pthread_once()

Dynamic package initialization

Synopsis:

#include <pthread.h>

pthread_once_t once_control = PTHREAD_ONCE_INIT;

int pthread_once( pthread_once_t* once_control,
                  void (*init_routine)(void) );

Library:

libc

Description:

The pthread_once() function uses the once-control object once_control to determine whether the initialization routine init_routine should be called. The first call to pthread_once() by any thread in a process, with a given once_control, calls init_routine with no arguments. Subsequent calls of pthread_once() with the same once_control won't call init_routine.

The once-control object once_control must be initialized before use by assigning it the macro PTHREAD_ONCE_INIT.

No thread will execute past this function until the init_routine returns.

Returns:

EOK

Success.

EINVAL

Uninitialized once-control object once_control.

Examples:

This example shows how once initialization can be used to initialize a library; both library_entry_point1() and library_entry_point2() need to initialize the library, but that only needs to happen once.

#include <stdio.h>
#include <pthread.h>

pthread_once_t once_control = PTHREAD_ONCE_INIT;

void library_init( void )
{
    /* initialize the library */
}

void library_entry_point1( void )
{
    pthread_once( &once_control, library_init );
    
    /* do stuff for library_entry_point1... */
}

void library_entry_point2( void )
{
    pthread_once( &once_control, library_init );
    
    /* do stuff for library_entry_point1... */
}

This initializes the library once; if multiple threads call pthread_once(), only one will actually enter the library_init() function. The other threads will block at the pthread_once() call until library_init() has returned. pthread_once() also ensures that library_init() is only ever called once; subsequent calls to the library entry points will skip the call to library_init().

Classification:

POSIX 1003.1 (Threads)