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GETGROUPLIST(3) 		    Linux Programmer's Manual			  GETGROUPLIST(3)

NAME
       getgrouplist - get list of groups to which a user belongs

SYNOPSIS
       #include <grp.h>

       int getgrouplist(const char *user, gid_t group,
			gid_t *groups, int *ngroups);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getgrouplist(): _BSD_SOURCE

DESCRIPTION
       The  getgrouplist() function scans the group database (see group(5)) to obtain the list of
       groups that user belongs to.  Up to *ngroups of these groups are  returned  in  the  array
       groups.

       If  it  was  not  among	the  groups defined for user in the group database, then group is
       included in the list of groups returned by  getgrouplist();  typically  this  argument  is
       specified as the group ID from the password record for user.

       The  ngroups  argument is a value-result argument: on return it always contains the number
       of groups found for user, including group; this value may be greater than  the  number  of
       groups stored in groups.

RETURN VALUE
       If  the number of groups of which user is a member is less than or equal to *ngroups, then
       the value *ngroups is returned.

       If the user is a member of more than *ngroups groups, then getgrouplist() returns -1.   In
       this case the value returned in *ngroups can be used to resize the buffer passed to a fur-
       ther call getgrouplist().

VERSIONS
       This function is present since glibc 2.2.4.

CONFORMING TO
       This function is nonstandard; it appears on most BSDs.

BUGS
       In glibc versions before 2.3.3, the implementation of this  function  contains  a  buffer-
       overrun	bug:  it  returns  the complete list of groups for user in the array groups, even
       when the number of groups exceeds *ngroups.

EXAMPLE
       The program below displays the group list for the user named  in  its  first  command-line
       argument.   The second command-line argument specifies the ngroups value to be supplied to
       getgrouplist().	The following shell session shows examples of the use of this program:

	   $ ./a.out cecilia 0
	   getgrouplist() returned -1; ngroups = 3
	   $ ./a.out cecilia 3
	   ngroups = 3
	   16 (dialout)
	   33 (video)
	   100 (users)

   Program source

       #include <stdio.h>
       #include <stdlib.h>
       #include <grp.h>
       #include <pwd.h>

       int
       main(int argc, char *argv[])
       {
	   int j, ngroups;
	   gid_t *groups;
	   struct passwd *pw;
	   struct group *gr;

	   if (argc != 3) {
	       fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   ngroups = atoi(argv[2]);

	   groups = malloc(ngroups * sizeof (gid_t));
	   if (groups == NULL) {
	       perror("malloc");
	       exit(EXIT_FAILURE);
	   }

	   /* Fetch passwd structure (contains first group ID for user) */

	   pw = getpwnam(argv[1]);
	   if (pw == NULL) {
	       perror("getpwnam");
	       exit(EXIT_SUCCESS);
	   }

	   /* Retrieve group list */

	   if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
	       fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",
		       ngroups);
	    exit(EXIT_FAILURE);
	   }

	   /* Display list of retrieved groups, along with group names */

	   fprintf(stderr, "ngroups = %d\n", ngroups);
	   for (j = 0; j < ngroups; j++) {
	       printf("%d", groups[j]);
	       gr = getgrgid(groups[j]);
	       if (gr != NULL)
		   printf(" (%s)", gr->gr_name);
	       printf("\n");
	   }

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       getgroups(2), setgroups(2), getgrent(3), group(5), passwd(5)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU					    2008-07-03				  GETGROUPLIST(3)
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