man3/getgrent_r.3

.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl)
.\"
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.TH GETGRENT_R 3 2020-06-09 "GNU" "Linux Programmer's Manual"
.SH NAME
getgrent_r, fgetgrent_r \- get group file entry reentrantly
.SH SYNOPSIS
.nf
.B #include <grp.h>
.PP
.BI "int getgrent_r(struct group *" gbuf ", char *" buf ,
.BI "               size_t " buflen ", struct group **" gbufp );
.PP
.BI "int fgetgrent_r(FILE *" stream ", struct group *" gbuf ", char *" buf ,
.BI "                size_t " buflen ", struct group **" gbufp );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR getgrent_r ():
_GNU_SOURCE
.\" FIXME . The FTM requirements seem inconsistent here.  File a glibc bug?
.br
.BR fgetgrent_r ():
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _SVID_SOURCE
.SH DESCRIPTION
The functions
.BR getgrent_r ()
and
.BR fgetgrent_r ()
are the reentrant versions of
.BR getgrent (3)
and
.BR fgetgrent (3).
The former reads the next group entry from the stream initialized by
.BR setgrent (3).
The latter reads the next group entry from
.IR stream .
.PP
The \fIgroup\fP structure is defined in
.I <grp.h>
as follows:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
For more information about the fields of this structure, see
.BR group (5).
.PP
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
.I gbuf
that can hold a \fIstruct group\fP.
And next the buffer
.I buf
of size
.I buflen
that can hold additional strings.
The result of these functions, the \fIstruct group\fP read from the stream,
is stored in the provided buffer
.IR *gbuf ,
and a pointer to this \fIstruct group\fP is returned in
.IR *gbufp .
.SH RETURN VALUE
On success, these functions return 0 and
.I *gbufp
is a pointer to the \fIstruct group\fP.
On error, these functions return an error value and
.I *gbufp
is NULL.
.SH ERRORS
.TP
.B ENOENT
No more entries.
.TP
.B ERANGE
Insufficient buffer space supplied.
Try again with larger buffer.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbw27
l l l.
Interface	Attribute	Value
T{
.BR getgrent_r ()
T}	Thread safety	MT-Unsafe race:grent locale
T{
.BR fgetgrent_r ()
T}	Thread safety	MT-Safe
.TE
.sp 1
In the above table,
.I grent
in
.I race:grent
signifies that if any of the functions
.BR setgrent (3),
.BR getgrent (3),
.BR endgrent (3),
or
.BR getgrent_r ()
are used in parallel in different threads of a program,
then data races could occur.
.SH CONFORMING TO
These functions are GNU extensions, done in a style resembling
the POSIX version of functions like
.BR getpwnam_r (3).
Other systems use the prototype
.PP
.in +4n
.EX
struct group *getgrent_r(struct group *grp, char *buf,
                         int buflen);
.EE
.in
.PP
or, better,
.PP
.in +4n
.EX
int getgrent_r(struct group *grp, char *buf, int buflen,
               FILE **gr_fp);
.EE
.in
.SH NOTES
The function
.BR getgrent_r ()
is not really reentrant since it shares the reading position
in the stream with all other threads.
.SH EXAMPLES
.EX
#define _GNU_SOURCE
#include <grp.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFLEN 4096

int
main(void)
{
    struct group grp, *grpp;
    char buf[BUFLEN];
    int i;

    setgrent();
    while (1) {
        i = getgrent_r(&grp, buf, BUFLEN, &grpp);
        if (i)
            break;
        printf("%s (%d):", grpp\->gr_name, grpp\->gr_gid);
        for (i = 0; ; i++) {
            if (grpp\->gr_mem[i] == NULL)
                break;
            printf(" %s", grpp\->gr_mem[i]);
        }
        printf("\en");
    }
    endgrent();
    exit(EXIT_SUCCESS);
}
.EE
.\" perhaps add error checking - should use strerror_r
.\" #include <errno.h>
.\" #include <stdlib.h>
.\"         if (i) {
.\"               if (i == ENOENT)
.\"                     break;
.\"               printf("getgrent_r: %s", strerror(i));
.\"               exit(EXIT_FAILURE);
.\"         }
.SH SEE ALSO
.BR fgetgrent (3),
.BR getgrent (3),
.BR getgrgid (3),
.BR getgrnam (3),
.BR putgrent (3),
.BR group (5)