getutent(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

GETUTENT(3)             Linux Programmer's Manual            GETUTENT(3)

NAME         top

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

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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.

RETURN VALUE         top

       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.

       In the event of an error, these functions errno set to indicate
       the cause.

ERRORS         top

       ENOMEM Out of memory.

       ESRCH  Record not found.

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

FILES         top

       /var/run/utmp
              database of currently logged-in users

       /var/log/wtmp
              database of past user logins

ATTRIBUTES         top

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

       ┌────────────┬───────────────┬──────────────────────────────┐
       │Interface   Attribute     Value                        │
       ├────────────┼───────────────┼──────────────────────────────┤
       │getutent()  │ Thread safety │ MT-Unsafe init race:utent    │
       │            │               │ race:utentbuf sig:ALRM timer │
       ├────────────┼───────────────┼──────────────────────────────┤
       │getutid(),  │ Thread safety │ MT-Unsafe init race:utent    │
       │getutline() │               │ sig:ALRM timer               │
       ├────────────┼───────────────┼──────────────────────────────┤
       │pututline() │ Thread safety │ MT-Unsafe race:utent         │
       │            │               │ sig:ALRM timer               │
       ├────────────┼───────────────┼──────────────────────────────┤
       │setutent(), │ Thread safety │ MT-Unsafe race:utent         │
       │endutent(), │               │                              │
       │utmpname()  │               │                              │
       └────────────┴───────────────┴──────────────────────────────┘
       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.

CONFORMING TO         top

       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()).

NOTES         top

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

       #include <utmp.h>

       int getutent_r(struct utmp *ubuf, struct utmp **ubufp);
       int getutid_r(struct utmp *ut,
                     struct utmp *ubuf, struct utmp **ubufp);
       int getutline_r(struct utmp *ut,
                       struct utmp *ubuf, struct utmp **ubufp);

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

       getutent_r(), getutid_r(), getutline_r():
           _GNU_SOURCE
           || /* since glibc 2.19: */ _DEFAULT_SOURCE
           || /* glibc <= 2.19: */    _SVID_SOURCE || _BSD_SOURCE

       These functions are GNU extensions, analogs of the functions of
       the same name without the _r suffix.  The ubuf argument gives
       these functions a place to store their result.  On success, they
       return 0, and a pointer to the result is written in *ubufp.  On
       error, these functions return -1.  There are no utmpx equivalents
       of the above functions.  (POSIX.1 does not specify such
       functions.)

EXAMPLES         top

       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);
       }

SEE ALSO         top

       getutmp(3), utmp(5)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

                               2020-06-09                    GETUTENT(3)

Pages that refer to this page: getlogin(3)getutmp(3)glob(3)login(3)updwtmp(3)wordexp(3)utmp(5)