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
   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 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
   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_member(3),  group(5),
   passwd(5)

COLOPHON

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   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/.

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