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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():
         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 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().

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>\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); }