Multithreading

Introduction#

In C11 there is a standard thread library, <threads.h>, but no known compiler that yet implements it. Thus, to use multithreading in C you must use platform specific implementations such as the POSIX threads library (often referred to as pthreads) using the pthread.h header.

Syntax#

• thrd_t // Implementation-defined complete object type identifying a thread
• int thrd_create( thrd_t *thr, thrd_start_t func, void *arg ); // Creates a thread
• int thrd_equal( thrd_t thr0, thrd_t thr1 ); // Check if arguments refer to the same thread
• thr_t thrd_current(void); // Returns identifier of the thread that calls it
• int thrd_sleep( const struct timespec *duration, struct timespec *remaining ); // Suspend call thread execution for at least a given time
• void thrd_yield(void); // Permit other threads to run instead of the thread that calls it
• _Noreturn void thrd_exit( int res ); // Terminates the thread the thread that calls it
• int thrd_detatch( thrd_t thr; // Detaches a given thread from the current environment
• int thrd_join( thrd_t thr, int *res ); // Blocks the current thread until the given thread finishes

Remarks#

Using threads can introduce extra undefined behavior such as a https://stackoverflow.com/documentation/c/364/undefined-behavior/2622/data-race#t=201706130820201457052. For race-free access to variables that are shared between different threads C11 provides the mtx_lock() mutex functionality or the (optional) https://stackoverflow.com/documentation/c/4924/atomics#t=201706150835215525448 data-types and associated functions in stdatomic.h.

C11 Threads simple example

#include <threads.h>
#include <stdio.h>

int run(void *arg)
{
    printf("Hello world of C11 threads.");

    return 0;
}

int main(int argc, const char *argv[])
{
    thrd_t thread;
    int result;

    thrd_create(&thread, run, NULL);

    thrd_join(&thread, &result);

    printf("Thread return %d at the end\n", result);
}