#include <pthread.h>
int pthread_once(pthread_once_t *once_control,
void (*init_routine)(void));
pthread_once_t once_control = PTHREAD_ONCE_INIT;
The first call to pthread_once() by any thread in a process, with a given once_control, shall call the init_routine with no arguments. Subsequent calls of pthread_once() with the same once_control shall not call the init_routine. On return from pthread_once(), init_routine shall have completed. The once_control parameter shall determine whether the associated initialization routine has been called.
The pthread_once() function is not a cancellation point. However, if init_routine is a cancellation point and is canceled, the effect on once_control shall be as if pthread_once() was never called.
The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header.
The behavior of pthread_once() is undefined if once_control has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT.
Upon successful completion, pthread_once() shall return zero; otherwise, an error number shall be returned to indicate the error.
The pthread_once() function may fail if:
The pthread_once() function shall not return an error code of [EINTR].
The following sections are informative.
Some C libraries are designed for dynamic initialization. That is, the global initialization for the library is performed when the first procedure in the library is called. In a single-threaded program, this is normally implemented using a static variable whose value is checked on entry to a routine, as follows:
static int random_is_initialized = 0;
extern int initialize_random();
int random_function()
{
if (random_is_initialized == 0) {
initialize_random();
random_is_initialized = 1;
}
... /* Operations performed after initialization. */
}
To keep the same structure in a multi-threaded program, a new primitive is needed. Otherwise, library initialization has to be accomplished by an explicit call to a library-exported initialization function prior to any use of the library.
For dynamic library initialization in a multi-threaded process, a simple initialization flag is not sufficient; the flag needs to be protected against modification by multiple threads simultaneously calling into the library. Protecting the flag requires the use of a mutex; however, mutexes have to be initialized before they are used. Ensuring that the mutex is only initialized once requires a recursive solution to this problem.
The use of pthread_once() not only supplies an implementation-guaranteed means of dynamic initialization, it provides an aid to the reliable construction of multi-threaded and realtime systems. The preceding example then becomes:
#include <pthread.h>
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern int initialize_random();
int random_function()
{
(void) pthread_once(&random_is_initialized, initialize_random);
... /* Operations performed after initialization. */
}
Note that a pthread_once_t cannot be an array because some compilers do not accept the construct &<array_name>.
The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>