In the main program we need to declare the mutex, condition variable and the global variable:

pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t myConVar = PTHREAD_COND_INITIALIZER;
int threadid=0;

The main program starts the two threads going and then waits:

int main(int argc, char** argv) {
    srand(time(0));
    pthread_t pthreadA;
    pthread_create(&pthreadA, NULL, threadA, NULL);
    pthread_t pthreadB;
    pthread_create(&pthreadB, NULL, threadB, NULL);
    pthread_mutex_lock(&my_mutex);
    while (threadid == 0) 
            pthread_cond_wait(&myConVar, &my_mutex);
    printf("%d", threadid);
    pthread_mutex_unlock(&my_mutex);
    return (EXIT_SUCCESS);
}

Notice that the call to pthread_cond_wait is in the form of a while loop. The reason is that the wait can end without any thread signaling. To make sure that it isn’t a spurious wake-up we have to check that threadid has been set to something other than zero. If it is still zero then the wait is restarted. Finally we print the value of threadid and unlock the mutex. Notice you can’t unlock the mutex until the value of threadid has been printed as one of the other threads might update it.

The complete program is:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t myConVar = PTHREAD_COND_INITIALIZER;
int threadid = 0;
void * threadA(void *p) {
    sleep(rand() % 5);
    pthread_mutex_lock(&my_mutex);
    threadid = 1;
    pthread_cond_signal(&myConVar);
    pthread_mutex_unlock(&my_mutex);
}
void * threadB(void *p) {
    sleep(rand() % 5);
    pthread_mutex_lock(&my_mutex);
    threadid = 2;
    pthread_cond_signal(&myConVar);
    pthread_mutex_unlock(&my_mutex);
}
int main(int argc, char** argv) {
    srand(time(0));
    pthread_t pthreadA;
    pthread_create(&pthreadA, NULL, threadA, NULL);
    pthread_t pthreadB;
    pthread_create(&pthreadB, NULL, threadB, NULL);
    pthread_mutex_lock(&my_mutex);
    while (threadid == 0)
               pthread_cond_wait(&myConVar, &my_mutex);
    printf("%d", threadid);
    pthread_mutex_unlock(&my_mutex);
    return (EXIT_SUCCESS);
}

Now that you have seen how this works you should be able to modify the program so that the main program only resumes when all of the threads have completed.

All you have to do is change the update of the global variable and the signal to:

threadid++;
if(threadid==2) pthread_cond_signal(&myConVar);

With this change each thread increments the global variable but only wakes the main thread up when it reaches 2 and both threads have terminated.

You might want to use this in place of joins if you want to create detached threads.

Now available as a paperback or ebook from Amazon.

Applying C For The IoT With Linux

1. C,IoT, POSIX & LINUX
2. Kernel Mode, User Mode & Syscall
   
3. Execution, Permissions & Systemd
   Extract Running Programs With Systemd
4. Signals & Exceptions
   Extract  Signals
5. Integer Arithmetic
   Extract: Basic Arithmetic As Bit Operations
   Extract: BCD Arithmetic  ***NEW
6. Fixed Point
   Extract: Simple Fixed Point Arithmetic
7. Floating Point 
8. File Descriptors
   Extract: Simple File Descriptors 
   Extract: Pipes 
9. The Pseudo-File System
   Extract: The Pseudo File System
   Extract: Memory Mapped Files 
10. Graphics
    Extract: framebuffer
11. Sockets
    Extract: Sockets The Client
    Extract: Socket Server
12. Threading
    Extract:  Pthreads
    Extract:  Condition Variables
    Extract:  Deadline Scheduling
13. Cores Atomics & Memory Management
    Extract: Applying C - Cores 
14. Interupts & Polling
    Extract: Interrupts & Polling 
15. Assembler
    Extract: Assembler

Also see the companion book: Fundamental C

<ASIN:1871962609>

<ASIN:1871962617>

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