Can't execute first thread with mutex - c

Question

I want to finish my threads with a mutex. The first thread won't execute, thread 2 & 3 execute.
Does anyone know what this problem could be? Sometimes thread 1 is executed but then 2 or 3 is not executed. I don't know what the problem is here.
Thread created successfully
Thread created successfully
Thread created successfully
----------------------------------------------------
J:0
NUM_REQUESTS (before function): 0
J:0
----------------------------------------------------
----------------------------------------------------
J:1
Third thread processing done
WRITE DATA TO LIST!
NUM_REQUESTS(function): 1
NUM_REQUESTS (before function): 0
J:1
----------------------------------------------------
----------------------------------------------------
J:2
Second thread processing done
WRITE DATA TO LIST!
NUM_REQUESTS(function): 0
NUM_REQUESTS (before function): 0
J:2
----------------------------------------------------
Program:
#include<stdio.h>
#include<string.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
pthread_mutex_t request_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t got_request = PTHREAD_COND_INITIALIZER;
pthread_t tid[3];
int thr_id[3];
int ret1,ret2,ret3;
int i = 0;
int err;
int *ptr[2];
int num_requests = 0;
int rc = 0;
This is the function of the threads, the first thread is not executed!
void* doSomeThing(void *arg)
{
unsigned long i = 0;
pthread_t id = pthread_self();
for(i=0; i<1000000;i++);
if(pthread_equal(id,tid[0]))
{
printf("First thread processing done\n");
printf("WRITE DATA TO LIST!\n");
num_requests--;
printf("NUM_REQUESTS(function): %d\n",num_requests);
rc = pthread_mutex_unlock(&request_mutex);
pthread_exit(&tid[0]);
}else if(pthread_equal(id,tid[1])){
printf("Second thread processing done\n");
num_requests--;
printf("WRITE DATA TO LIST!\n");
printf("NUM_REQUESTS(function): %d\n",num_requests);
rc = pthread_mutex_unlock(&request_mutex);
pthread_exit(&tid[1]);
}else if(pthread_equal(id,tid[2])){
printf("Third thread processing done\n");
printf("WRITE DATA TO LIST!\n");
printf("NUM_REQUESTS(function): %d\n",num_requests);
num_requests--;
rc = pthread_mutex_unlock(&request_mutex);
pthread_exit(&tid[2]);
}
return NULL;
}
This is where i create the output of the threads
void add_request(int j,pthread_mutex_t* p_mutex,pthread_cond_t* p_cond_var)
{
printf("----------------------------------------------------\n");
printf("J:%d\n",j);
if(num_requests > 3){
printf("WAIT TILL THREADS ARE FREE!\n");
}else{
rc = pthread_mutex_lock(&request_mutex);
printf("NUM_REQUESTS (before function): %d\n",num_requests);
num_requests++;
rc = pthread_mutex_unlock(&request_mutex);
rc = pthread_mutex_lock(&request_mutex);
rc = pthread_cond_signal(p_cond_var);
printf("J:%d\n",j);
printf("----------------------------------------------------\n");
}
}
In the main i only create the threads and use the add_request function to execute the threads
int main(void)
{
//create 3 threads
while(i < 3)
{
thr_id[i] = i;
err = pthread_create(&(tid[i]), NULL, &doSomeThing, (void*)&thr_id[i]);
if (err != 0)
printf("can't create thread :[%s]", strerror(err));
else
printf("Thread created successfully\n");
i++;
}
int j;
for(j=0;j<3;j++){
add_request(j, &request_mutex, &got_request);
}
return 0;
}

Answer 1

what are you trying to do?
At least one problem I see it that you unlock the mutex without locking it...
When you create a thread it starts spontaneously (more or less) so you can't assume add_request will be called after doSomething().
You can lock and then unlock. Also consider using ptheard_join if you want to wait until the threads finish.
EDIT
this is what you want to do https://computing.llnl.gov/tutorials/pthreads/samples/condvar.c
-taken from https://computing.llnl.gov/tutorials/pthreads/
Good luck