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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getutent(3) Library Functions Manual getutent(3)
getutent, getutid, getutline, pututline, setutent, endutent,
utmpname - access utmp file entries
Standard C library (libc, -lc)
#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 STANDARDS.
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).
┌─────────────┬───────────────┬─────────────────────────────────┐
│ 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.
None.
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()).
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.)
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 <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"));
entry.ut_time = time(NULL);
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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Linux man-pages 6.9.1 2024-05-02 getutent(3)
Pages that refer to this page: getlogin(3), getutmp(3), glob(3), login(3), procps_misc(3), updwtmp(3), wordexp(3), utmp(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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