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getgrouplist(3)            Library Functions Manual            getgrouplist(3)

NAME
       getgrouplist - get list of groups to which a user belongs

LIBRARY
       Standard C library (libc, -lc)

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 to getgrou-
       plist().

VERSIONS
       This function is present since glibc 2.2.4.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       ┌─────────────────────────────────────┬───────────────┬────────────────┐
       │InterfaceAttributeValue          │
       ├─────────────────────────────────────┼───────────────┼────────────────┤
       │getgrouplist()                       │ Thread safety │ MT-Safe locale │
       └─────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS
       This function is nonstandard; it appears on most BSDs.

BUGS
       Before glibc 2.3.3, the implementation of this function contains a buf-
       fer-overrun bug: it returns the complete list of groups for user in the
       array groups, even when the number of groups exceeds *ngroups.

EXAMPLES
       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 <grp.h>
       #include <pwd.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(int argc, char *argv[])
       {
           int ngroups;
           struct passwd *pw;
           struct group *gr;
           gid_t *groups;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           ngroups = atoi(argv[2]);

           groups = malloc(sizeof(*groups) * ngroups);
           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 (size_t 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)

Linux man-pages 6.03              2023-02-05                   getgrouplist(3)

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