83 lines
1.9 KiB
C
83 lines
1.9 KiB
C
/*
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* semaphore_wait.c
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*
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* Demonstrate use of semaphores for synchronization.
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*/
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#include <sys/types.h>
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#include <unistd.h>
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#include <pthread.h>
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#include <semaphore.h>
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#include <signal.h>
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#include "errors.h"
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sem_t semaphore;
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/*
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* Thread start routine to wait on a semaphore.
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*/
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void *sem_waiter (void *arg)
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{
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long num = (long)arg;
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printf ("Thread %d waiting\n", num);
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if (sem_wait (&semaphore) == -1)
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errno_abort ("Wait on semaphore");
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printf ("Thread %d resuming\n", num);
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return NULL;
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}
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int main (int argc, char *argv[])
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{
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int thread_count;
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pthread_t sem_waiters[5];
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int status;
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#if !defined(_POSIX_SEMAPHORES)
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printf ("This system does not support POSIX semaphores\n");
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return -1;
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#else
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if (sem_init (&semaphore, 0, 0) == -1)
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errno_abort ("Init semaphore");
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/*
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* Create 5 threads to wait concurrently on the semaphore.
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*/
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for (thread_count = 0; thread_count < 5; thread_count++) {
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status = pthread_create (
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&sem_waiters[thread_count], NULL,
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sem_waiter, (void*)thread_count);
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if (status != 0)
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err_abort (status, "Create thread");
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}
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sleep (2);
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/*
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* "Broadcast" the semaphore by repeatedly posting until the
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* count of waiters goes to 0.
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*/
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while (1) {
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int sem_value;
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if (sem_getvalue (&semaphore, &sem_value) == -1)
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errno_abort ("Get semaphore value");
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if (sem_value >= 0)
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break;
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printf ("Posting from main: %d\n", sem_value);
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if (sem_post (&semaphore) == -1)
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errno_abort ("Post semaphore");
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}
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/*
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* Wait for all threads to complete.
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*/
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for (thread_count = 0; thread_count < 5; thread_count++) {
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status = pthread_join (sem_waiters[thread_count], NULL);
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if (status != 0)
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err_abort (status, "Join thread");
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}
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return 0;
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#endif
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}
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