Menu
I am working on some code which uses the pthread and semaphore libraries. The sem_init function works fine on my Ubuntu machine, but on OS X the sem_init function has absolutely no effect. Is there something wrong with the library or is there a different way of doing it? This is the code I am using to test.
sem_t sem1; sem_t sem2; sem_t sem3; sem_t sem4; sem_t sem5; sem_t sem6; sem_init(&sem1, 1, 1); sem_init(&sem2, 1, 2); sem_init(&sem3, 1, 3); sem_init(&sem4, 1, 4); sem_init(&sem5, 1, 5); sem_init(&sem6, 1, 6); The values appear to be random numbers, and they do not change after the sem_init call.
690
The sem_init() function initializes an unnamed semaphore and sets its initial value. The maximum value of the semaphore is set to SEM_VALUE_MAX. The title for the semaphore is set to the character representation of the address of the semaphore.
h> int sem_init (sem_t *sem, int pshared, unsigned int value); sem_init is the equivalent of sem_open for unnamed semaphores. One defines a variable of type sem_t and passes its pointer as sem in the sem_init call. Or, one can define a pointer and allocate memory dynamically using malloc or a similar function call.
Use sem_wait(3RT) to block the calling thread until the semaphore count pointed to by sem becomes greater than zero, then atomically decrement the count.
sem_init() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.
Unnamed semaphores are not supported, you need to use named semaphores.
To use named semaphores instead of unnamed semaphores, use sem_open instead of sem_init, and use sem_close and sem_unlink instead of sem_destroy.
79
A better solution (these days) than named semaphores on OS X is Grand Central Dispatch's dispatch_semaphore_t. It works very much like the unnamed POSIX semaphores.
Initialize the semaphore:
#include <dispatch/dispatch.h> dispatch_semaphore_t semaphore; semaphore = dispatch_semaphore_create(1); // init with value of 1 Wait & post (signal):
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); ... dispatch_semaphore_signal(semaphore); Destroy:
dispatch_release(semaphore); The header file is well documented and I found it quite easy to use.
33
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
