getutent(3) Library Functions Manual getutent(3)
NAME
getutent, getutid, getutline, pututline, setutent, endutent, utmpname -
access utmp file entries
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 be-
fore 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-
tions.
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, LO-
GIN_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
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.
ERRORS
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
/var/run/utmp
database of currently logged-in users
/var/log/wtmp
database of past user logins
ATTRIBUTES
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 sig:ALRM │
│ getutline() │ │ 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.
STANDARDS
None.
HISTORY
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 in-
troduces 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 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
/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
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
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);
}
SEE ALSO
getutmp(3), utmp(5)
Linux man-pages 6.9.1 2024-05-02 getutent(3)
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