Manpages - getgrent_r.3

getgrent_r, fgetgrent_r - get group file entry reentrantly

  #include <grp.h>

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

Feature Test Macro Requirements for glibc (see *feature_test_macros*(7)):

*getgrent_r*():

      _GNU_SOURCE

*fgetgrent_r*(): Since glibc 2.19: _DEFAULT_SOURCE Glibc 2.19 and earlier: _SVID_SOURCE

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 stream.

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;         /* NULL-terminated array of pointers
                                 to names of group members */
  };

For more information about the fields of this structure, see *group*(5).

The nonreentrant 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.

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.

ENOENT

No more entries.

ERANGE

Insufficient buffer space supplied. Try again with larger buffer.

For an explanation of the terms used in this section, see *attributes*(7).

In the above table, grent in race:grent signifies that if any of the functions *setgrent*(3), *getgrent*(3), *endgrent*(3), or *getgrent_r*() are used in parallel in different threads of a program, then data races could occur.

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

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

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

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

      setgrent();
      while (1) {
          i = getgrent_r(&grp, buf, sizeof(buf), &grpp);
          if (i)
              break;
          printf("%s (%jd):", grpp->gr_name, (intmax_t) grpp->gr_gid);
          for (int j = 0; ; j++) {
              if (grpp->gr_mem[j] == NULL)
                  break;
              printf(" %s", grpp->gr_mem[j]);
          }
          printf("\n");
      }
      endgrent();
      exit(EXIT_SUCCESS);
  }

*fgetgrent*(3), *getgrent*(3), *getgrgid*(3), *getgrnam*(3), *putgrent*(3), *group*(5)

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