3/20/08

Semaphore Example in C

This is a Semaphore example in C. I extended the C Thread example to use semaphores. Every thread created will first try to acquire the semaphore lock and then start printing numbers. After printing is done lock is left so other threads can continue their job. Semaphores is a powerful tool for programmers.
Here is the semaphore example source code in C.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <pthread.h>
#include <semaphore.h>
#define MAX_THREAD 100

typedef struct {
int start,end;
} param;
sem_t mysem;
void *count(void *arg){
sem_wait(&mysem);
int i =0;
param *p=(param *)arg;
printf("\nprintfrom %d to %d\n",p->start,p->end);
for(i =p->start ; i< p->end ; i++){
printf(" i = %d",i);sleep(1);
}

sem_post(&mysem);
return (void*)(1);
}

int main(int argc, char* argv[]) {

if ( sem_init(&mysem,0,1) ) {
perror("init");
}
int n,i;
pthread_t *threads;
param *p;

if (argc != 2) {
printf ("Usage: %s n\n",argv[0]);
printf ("\twhere n is no. of threads\n");
exit(1);
}

n=atoi(argv[1]);

if ((n < 1) || (n > MAX_THREAD)) {
printf ("arg[1] should be 1 - %d.\n",MAX_THREAD);
exit(1);
}

threads=(pthread_t *)malloc(n*sizeof(*threads));

p=(param *)malloc(sizeof(param)*n);
/* Assign args to a struct and start thread */
for (i=0; i<n; i++) {
p[i].start=i*100;
p[i].end=(i+1)*100;
pthread_create(&threads[i],NULL,count,(void *)(p+i));
}
printf("\nWait threads\n");
sleep(1);
/* Wait for all threads. */
int *x = malloc(sizeof(int));
for (i=0; i<n; i++) {
pthread_join(threads[i],(void*)x);
}
free(p);
exit(0);
}

Del.icio.us

1 comment:

Anonymous said...

for me (compiled with gcc, using linux mint), the program waits an entire 100 seconds then displays the output from 1 to 100 together.
the printf just before sleep does not seem to be printing... atleast till all the sleeps are over.
so i changed line 18 it to:
printf(" i = %d",i);fflush(stdout);sleep(1);

so that it updates the stdout after each second.

Related Posts Plugin for WordPress, Blogger...