pthread_once

 pthread_once - once-only initialization


pthread_once_t once_control = 0;

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


pthread_once函数在用于仅仅调用一次初始化
它的代码比想象中的复杂一些
本来直接用一个原子的test and set bit操作就可以防止执行多次，但是pthread_once
的一个重要功能就是让其它线程睡眠等待而不是直接不等待init_routine执行完就继续
还要考虑正在执行的时候被fork或者被cancel的问题

.globl    __pthread_once
    .type    __pthread_once,@function
    .align    16
__pthread_once:
    movl    4(%esp), %ecx

  只有执行过init_routine之后bit 1才会被设置
    testl    $2, (%ecx)
    jz    1f
    xorl    %eax, %eax
    ret

1:    pushl    %ebx
    pushl    %esi
    movl    %ecx, %ebx
    xorl    %esi, %esi

    /* Not yet initialized or initialization in progress.
   Get the fork generation counter now.  */

6:    movl    (%ebx), %eax
5:    movl    %eax, %edx


    testl    $2, %eax
    jnz    4f

  __fork_generation在fork的时候会被 +4
    andl    $3, %edx
    orl    __fork_generation, %edx
    orl    $1, %edx
  原子地设置并交换bit 0,该位表示有线程正在执行
    LOCK
    cmpxchgl %edx, (%ebx)
    jnz    5b


    /* Check whether another thread already runs the initializer.  */
    testl    $1, %eax
    jz    3f    /* No -> do it.  */


    fork的问题， 如果正在执行init_routine的时候fork, 其子进程
    调用pthread_once就会到这，这样会执行init_routine，下面那个例子可以看出来

    xorl    %edx, %eax
    testl    $0xfffffffc, %eax
    jnz    3f    /* Different for generation -> run initializer.  */

    /* Somebody else got here first.  Wait.  */

   
等待别的线程执行init_routine然后唤醒
    movl    $FUTEX_WAIT|FUTEX_PRIVATE_FLAG, %ecx
    movl    $SYS_futex, %eax
    ENTER_KERNEL

    
   
 
     之所以要重新检查，是因为init_routine在执行的时候
   它可能被杀死

    jmp    6b

3:
    subl    $UNWINDBUFSIZE+8, %esp
    movl    %ecx, %ebx        /* PIC register value.  */

    set
    leal    8+UWJMPBUF(%esp), %eax
    movl    $0, 4(%esp)
    movl    %eax, (%esp)
    call    __sigsetjmp@PLT
    testl    %eax, %eax
    jne    7f

  setjmp == 0，注册cancel jmpbuf
  这样，线程在退出的时候会到这里来，上面那条
  jne 7f会处理这种情况

    leal    8(%esp), %eax
    call    HIDDEN_JUMPTARGET(__pthread_register_cancel)

    /* Call the user-provided initialization function.  */
    call    *24+UNWINDBUFSIZE(%esp)

    /* Pop the cleanup handler.  */
    leal    8(%esp), %eax
    call    HIDDEN_JUMPTARGET(__pthread_unregister_cancel)
    addl    $UNWINDBUFSIZE+8, %esp

    /* Sucessful run of the initializer.  Signal that we are done.  */
    movl    12(%esp), %ebx

  函数执行完毕！

    LOCK
    addl    $1, (%ebx)

    /* Wake up all other threads.  */
    movl    $0x7fffffff, %edx

    movl    $FUTEX_WAKE|FUTEX_PRIVATE_FLAG, %ecx
    movl    $SYS_futex, %eax
    ENTER_KERNEL

4:    popl    %esi
    popl    %ebx
    xorl    %eax, %eax
    ret

7:    /* __sigsetjmp returned for the second time.  */
    这表明执行init_routine的时候线程被取消
    这需要唤醒别的线程
    但是并不设置执行完毕的bit 1位
    movl    20+UNWINDBUFSIZE(%esp), %ebx
    movl    $0, (%ebx)

    movl    $0x7fffffff, %edx

    movl    $FUTEX_WAKE|FUTEX_PRIVATE_FLAG, %ecx

    movl    $SYS_futex, %eax
    ENTER_KERNEL


    leal    8(%esp), %eax
    线程准备结束，有关线程如何结束，可以参考
     pthread_create
    call    HIDDEN_JUMPTARGET (__pthread_unwind_next)
    /* NOTREACHED */
    hlt

fork的例子

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#define NUMTHREADS 3

int number = 0;

pthread_once_t onceControl = PTHREAD_ONCE_INIT;

void initRoutine(void)
{
    ++number;
    sleep(2);
    ++number;
}

void *threadfunc(void *parm)
{
    pthread_once(&onceControl, initRoutine);
    return 0;
}

int main(int argc, char **argv)
{
    pthread_t  thread[NUMTHREADS];
    int rc = 0;
    int i = NUMTHREADS;
    int is_child;

    for (i=0; i < NUMTHREADS; ++i) {
        rc = pthread_create(&thread[i], NULL, threadfunc, NULL);
    }

    sleep(1);

    is_child = !fork();
    if (is_child)
    {
        printf("onceControl: %d\n", onceControl);
        pthread_once(&onceControl, initRoutine);
    } else {
        for (i = 0; i < NUMTHREADS; ++i) {
            rc = pthread_join(thread[i], 0);
        }
        wait(0);
    }

    printf("%s: number = %d\n", is_child?"child":"parent", number);
    printf("onceControl: %d\n", onceControl);

    return 0;
}