GETUTENT(3) Linux Programmer's Manual GETUTENT(3)
getutent, getutid, getutline, pututline, setutent, endutent, utmpname - access utmp file
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
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 struc-
ture 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 appropri-
ate 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 allo-
cated 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.
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
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 prototype 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 provides utmpxname(), although this function is not specified
On some other systems, the utmpx structure is a superset of the utmp structure, with addi-
tional fields, and larger versions of the existing fields, and parallel files are main-
tained, 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 coun-
terparts without the "x" (e.g., getutxent() is an alias for getutent()).
The above functions are not thread-safe. Glibc adds reentrant versions
#define _GNU_SOURCE /* or _SVID_SOURCE or _BSD_SOURCE;
see feature_test_macros(7) */
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 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).
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");
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