linux man page for getgrouplist

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  getgrou-
       plist(); 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 further 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  speci-
       fies 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>
", 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
",
		       ngroups);
	    exit(EXIT_FAILURE);
	   }

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

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

	   exit(EXIT_SUCCESS);
       }

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

COLOPHON
       This  page is part of release 3.27 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)