getpwent_r(3) Library Functions Manual getpwent_r(3)

getpwent_r, fgetpwent_r - get passwd file entry reentrantly

Standard C library (libc, -lc)

#include <pwd.h>
int getpwent_r(struct passwd *restrict pwbuf,
               char buf[restrict .buflen], size_t buflen,
               struct passwd **restrict pwbufp);
int fgetpwent_r(FILE *restrict stream, struct passwd *restrict pwbuf,
               char buf[restrict .buflen], size_t buflen,
               struct passwd **restrict pwbufp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    Since glibc 2.19:
    glibc 2.19 and earlier:


    Since glibc 2.19:
    glibc 2.19 and earlier:

The functions getpwent_r() and fgetpwent_r() are the reentrant versions of getpwent(3) and fgetpwent(3). The former reads the next passwd entry from the stream initialized by setpwent(3). The latter reads the next passwd entry from stream.

The passwd structure is defined in <pwd.h> as follows:

struct passwd {
    char    *pw_name;      /* username */
    char    *pw_passwd;    /* user password */
    uid_t    pw_uid;       /* user ID */
    gid_t    pw_gid;       /* group ID */
    char    *pw_gecos;     /* user information */
    char    *pw_dir;       /* home directory */
    char    *pw_shell;     /* shell program */

For more information about the fields of this structure, see passwd(5).

The nonreentrant functions return a pointer to static storage, where this static storage contains further pointers to user name, password, gecos field, home directory and shell. The reentrant functions described here return all of that in caller-provided buffers. First of all there is the buffer pwbuf that can hold a struct passwd. And next the buffer buf of size buflen that can hold additional strings. The result of these functions, the struct passwd read from the stream, is stored in the provided buffer *pwbuf, and a pointer to this struct passwd is returned in *pwbufp.

On success, these functions return 0 and *pwbufp is a pointer to the struct passwd. On error, these functions return an error value and *pwbufp is NULL.

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
getpwent_r () Thread safety MT-Unsafe race:pwent locale
fgetpwent_r () Thread safety MT-Safe

In the above table, pwent in race:pwent signifies that if any of the functions setpwent(), getpwent(), endpwent(), or getpwent_r() are used in parallel in different threads of a program, then data races could occur.

Other systems use the prototype

struct passwd *
getpwent_r(struct passwd *pwd, char *buf, int buflen);

or, better,

getpwent_r(struct passwd *pwd, char *buf, int buflen,
           FILE **pw_fp);


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

The function getpwent_r() is not really reentrant since it shares the reading position in the stream with all other threads.

#define _GNU_SOURCE
#include <pwd.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFLEN 4096
    struct passwd pw;
    struct passwd *pwp;
    char buf[BUFLEN];
    int i;
    while (1) {
        i = getpwent_r(&pw, buf, sizeof(buf), &pwp);
        if (i)
        printf("%s (%jd)\tHOME %s\tSHELL %s\n", pwp->pw_name,
               (intmax_t) pwp->pw_uid, pwp->pw_dir, pwp->pw_shell);

fgetpwent(3), getpw(3), getpwent(3), getpwnam(3), getpwuid(3), putpwent(3), passwd(5)

2024-06-15 Linux man-pages 6.9.1