path: root/man3/getutent.3

'\" t
.\" Copyright 1995 Mark D. Roth (roth@uiuc.edu)
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Solaris manpages
.\"
.\" Modified Thu Jul 25 14:43:46 MET DST 1996 by Michael Haardt
.\"     <michael@cantor.informatik.rwth-aachen.de>
.\"
.TH getutent 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
getutent, getutid, getutline, pututline, setutent, endutent,
utmpname \- access utmp file entries
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <utmp.h>
.PP
.B struct utmp *getutent(void);
.BI "struct utmp *getutid(const struct utmp *" ut );
.BI "struct utmp *getutline(const struct utmp *" ut );
.PP
.BI "struct utmp *pututline(const struct utmp *" ut );
.PP
.B void setutent(void);
.B void endutent(void);
.PP
.BI "int utmpname(const char *" file );
.fi
.SH DESCRIPTION
New applications should use the POSIX.1-specified "utmpx" versions of
these functions; see STANDARDS.
.PP
.BR utmpname ()
sets the name of the utmp-format file for the other utmp
functions to access.
If
.BR utmpname ()
is not used to set the filename
before the other functions are used, they assume \fB_PATH_UTMP\fP, as
defined in \fI<paths.h>\fP.
.PP
.BR 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.
.PP
.BR endutent ()
closes the utmp file.
It should be called when the user
code is done accessing the file with the other functions.
.PP
.BR 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
.BR utmp (5).
.PP
.BR getutid ()
searches forward from the current file position in the utmp
file based upon \fIut\fP.
If \fIut\->ut_type\fP is one of \fBRUN_LVL\fP,
\fBBOOT_TIME\fP, \fBNEW_TIME\fP, or \fBOLD_TIME\fP,
.BR getutid ()
will
find the first entry whose \fIut_type\fP field matches \fIut\->ut_type\fP.
If \fIut\->ut_type\fP is one of \fBINIT_PROCESS\fP, \fBLOGIN_PROCESS\fP,
\fBUSER_PROCESS\fP, or \fBDEAD_PROCESS\fP,
.BR getutid ()
will find the
first entry whose
.I ut_id
field matches \fIut\->ut_id\fP.
.PP
.BR getutline ()
searches forward from the current file position in the utmp file.
It scans entries whose
.I ut_type
is \fBUSER_PROCESS\fP
or \fBLOGIN_PROCESS\fP and returns the first one whose
.I ut_line
field
matches \fIut\->ut_line\fP.
.PP
.BR pututline ()
writes the
.I utmp
structure \fIut\fP into the utmp file.
It uses
.BR getutid ()
to search for the proper place in the file to insert
the new entry.
If it cannot find an appropriate slot for \fIut\fP,
.BR pututline ()
will append the new entry to the end of the file.
.SH RETURN VALUE
.BR getutent (),
.BR getutid (),
and
.BR getutline ()
return a pointer to a \fIstruct utmp\fP on success,
and NULL on failure (which includes the "record not found" case).
This \fIstruct utmp\fP is allocated in static storage, and may be
overwritten by subsequent calls.
.PP
On success
.BR pututline ()
returns
.IR ut ;
on failure, it returns NULL.
.PP
.BR utmpname ()
returns 0 if the new name was successfully stored, or \-1 on failure.
.PP
On failure, these functions
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B ENOMEM
Out of memory.
.TP
.B ESRCH
Record not found.
.PP
.BR setutent (),
.BR pututline (),
and the
.BR getut* ()
functions can also fail for the reasons described in
.BR open (2).
.SH FILES
.TP
.I /var/run/utmp
database of currently logged-in users
.TP
.I /var/log/wtmp
database of past user logins
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR getutent ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe init race:utent
race:utentbuf sig:ALRM timer
T}
T{
.na
.nh
.BR getutid (),
.BR getutline ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe init race:utent
sig:ALRM timer
T}
T{
.na
.nh
.BR pututline ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:utent
sig:ALRM timer
T}
T{
.na
.nh
.BR setutent (),
.BR endutent (),
.BR utmpname ()
T}	Thread safety	MT-Unsafe race:utent
.TE
.sp 1
In the above table,
.I utent
in
.I race:utent
signifies that if any of the functions
.BR setutent (),
.BR getutent (),
.BR getutid (),
.BR getutline (),
.BR pututline (),
.BR utmpname (),
or
.BR endutent ()
are used in parallel in different threads of a program,
then data races could occur.
.SH STANDARDS
None.
.SH HISTORY
XPG2, SVr4.
.PP
In XPG2 and SVID 2 the function
.BR pututline ()
is documented to return void, and that is what it does on many systems
(AIX, HP-UX).
HP-UX introduces a new function
.BR _pututline ()
with the prototype given above for
.BR pututline ().
.PP
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
.PP
.RS 4
.EX
.B #include <utmpx.h>
.PP
.B struct utmpx *getutxent(void);
.B struct utmpx *getutxid(const struct utmpx *);
.B struct utmpx *getutxline(const struct utmpx *);
.B struct utmpx *pututxline(const struct utmpx *);
.B void setutxent(void);
.B void endutxent(void);
.EE
.RE
.PP
These functions are provided by glibc,
and perform the same task as their equivalents without the "x", but use
.IR "struct utmpx" ,
defined on Linux to be the same as
.IR "struct utmp" .
For completeness, glibc also provides
.BR utmpxname (),
although this function is not specified by POSIX.1.
.PP
On some other systems,
the \fIutmpx\fP structure is a superset of the \fIutmp\fP structure,
with additional fields, and larger versions of the existing fields,
and parallel files are maintained, often
.I /var/*/utmpx
and
.IR /var/*/wtmpx .
.PP
Linux glibc on the other hand does not use a parallel \fIutmpx\fP file
since its \fIutmp\fP structure is already large enough.
The "x" functions listed above are just aliases for
their counterparts without the "x" (e.g.,
.BR getutxent ()
is an alias for
.BR getutent ()).
.SH NOTES
.SS glibc notes
The above functions are not thread-safe.
glibc adds reentrant versions
.PP
.nf
.B #include <utmp.h>
.PP
.BI "int getutent_r(struct utmp *" ubuf ", struct utmp **" ubufp );
.BI "int getutid_r(struct utmp *" ut ,
.BI "              struct utmp *" ubuf ", struct utmp **" ubufp );
.BI "int getutline_r(struct utmp *" ut ,
.BI "                struct utmp *" ubuf ", struct utmp **" ubufp );
.fi
.PP
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.PP
.BR getutent_r (),
.BR getutid_r (),
.BR getutline_r ():
.nf
    _GNU_SOURCE
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */    _SVID_SOURCE || _BSD_SOURCE
.fi
.PP
These functions are GNU extensions, analogs of the functions of the
same name without the _r suffix.
The
.I 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
.IR *ubufp .
On error, these functions return \-1.
There are no utmpx equivalents of the above functions.
(POSIX.1 does not specify such functions.)
.SH EXAMPLES
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
.BR getpwuid (3)
and
.BR ttyname (3).
.PP
.\" SRC BEGIN (getutent.c)
.EX
#include <pwd.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <utmp.h>
\&
int
main(void)
{
    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);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR getutmp (3),
.BR utmp (5)