getgrent_r

Section: C Library Functions (3)
Updated: 2026-02-08
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NAME

getgrent_r, fgetgrent_r - get group file entry reentrantly  

LIBRARY

Standard C library (libc,~-lc)  

SYNOPSIS

#include <grp.h>
int getgrent_r(size_t size;
               struct group *restrict gbuf,
               char buf[restrict size], size_t size,
               struct group **restrict gbufp);
int fgetgrent_r(size_t size;
               FILE *restrict stream, struct group *restrict gbuf,
               char buf[restrict size], size_t size,
               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

 

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 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

ENOENT

No more entries.

ERANGE

Insufficient buffer space supplied. Try again with larger buffer.

 

ATTRIBUTES

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.  

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);  

STANDARDS

GNU.  

HISTORY

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

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();
    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 (size_t j = 0; ; j++) {
            if (grpp->gr_mem[j] == NULL)
                break;
            printf(" %s", grpp->gr_mem[j]);
        }
        printf("[rs]n");
    }
    endgrent();
    exit(EXIT_SUCCESS); }  

SEE ALSO

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

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

VERSIONS

STANDARDS

HISTORY

NOTES

EXAMPLES

SEE ALSO