getgroups

## 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.
    


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