Manpages - getutent.3

getutent, getutid, getutline, pututline, setutent, endutent, utmpname - access utmp file entries

  #include <utmp.h>

  struct utmp *getutent(void);
  struct utmp *getutid(const struct utmp *ut);
  struct utmp *getutline(const struct utmp *ut);

  struct utmp *pututline(const struct utmp *ut);

  void setutent(void);
  void endutent(void);

  int utmpname(const char *file);

New applications should use the POSIX.1-specified “utmpx” versions of these functions; see CONFORMING TO.

utmpname*() sets the name of the utmp-format file for the other utmp functions to access. If *utmpname*() is not used to set the filename before the other functions are used, they assume *_PATH_UTMP, as defined in <paths.h>.

*setutent*() rewinds the file pointer to the beginning of the utmp file. It is generally a good idea to call it before any of the other functions.

*endutent*() closes the utmp file. It should be called when the user code is done accessing the file with the other functions.

*getutent*() reads a line from the current file position in the utmp file. It returns a pointer to a structure containing the fields of the line. The definition of this structure is shown in *utmp*(5).

getutid*() searches forward from the current file position in the utmp file based upon ut. If ut->ut_type is one of *RUN_LVL, BOOT_TIME, NEW_TIME, or OLD_TIME, getutid*() will find the first entry whose ut_type field matches ut->ut_type. If ut->ut_type is one of *INIT_PROCESS, LOGIN_PROCESS, USER_PROCESS, or DEAD_PROCESS, *getutid*() will find the first entry whose ut_id field matches ut->ut_id.

getutline*() searches forward from the current file position in the utmp file. It scans entries whose ut_type is *USER_PROCESS or LOGIN_PROCESS and returns the first one whose ut_line field matches ut->ut_line.

*pututline*() writes the utmp structure ut into the utmp file. It uses *getutid*() to search for the proper place in the file to insert the new entry. If it cannot find an appropriate slot for ut, *pututline*() will append the new entry to the end of the file.

*getutent*(), *getutid*(), and *getutline*() return a pointer to a struct utmp on success, and NULL on failure (which includes the “record not found” case). This struct utmp is allocated in static storage, and may be overwritten by subsequent calls.

On success *pututline*() returns ut; on failure, it returns NULL.

*utmpname*() returns 0 if the new name was successfully stored, or -1 on failure.

On failure, these functions errno set to indicate the error.

ENOMEM

Out of memory.

ESRCH

Record not found.

*setutent*(), *pututline*(), and the *getut**() functions can also fail for the reasons described in *open*(2).

/var/run/utmp

database of currently logged-in users

/var/log/wtmp

database of past user logins

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

In the above table, utent in race:utent signifies that if any of the functions *setutent*(), *getutent*(), *getutid*(), *getutline*(), *pututline*(), *utmpname*(), or *endutent*() are used in parallel in different threads of a program, then data races could occur.

XPG2, SVr4.

In XPG2 and SVID 2 the function *pututline*() is documented to return void, and that is what it does on many systems (AIX, HP-UX). HP-UX introduces a new function *_pututline*() with the prototype given above for *pututline*().

All these functions are obsolete now on non-Linux systems. POSIX.1-2001 and POSIX.1-2008, following SUSv1, does not have any of these functions, but instead uses

    #include <utmpx.h>

    struct utmpx *getutxent(void);
    struct utmpx *getutxid(const struct utmpx *);
    struct utmpx *getutxline(const struct utmpx *);
    struct utmpx *pututxline(const struct utmpx *);
    void setutxent(void);
    void endutxent(void);

These functions are provided by glibc, and perform the same task as their equivalents without the “x”, but use struct utmpx, defined on Linux to be the same as struct utmp. For completeness, glibc also provides *utmpxname*(), although this function is not specified by POSIX.1.

On some other systems, the utmpx structure is a superset of the utmp structure, with additional fields, and larger versions of the existing fields, and parallel files are maintained, often /var/*/utmpx and /var/*/wtmpx.

Linux glibc on the other hand does not use a parallel utmpx file since its utmp structure is already large enough. The “x” functions listed above are just aliases for their counterparts without the “x” (e.g., *getutxent*() is an alias for *getutent*()).

The following example adds and removes a utmp record, assuming it is run from within a pseudo terminal. For usage in a real application, you should check the return values of *getpwuid*(3) and *ttyname*(3).

  #include <string.h>
  #include <stdlib.h>
  #include <pwd.h>
  #include <unistd.h>
  #include <utmp.h>
  #include <time.h>

  int
  main(int argc, char *argv[])
  {
      struct utmp entry;

      system("echo before adding entry:;who");

      entry.ut_type = USER_PROCESS;
      entry.ut_pid = getpid();
      strcpy(entry.ut_line, ttyname(STDIN_FILENO) + strlen("/dev/"));
      /* only correct for ptys named /dev/tty[pqr][0-9a-z] */
      strcpy(entry.ut_id, ttyname(STDIN_FILENO) + strlen("/dev/tty"));
      time(&entry.ut_time);
      strcpy(entry.ut_user, getpwuid(getuid())->pw_name);
      memset(entry.ut_host, 0, UT_HOSTSIZE);
      entry.ut_addr = 0;
      setutent();
      pututline(&entry);

      system("echo after adding entry:;who");

      entry.ut_type = DEAD_PROCESS;
      memset(entry.ut_line, 0, UT_LINESIZE);
      entry.ut_time = 0;
      memset(entry.ut_user, 0, UT_NAMESIZE);
      setutent();
      pututline(&entry);

      system("echo after removing entry:;who");

      endutent();
      exit(EXIT_SUCCESS);
  }

*getutmp*(3), *utmp*(5)

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