lock-file
Purpose: Locks file exclusively.
lock-file <file path> id [ define ] <lock id> status [ define ] <status>
lock-file attempts to create a file (deleting all contents of the previous file, if existed), and exclusively lock it. If successful, no other process can do the same unless current process ends or calls
unlock-file. This statement is non-blocking, thus you can check in a sleepy loop for success.
<file path> should be either an existing or non-existing file with a valid file path.
<lock id> (in "id" clause) is a file descriptor associated with locked file.
<status> (in "status" clause) represents the status of file locking: VV_OKAY if successfully locked, VV_ERR_FAILED if cannot lock (likely because another process holds a lock), VV_ERR_INVALID if the path of the file is invalid (i.e. if it is empty), VV_ERR_CREATE if lock file cannot be created.
Generally, this statement is used for process control. A process would use lock-file to start working on a job that only one process can do at a time; once done, by using
unlock-file statement, another process will be able to issue a successful lock-file call. Typically, lock-file is issued in a sleepy loop, waiting for a resource to be released.
Note that file lock is not carried across children processes (i.e. if your process creates children, such as with
exec-program, then such children must obtain their own lock). In addition, if a process serving a request terminates before the request could issue
unlock-file, the lock will be automatically released.
You can use any file name (likely choose a name that signifies the purpose of a lock, as long as you have permissions to create it), and create any number of them. This way you can create as many "binary semaphore"-like objects as you like.
Examples
get-app directory to define dir
write-string define fname
@<<p-out dir>>/.lock
end-write-string
num lockid;
while (1) {
lock-file fname id lockid status define lockst
if (lockst == VV_OKAY) {
20
@WORKING
sleep (20);
@DONE
break;
} else if (lockst == VV_ERR_FAILED) {
sleep(1);
@WAITING
continue;
} else if (lockst == VV_ERR_OPEN || lockst == VV_ERR_INVALID) {
@BAD LOCK
return;
}
}
unlock-file id lockid
return;
See also
Files (
close-file copy-file delete-file file-position file_storage file_uploading lock-file open-file read-file read-line rename-file stat-file temporary_file uniq-file unlock-file write-file )
SEE ALL (
documentation)