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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) );

Arguments:

once_control

A pointer to a pthread_once_t object that the function uses to determine whether or not to run the initialization code.

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You must set the pthread_once_t object to the macro PTHREAD_ONCE_INIT before using it for the first time.

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init_routine

The function that you want to call to do any required initialization.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

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.

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

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Returns:

EOK

Success.

EINVAL

Uninitialized once-control object once_control.

Examples:

This example shows how you can use once-initialization to initialize a library; both library_entry_point1() and library_entry_point2() need to initialize the library, but that needs to happen only 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 actually enters the library_init() function. The other threads block at the pthread_once() call until library_init() has returned. The pthread_once() function also ensures that library_init() is only ever called once; subsequent calls to the library entry points skip the call to library_init().

Classification:

POSIX 1003.1 THR

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	Yes
Thread	Yes

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