# Threads (native)

# Inititialization by one thread

In most cases all data that is accessed by several threads should be initialized before the threads are created. This ensures that all threads start with a clear state and no race condition occurs.

If this is not possible once_flag and call_once can be used

#include <threads.h>
#include <stdlib.h>

// the user data for this example
double const* Big = 0;

// the flag to protect big, must be global and/or static
static once_flag onceBig = ONCE_INIT;

void destroyBig(void) {

void initBig(void) {
    // assign to temporary with no const qualification
    double* b = malloc(largeNum);
    if (!b) {
       perror("allocation failed for Big");
    // now initialize and store Big
    initializeBigWithSophisticatedValues(largeNum, b);
    Big = b;
    // ensure that the space is freed on exit or quick_exit

// the user thread function that relies on Big
int myThreadFunc(void* a) {
   call_once(&onceBig, initBig);
   // only use Big from here on
   return 0;

The once_flag is used to coordinate different threads that might want to initialize the same data Big. The call to call_once guarantees that

  • initBig is called exactly once
  • call_once blocks until such a call to initBig has been made, either by the same or another thread.

Besides allocation, a typical thing to do in such a once-called function is a dynamic initialization of a thread control data structures such as mtx_t or cnd_t that can't be initialized statically, using mtx_init or cnd_init, respectively.

# Start several threads

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

struct my_thread_data {
   double factor;

int my_thread_func(void* a) {
   struct my_thread_data* d = a;
   // do something with d
   printf("we found %g\n", d->factor);
   // return an success or error code
   return d->factor > 1.0;

int main(int argc, char* argv[argc+1]) {
    unsigned n = 4;
    if (argc > 1) n = strtoull(argv[1], 0, 0);
    // reserve space for the arguments for the threads
    struct my_thread_data D[n];     // can't be initialized
    for (unsigned i = 0; i < n; ++i) {
         D[i] = (struct my_thread_data){ .factor = 0.5*i, };
    // reserve space for the ID's of the threads
    thrd_t id[4];
    // launch the threads
    for (unsigned i = 0; i < n; ++i) {
         thrd_create(&id[i], my_thread_func, &D[i]);
    // Wait that all threads have finished, but throw away their
    // return values
    for (unsigned i = 0; i < n; ++i) {
         thrd_join(id[i], 0);
    return EXIT_SUCCESS;

# Syntax

  • #ifndef __STDC_NO_THREADS__
  • # include <threads.h>
  • #endif
  • void call_once(once_flag *flag, void (*func)(void));
  • int cnd_broadcast(cnd_t *cond);
  • void cnd_destroy(cnd_t *cond);
  • int cnd_init(cnd_t *cond);
  • int cnd_signal(cnd_t *cond);
  • int cnd_timedwait(cnd_t *restrict cond, mtx_t *restrict mtx, const struct timespec *restrict ts);
  • int cnd_wait(cnd_t *cond, mtx_t *mtx);
  • void mtx_destroy(mtx_t *mtx);
  • int mtx_init(mtx_t *mtx, int type);
  • int mtx_lock(mtx_t *mtx);
  • int mtx_timedlock(mtx_t *restrict mtx, const struct timespec *restrict ts);
  • int mtx_trylock(mtx_t *mtx);
  • int mtx_unlock(mtx_t *mtx);
  • int thrd_create(thrd_t *thr, thrd_start_t func, void *arg);
  • thrd_t thrd_current(void);
  • int thrd_detach(thrd_t thr);
  • int thrd_equal(thrd_t thr0, thrd_t thr1);
  • _Noreturn void thrd_exit(int res);
  • int thrd_join(thrd_t thr, int *res);
  • int thrd_sleep(const struct timespec *duration, struct timespec* remaining);
  • void thrd_yield(void);
  • int tss_create(tss_t *key, tss_dtor_t dtor);
  • void tss_delete(tss_t key);
  • void *tss_get(tss_t key);
  • int tss_set(tss_t key, void *val);

# Remarks

C11 threads are an optional feature. Their absence can be tested with __STDC__NO_THREAD__. Currently (Jul 2016) this feature is not yet implemented by all C libraries that otherwise support C11.

# C libraries that are known to support C11 threads are:

# C libraries that don't support C11 threads, yet: