getpwent_r, fgetpwent_r - get passwd file entry reentrantly

       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.

       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 structpasswd.  And next the buffer buf of size buflen that can hold additional strings.
       The result of these functions, the structpasswd read from the stream, is stored in the  provided  buffer
       *pwbuf, and a pointer to this structpasswd is returned in *pwbufp.

ENOENT No more entries.

       ERANGE Insufficient buffer space supplied.  Try again with larger buffer.

       #define _GNU_SOURCE
       #include <pwd.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>

       #define BUFLEN 4096

       int
       main(void)
       {
           struct passwd pw;
           struct passwd *pwp;
           char buf[BUFLEN];
           int i;

           setpwent();
           while (1) {
               i = getpwent_r(&pw, buf, sizeof(buf), &pwp);
               if (i)
                   break;
               printf("%s (%jd)\tHOME %s\tSHELL %s\n", pwp->pw_name,
                      (intmax_t) pwp->pw_uid, pwp->pw_dir, pwp->pw_shell);
           }
           endpwent();
           exit(EXIT_SUCCESS);
       }

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

       Standard C library (libc, -lc)

       getpwent_r, fgetpwent_r - get passwd file entry reentrantly

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

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

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

Linux man-pages 6.9.1                              2024-06-15                                      getpwent_r(3)

       None.

#include<pwd.h>intgetpwent_r(structpasswd*restrictpwbuf,charbuf[restrict.buflen],size_tbuflen,structpasswd**restrictpwbufp);intfgetpwent_r(FILE*restrictstream,structpasswd*restrictpwbuf,charbuf[restrict.buflen],size_tbuflen,structpasswd**restrictpwbufp);

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

       getpwent_r(),
           Since glibc 2.19:
               _DEFAULT_SOURCE
           glibc 2.19 and earlier:
               _BSD_SOURCE || _SVID_SOURCE

       fgetpwent_r():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           glibc 2.19 and earlier:
               _SVID_SOURCE

       Other systems use the prototype

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

       or, better,

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