access utmp file entries

       #include <utmp.h>

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

       struct utmp *pututline(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 func-

       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
       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
       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

       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, Linux
       libc5).  HP-UX introduces a new function _pututline() with  the  proto-
       type given above for pututline() (also found in Linux libc5).

       All   these   functions   are   obsolete   now  on  non-Linux  systems.
       POSIX.1-2001, 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 pro-
       vides 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

       Linux  glibc on the other hand does not use a parallel utmpx file since
       its utmp structure is already large enough.  The functions  getutxent()
       etc. are aliases for getutent() etc.

   Glibc Notes
       The above functions are not thread-safe.  Glibc adds reentrant versions

       #define _GNU_SOURCE    /* or _SVID_SOURCE or _BSD_SOURCE;
                                 see feature_test_macros(7) */
       #include <utmp.h>

       int getutent_r(struct utmp *ubuf, struct utmp **ubufp);

       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>

       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"));
           strcpy(entry.ut_user, getpwuid(getuid())->pw_name);
           memset(entry.ut_host, 0, UT_HOSTSIZE);
           entry.ut_addr = 0;

           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);

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


       getutmp(3), utmp(5)

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