Unexpected behavior from Semaphore

[azzmosphere]

Hi I have been playing with a bit of code to teach myself how to use semaphores on my MAC OSX, however it does not appear to be working, here is a screen shot of what I am getting:

1. The value of the semaphors is 0
1. The value of the semaphors is 0
Bad file descriptor mutex returned -1

Here is the code that I have been playing with:

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

#define NITER 1000000

int count = 0;
sem_t *mutex;

void * ThreadAdd(void * a)
{

int value;
sem_getvalue(mutex, &value);
printf("1. The value of the semaphors is %d\n", value);

int r =0;
if((r = sem_trywait(mutex)) != 0)
{
fprintf(stderr,"%s mutex returned %d\n",strerror(errno),r);
exit(EXIT_FAILURE);
}

int i, tmp;
for(i = 0; i < NITER; i++)
{
tmp = count; /* copy the global count locally */
tmp = tmp+1; /* increment the local copy */
count = tmp; /* store the local value into the global count */
}
sem_post (mutex);

sem_getvalue(mutex, &value);
printf("2. The value of the semaphors is %d\n", value);
}

int main(int argc, char * argv[])
{
pthread_t tid1, tid2;
sem_init(mutex, 0, 1);
//mutex = sem_open("mutex",O_CREAT, S_IRUSR | S_IWUSR, 10);


if(pthread_create(&tid1, NULL, ThreadAdd, NULL))
{
printf("\n ERROR creating thread 1");
exit(EXIT_FAILURE);
}

if(pthread_create(&tid2, NULL, ThreadAdd, NULL))
{
printf("\n ERROR creating thread 2");
exit(EXIT_FAILURE);
}

if(pthread_join(tid1, NULL)) /* wait for the thread 1 to finish */
{
printf("\n ERROR joining thread");
exit(EXIT_FAILURE);
}

if(pthread_join(tid2, NULL)) /* wait for the thread 2 to finish */
{
printf("\n ERROR joining thread");
exit(EXIT_FAILURE);
}

if (count < 2 * NITER)
printf("\n BOOM! count is [%d], should be %d\n", count, 2*NITER);
else
printf("\n OK! count is [%d]\n", count);

sem_close(mutex);
//sem_unlink("mutex");
pthread_exit(NULL);
}


and here is how I have been compiling the code


gcc -o filename filename.c -lpthread

I feel like I am missing something really obvious here but don't know what. Has anybody got any idea's?

[weaknessforcats]

pthread_t tid1, tid2;
sem_init(mutex, 0, 1);
//mutex = sem_open("mutex",O_CREAT, S_IRUSR | S_IWUSR, 10);


if(pthread_create(&tid1, NULL, ThreadAdd, NULL))
{

So exactly what are the contents of tid1 and tid2?

I see them defined but never initialized.

[azzmosphere]

HI thanks for your response but I have figured out what is going wrong here. I had a look at tid1 and tid2 and re-read the man page fore pthread_create. From what I understand the command "pthread_create(&tid1, NULL, ThreadAdd, NULL)" will initialize tid1 to default values as the attr argument is NULL. The actual problem in the code was that MAC OS does not support POSIX style unamed semaphores but does support the older Mach style which is a counting semaphore therefore the solution was either used named semaphore or the Mach. So I modified the original code to do this:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

// Mach semaphore
#include <mach/semaphore.h>
#include <mach/task.h>


#define NITER 1000000

int count = 0;
semaphore_t sem = 0;

void * ThreadAdd(void * a)
{

semaphore_wait(sem);
int i, tmp;
for(i = 0; i < NITER; i++)
{
tmp = count; /* copy the global count locally */
tmp = tmp+1; /* increment the local copy */
count = tmp; /* store the local value into the global count */
}
semaphore_signal(sem);
}

int main(int argc, char * argv[])
{
pthread_t tid1, tid2;

int initialValue = 1;
semaphore_create(mach_task_self(), &sem, SYNC_POLICY_FIFO, initialValue);



if(pthread_create(&tid1, NULL, ThreadAdd, NULL))
{
printf("\n ERROR creating thread 1");
exit(EXIT_FAILURE);
}

if(pthread_create(&tid2, NULL, ThreadAdd, NULL))
{
printf("\n ERROR creating thread 2");
exit(EXIT_FAILURE);
}

if(pthread_join(tid1, NULL)) /* wait for the thread 1 to finish */
{
printf("\n ERROR joining thread");
exit(EXIT_FAILURE);
}

if(pthread_join(tid2, NULL)) /* wait for the thread 2 to finish */
{
printf("\n ERROR joining thread");
exit(EXIT_FAILURE);
}

if (count < 2 * NITER)
printf("\n BOOM! count is [%d], should be %d\n", count, 2*NITER);
else
printf("\n OK! count is [%d]\n", count);

pthread_exit(NULL);
}

and sha-bam problem solved.