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():
	   Since glibc 2.19:
	       _DEFAULT_SOURCE
	   Glibc 2.19 and earlier:
	       _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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       +---------------+---------------+----------------+
       |Interface      | Attribute     | Value		|
       +---------------+---------------+----------------+
       |getgrouplist() | Thread safety | MT-Safe locale |
       +---------------+---------------+----------------+
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_member(3), group(5), passwd(5)

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

GNU
								    2017-09-15							   GETGROUPLIST(3)