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Top Forums Shell Programming and Scripting Avoiding 'sh -c' when running ps from CRON Post 302605772 by methyl on Thursday 8th of March 2012 12:43:17 PM
Old 03-08-2012
In the semaphore method, the script first checks for the presence of a semaphore file and exits if the file is present. If the file is not present the script immediately creates the semaphore file and subseqently deletes the semaphore file on exit.
Some people call this an interlock file (or even a mutex).

This semaphore file also gives you a reference for when the script started so a monitoring process could detect an overrunning process.
 

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sem_unlink(2)							System Calls Manual						     sem_unlink(2)

NAME
sem_unlink - unlink a named POSIX semaphore SYNOPSIS
DESCRIPTION
is used to unlink named semaphores. A successful call to marks the semaphore, specified by name, for removal. Calling does not affect processes, including the calling process, which currently have a descriptor, obtained from a call to Named semaphores are uniquely identi- fied by character strings. All character string names will be pre-processed to ensure variations of a pathname resolve to the same sema- phore name. If the semaphore is successfully marked for removal by a call to the semaphore will be removed when all processes remove their descriptors to the specified semaphore by calling Subsequent calls to using the string name will refer to a new semaphore. To use this function, link in the realtime library by specifying on the compiler or linker command line. EXAMPLES
The following call to will remove the named semaphore named by the string name. If the semaphore is currently referenced by one or more processes, the semaphore will be marked for removal and removed when there are no more processes referencing it. RETURN VALUE
If the semaphore was unlinked successfully, returns 0. If the semaphore could not be unlinked, the call returns -1 and sets to indicate the error. ERRORS
fails and does not perform the requested operation if any of the following conditions are encountered: [EACCES] The named semaphore exists and the process does not have the permissions to unlink the semaphore. [ENAMETOOLONG] The name string is longer than [ENOENT] The flag is not set in oflag (see sem_open(2)) and the named semaphore does not exist. SEE ALSO
sem_close(2), sem_open(2), privileges(5), <semaphore.h>. STANDARDS CONFORMANCE
sem_unlink(2)
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