getgrent_r, fgetgrent_r - get group file entry reentrantly

#include <grp.h>

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

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

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

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.

#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)