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Linux 2.6 - man page for getgroups (linux section 3posix)

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

       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 other-
       wise 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.

       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 IEEE Std 1003.1-2001.

       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 infor-
       mation.	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 mathemati-
       cal sense of "set"). (The history of a process and its parents could affect the details of
       the result.)

       Application  writers  should  note that {NGROUPS_MAX} is not necessarily a constant on all


       getegid() , setgid() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>,

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				     GETGROUPS(P)

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