GETGRENT(3)

NAME
getgrent_r, fgetgrent_r - get group file entry reentrantly

SYNOPSIS
#define _GNU_SOURCE
#include <grp.h>

int getgrent_r(struct group *gbuf, char *buf,
               size_t buflen, struct group **gbufp);

int fgetgrent_r(FILE *fp, struct group *gbuf, char *buf,
                size_t buflen, struct group **gbufp);

DESCRIPTION
The functions getgrent_r() and fgetgrent_r() are the reentrant versions of getgrent(3) and fgetgrent(3). The former reads the next group entry from the stream initialized by setgrent(3). The latter reads the next group entry from the stream fp given as parameter.

The group structure is defined in <grp.h> as follows:

    struct group {
          char    *gr_name;      /* group name */
          char    *gr_passwd;    /* group password */
          gid_t   gr_gid;        /* group ID */
          char    **gr_mem;      /* group members */
    };

The non-reentrant functions return a pointer to static storage, where this static storage contains further pointers to group name, password and members. The reentrant functions described here return all of that in caller-provided buffers. First of all there is the buffer gbuf that can hold a struct group. And next the buffer buf of size buflen that can hold additional strings. The result of these functions, the struct group read from the stream, is stored in the provided buffer *gbuf, and a pointer to this struct group is returned in *gbufp.

RETURN VALUE
On success, these functions return 0 and *gbufp is a pointer to the struct group. On error, these functions return an error value and *gbufp is NULL.

ERRORS

ENOENT
    No more entries.

ERANGE
    Insufficient buffer space supplied. Try again with larger buffer.

EXAMPLE
#define _GNU_SOURCE
#include <grp.h>
#include <stdio.h>
#define BUFLEN 4096

int main() {
      struct group grp, *grpp;
      char buf[BUFLEN];
      int i;

      setgrent();
      while (1) {
            i = getgrent_r(&grp, buf, BUFLEN, &grpp);
            if (i)
                  break;
            printf("%s (%d):", grpp->gr_name, grpp->gr_gid);
            for (i = 0; ; i++) {
                  if (grpp->gr_mem[i] == NULL)
                        break;
                  printf(" %s", grpp->gr_mem[i]);
            }
            printf("\n");
      }
      endgrent();
      return 0;
}

CONFORMING TO
These functions are GNU extensions, done in a style resembling the POSIX version of functions like getpwnam_r(3). Other systems use prototype

struct group *
getgrent_r(struct group *grp, char *buf, int buflen);

or, better,

int
getgrent_r(struct group *grp, char *buf, int buflen,
           FILE **gr_fp);

NOTES
The function getgrent_r() is not really reentrant since it shares the reading position in the stream with all other threads.

SEE ALSO
fgetgrent(3), getgrent(3), getgrgid(3), getgrnam(3), putgrent(3), group(5)