GETUTENT(3) Linux Programmers Manual GETUTENT(3)
getutent, getutid, getutline, pututline, setutent, endutent, utmpname -
access utmp file entries
struct utmp *getutent(void);
struct utmp *getutid(struct utmp *ut);
struct utmp *getutline(struct utmp *ut);
struct utmp *pututline(struct utmp *ut);
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
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 INIT_PRO
CESS, 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
ENOMEM Out of memory.
ESRCH Record not found.
setutent(), pututent(), 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
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
struct utmpx *getutxent(void);
struct utmpx *getutxid(const struct utmpx *);
struct utmpx *getutxline(const struct utmpx *);
struct utmpx *pututxline(const struct utmpx *);
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.
The above functions are not thread-safe. Glibc adds reentrant versions
#define _GNU_SOURCE /* or _SVID_SOURCE or _BSD_SOURCE */
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);
These functions are GNU extensions, analogs of the functions of the
same name without the _r suffix. The ubuf argument gives these func
tions 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).
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"));
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), feature_test_macros(7)
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