getgrent_r(3) Library Functions Manual getgrent_r(3)

getgrent_r, fgetgrent_r - get group file entry reentrantly

Standard C library (libc, -lc)

#include <grp.h>
int getgrent_r(struct group *restrict gbuf,
               char buf[restrict .buflen], size_t buflen,
               struct group **restrict gbufp);
int fgetgrent_r(FILE *restrict stream, struct group *restrict gbuf,
               char buf[restrict .buflen], size_t buflen,
               struct group **restrict gbufp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):




    Since glibc 2.19:
    glibc 2.19 and earlier:

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.

No more entries.
Insufficient buffer space supplied. Try again with larger buffer.

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

Interface Attribute Value
getgrent_r () Thread safety MT-Unsafe race:grent locale
fgetgrent_r () Thread safety MT-Safe

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.

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);


These functions are done in a style resembling the POSIX version of functions like getpwnam_r(3).

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 <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFLEN 4096
    struct group grp;
    struct group *grpp;
    char buf[BUFLEN];
    int i;
    while (1) {
        i = getgrent_r(&grp, buf, sizeof(buf), &grpp);
        if (i)
        printf("%s (%jd):", grpp->gr_name, (intmax_t) grpp->gr_gid);
        for (size_t j = 0; ; j++) {
            if (grpp->gr_mem[j] == NULL)
            printf(" %s", grpp->gr_mem[j]);

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

2024-05-02 Linux man-pages 6.8