Inititialization by one thread

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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

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.

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