Full Discussion: Sleep command did not worked
Top Forums UNIX for Beginners Questions & Answers Sleep command did not worked Post 303002769 by bakunin on Friday 1st of September 2017 04:07:33 AM
Quote:
Originally Posted by Don Cragun
Not quite. There is a race condition between the time the existence test ([ -e "$fSemaphore" ]) is executed and the time the semaphore file is created (touch "$fSemaphore") that can allow two or more copies of this code to run simultaneously and not realize that another invocation is running.
If i understood thread o/p correctly he starts instances of this script every second (set apart by a sleep 1), so this concern (although valid in general) won't apply here. In the general case you are right: race conditions need to be addressed.

Quote:
Originally Posted by Don Cragun
Note that both of these run a risk of leaving the semaphore file in place if the script is terminated by a kill signal (for the code above) or by any signal (for the code bakunin suggested).
This is true. I wanted to illustrate what a semaphore in general is, so i left out the "implementation details". Yes, trapping signals should be done in production code.

bakunin
 
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SEM_WAIT(2)						      BSD System Calls Manual						       SEM_WAIT(2)

NAME
sem_trywait, sem_wait -- lock a semaphore SYNOPSIS
#include <semaphore.h> int sem_trywait(sem_t *sem); int sem_wait(sem_t *sem); DESCRIPTION
The semaphore referenced by sem is locked. When calling sem_wait(), if the semaphore's value is zero, the calling thread will block until the lock is acquired or until the call is interrupted by a signal. Alternatively, the sem_trywait() function will fail if the semaphore is already locked, rather than blocking on the semaphore. If successful (the lock was acquired), sem_wait() and sem_trywait() will return 0. Otherwise, -1 is returned and errno is set, and the state of the semaphore is unchanged. ERRORS
sem_wait() and sem_trywait() succeed unless: [EAGAIN] The semaphore is already locked. [EDEADLK] A deadlock was detected. [EINTR] The call was interrupted by a signal. [EINVAL] sem is not a valid semaphore descriptor. NOTES
Applications may encounter a priority inversion while using semaphores. When a thread is waiting on a semaphore which is about to be posted by a lower-priority thread and the lower-priority thread is preempted by another thread (of medium priority), a priority inversion has occured, and the higher-priority thread will be blocked for an unlimited time period. Programmers using the realtime functionality of the system should take care to avoid priority inversions. SEE ALSO
sem_open(2), sem_post(2), semctl(2), semget(2), semop(2) HISTORY
sem_wait() and sem_trywait() are specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995). Darwin June 8, 2000 Darwin

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