NAME
uu_lock,
uu_unlock, uu_lockerr,
uu_lock_txfr —
acquire and release control of a serial
device
SYNOPSIS
#include
<sys/types.h>
#include <util.h>
int
uu_lock(const
char *ttyname);
int
uu_lock_txfr(const
char *ttyname, pid_t
pid);
int
uu_unlock(const
char *ttyname);
const char *
uu_lockerr(int
uu_lockresult);
DESCRIPTION
The
uu_lock()
function attempts to create a lock file called
/var/spool/lock/LCK.. with a suffix given by the
passed ttyname. If the file already exists, it is
expected to contain the process ID of the locking program.
If the file does not already exist, or the owning
process given by the process ID found in the lock file is no longer running,
uu_lock()
will write its own process ID into the file and return success.
uu_lock_txfr()
transfers lock ownership to another process.
uu_lock() must have previously been successful.
uu_unlock()
removes the lockfile created by uu_lock() for the
given ttyname. Care should be taken that
uu_lock() was successful before calling
uu_unlock().
uu_lockerr()
returns an error string representing the error
uu_lockresult, as returned from
uu_lock().
RETURN VALUES
uu_unlock() returns 0 on success and -1 on
failure.
uu_lock() may return any of the following
values:
UU_LOCK_INUSE: The lock is in use by
another process.
UU_LOCK_OK: The lock was successfully
created.
UU_LOCK_OPEN_ERR: The lock file could not
be opened via open(2).
UU_LOCK_READ_ERR: The lock file could not
be read via read(2).
UU_LOCK_CREAT_ERR: Can't create temporary
lock file via creat(3).
UU_LOCK_WRITE_ERR: The current process ID
could not be written to the lock file via a call to
write(2).
UU_LOCK_LINK_ERR: Can't link temporary
lock file via link(2).
UU_LOCK_TRY_ERR: Locking attempts are
failed after 5 tries.
If a value of UU_LOCK_OK is passed to
uu_lockerr(), an empty string is returned.
Otherwise, a string specifying the reason for failure is returned.
uu_lockerr() uses the current value of
errno to determine the exact error. Care should be
made not to allow errno to be changed between calls to
uu_lock() and
uu_lockerr().
uu_lock_txfr() may return any of the
following values:
UU_LOCK_OK: The transfer was successful.
The specified process now holds the device lock.
UU_LOCK_OWNER_ERR: The current process
does not already own a lock on the specified device.
UU_LOCK_WRITE_ERR: The new process ID
could not be written to the lock file via a call to
write(2).
ERRORS
If uu_lock() returns one of the error
values above, the global value errno can be used to
determine the cause. Refer to the respective manual pages for further
details.
uu_unlock() will set the global variable
errno to reflect the reason that the lock file could
not be removed. Refer to the description of
unlink(2) for further details.
SEE ALSO
BUGS
It is possible that a stale lock is not recognised as such if a new process is assigned the same process ID as the program that left the stale lock.
The calling process must have write permissions to the /var/spool/lock directory. There is no mechanism in place to ensure that the permissions of this directory are the same as those of the serial devices that might be locked.