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

       getgrouplist - get list of groups to which a user belongs

       #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

       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.

       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 getgrou‐
       plist() returns -1.  In this case, the value returned in	 *ngroups  can
       be used to resize the buffer passed to a further call getgrouplist().

       This function is present since glibc 2.2.4.

       This function is nonstandard; it appears on most BSDs.

       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

       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>

       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]);

	   ngroups = atoi(argv[2]);

	   groups = malloc(ngroups * sizeof (gid_t));
	   if (groups == NULL) {

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

	   pw = getpwnam(argv[1]);
	   if (pw == NULL) {

	   /* Retrieve group list */

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

	   /* 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);


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

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

GNU				  2008-07-03		       GETGROUPLIST(3)

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