lock-file - (files)

Description

lock-file attempts to create file with the full path of <file path> (deleting it 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 (see pause-program). <file path> should be either an existing or non-existing file with a valid file path. If existing, it will be deleted. <lock id> (in "id" clause) is a file descriptor associated with locked file. <status> (in "status" clause) represents the status of file locking: GG_OKAY if successfully locked, GG_ERR_FAILED if cannot lock (likely because another process holds a lock), GG_ERR_INVALID if the path of the file is invalid (i.e. if it is empty), GG_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 (see pause-program), 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

%% /locktest public // Get application home directory get-app directory to dir // Create lock file name write-string fname @<<print-out dir>>/.lock end-write-string // Enter loop in which something is done, OR, program waits for others to complete their work before doing its own set-number lockid start-loop // Attempt to lock file lock-file fname id lockid status lockst // Check the status of locking if-true lockst equal GG_OKAY // File successfully locked, simulate working for 20 seconds @WORKING pause-program 20000 @DONE // Exit while loop break-loop else-if lockst equal GG_ERR_FAILED // Another process holds the lock, wait, try again for 1 second pause-program 1000 @WAITING continue-loop else-if lockst equal GG_ERR_OPEN or lockst equal GG_ERR_INVALID // Errors @BAD LOCK exit-handler end-if end-loop // Once done, unlock file unlock-file id lockid %% %%