uucplock(3)

NAME
uu_lock, uu_unlock, uu_lockerr - acquire and release control of a serial device

SYNOPSIS
#include <sys/types.h>
#include <libutil.h>
int
uu_lock(const char *ttyname);
int
uu_unlock(const char *ttyname);
const char *
uu_lockerr(int uu_lockresult);

Link with -lutil on the cc(1) command line.

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_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_SEEK_ERR: The lock file was “stale”, but the call to lseek(2) necessary to write the current process id failed.

UU_LOCK_WRITE_ERR: The current process id could not be written to the lock file via a call to write(2).

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().

ERRORS
If uu_lock() returns one of the four 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
open(2), read(2), lseek(2), write(2)

BUGS
Locking is not atomic. Should a race condition occur, it's possible that the “losing” process fails to identify the “winning” process. If this happens, uu_lock() returns UU_LOCK_READ_ERR and errno is set to EINVAL.

It is possible that a stale lock is not recognised as such if a new processes is assigned the same processes 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.