getgrent_r3

Usage

#include <grp.h>

    _GNU_SOURCE

    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _SVID_SOURCE

Library

Standard C library (libc, -lc)

Overview

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 size 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

Attributes

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.

Versions

Other systems use the prototype

struct group *getgrent_r(struct group *grp, char buf[.size],
                         int size);

or, better,

int getgrent_r(struct group *grp, char buf[.size], int size,
               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.

Examples

#define _GNU_SOURCE
#include <grp.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFLEN 4096
int
main(void)
{
    struct group grp;
    struct group *grpp;
    char buf[BUFLEN];
    int i;
    setgrent();
    for (;;) {
        i = getgrent_r(&grp, buf, sizeof(buf), &grpp);
        if (i)
            break;
        printf("%s (%jd):", grpp->gr_name, (intmax_t) grpp->gr_gid);
        for (size_t j = 0; ; j++) {
            if (grpp->gr_mem[j] == NULL)
                break;
            printf(" %s", grpp->gr_mem[j]);
        }
        printf("\n");
    }
    endgrent();
    exit(EXIT_SUCCESS);
}