Manpages - getpwnam.3

getpwnam, getpwnam_r, getpwuid, getpwuid_r - get password file entry

  #include <sys/types.h>
  #include <pwd.h>

  struct passwd *getpwnam(const char *name);
  struct passwd *getpwuid(uid_t uid);

  int getpwnam_r(const char *restrict name",structpasswd*restrict"pwd,
   char *restrict buf, size_t buflen,
   struct passwd **restrict result);
  int getpwuid_r(uid_t uid, struct passwd *restrict pwd,
   char *restrict buf, size_t buflen,
   struct passwd **restrict result);

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

*getpwnam_r*(), *getpwuid_r*():

      _POSIX_C_SOURCE
          || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

The *getpwnam*() function returns a pointer to a structure containing the broken-out fields of the record in the password database (e.g., the local password file /etc/passwd, NIS, and LDAP) that matches the username name.

The *getpwuid*() function returns a pointer to a structure containing the broken-out fields of the record in the password database that matches the user ID uid.

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 */
  };

See *passwd*(5) for more information about these fields.

The *getpwnam_r*() and *getpwuid_r*() functions obtain the same information as *getpwnam*() and *getpwuid*(), but store the retrieved passwd structure in the space pointed to by pwd. The string fields pointed to by the members of the passwd structure are stored in the buffer buf of size buflen. A pointer to the result (in case of success) or NULL (in case no entry was found or an error occurred) is stored in *result.

The call

sysconf(_SC_GETPW_R_SIZE_MAX)

returns either -1, without changing errno, or an initial suggested size for buf. (If this size is too small, the call fails with ERANGE, in which case the caller can retry with a larger buffer.)

The *getpwnam*() and *getpwuid*() functions return a pointer to a passwd structure, or NULL if the matching entry is not found or an error occurs. If an error occurs, errno is set to indicate the error. If one wants to check errno after the call, it should be set to zero before the call.

The return value may point to a static area, and may be overwritten by subsequent calls to *getpwent*(3), *getpwnam*(), or *getpwuid*(). (Do not pass the returned pointer to *free*(3).)

On success, *getpwnam_r*() and *getpwuid_r*() return zero, and set *result to pwd. If no matching password record was found, these functions return 0 and store NULL in *result. In case of error, an error number is returned, and NULL is stored in *result.

0 or ENOENT or ESRCH or EBADF or EPERM or …

The given name or uid was not found.

EINTR

A signal was caught; see *signal*(7).

EIO

I/O error.

EMFILE

The per-process limit on the number of open file descriptors has been reached.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENOMEM

Insufficient memory to allocate passwd structure.

ERANGE

Insufficient buffer space supplied.

/etc/passwd

local password database file

For an explanation of the terms used in this section, see *attributes*(7).

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. The pw_gecos field is not specified in POSIX, but is present on most implementations.

The formulation given above under “RETURN VALUE” is from POSIX.1-2001. It does not call “not found” an error, and hence does not specify what value errno might have in this situation. But that makes it impossible to recognize errors. One might argue that according to POSIX errno should be left unchanged if an entry is not found. Experiments on various UNIX-like systems show that lots of different values occur in this situation: 0, ENOENT, EBADF, ESRCH, EWOULDBLOCK, EPERM, and probably others.

The pw_dir field contains the name of the initial working directory of the user. Login programs use the value of this field to initialize the HOME environment variable for the login shell. An application that wants to determine its user’s home directory should inspect the value of HOME (rather than the value getpwuid(getuid())->pw_dir) since this allows the user to modify their notion of “the home directory” during a login session. To determine the (initial) home directory of another user, it is necessary to use getpwnam(“username”)->pw_dir or similar.

The program below demonstrates the use of *getpwnam_r*() to find the full username and user ID for the username supplied as a command-line argument.

  #include <pwd.h>
  #include <stdint.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <errno.h>

  int
  main(int argc, char *argv[])
  {
      struct passwd pwd;
      struct passwd *result;
      char *buf;
      size_t bufsize;
      int s;

      if (argc != 2) {
          fprintf(stderr, "Usage: %s username\n", argv[0]);
          exit(EXIT_FAILURE);
      }

      bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
      if (bufsize == -1)          /* Value was indeterminate */
          bufsize = 16384;        /* Should be more than enough */

      buf = malloc(bufsize);
      if (buf == NULL) {
          perror("malloc");
          exit(EXIT_FAILURE);
      }

      s = getpwnam_r(argv[1], &pwd, buf, bufsize, &result);
      if (result == NULL) {
          if (s == 0)
              printf("Not found\n");
          else {
              errno = s;
              perror("getpwnam_r");
          }
          exit(EXIT_FAILURE);
      }

      printf("Name: %s; UID: %jd\n", pwd.pw_gecos,
              (intmax_t) pwd.pw_uid);
      exit(EXIT_SUCCESS);
  }

*endpwent*(3), *fgetpwent*(3), *getgrnam*(3), *getpw*(3), *getpwent*(3), *getspnam*(3), *putpwent*(3), *setpwent*(3), *passwd*(5)

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