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)):
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.
#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);
}
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.
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.
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.