pthreads +信号量，为什么这不正确执行？(pthreads + semaphores, why is this not executing properly?)

 这是我正在处理的任务。 它必须使用信号量，而不是互斥量。 
 
#include <stdio.h> 
#include <pthread.h> 
#include <assert.h> 
#include <unistd.h> 
#include <semaphore.h> 
#include <fcntl.h>
sem_t *ab, *ac, *ad, *de, *ce, *bf, *ef; 

void *a(void *arg) {
    printf("Entering A...\n");
    sleep(1);
    printf("Exiting A...\n");
    assert(sem_post(ab)==0);
    assert(sem_post(ac)==0);
    assert(sem_post(ad)==0);
    pthread_exit((void *)99);
}

void *b(void *arg) {
    assert(sem_wait(ab)==0);
    printf("Entering B...\n");
    sleep(1);
    printf("Exiting B...\n");
    assert(sem_post(bf)==0);
    pthread_exit((void *)99);
}

void *c(void *arg) {
    assert(sem_wait(ac)==0);
    printf("Entering C...\n");
    sleep(1);
    printf("Exiting C...\n");
    assert(sem_post(ce)==0);
    pthread_exit((void *)99);
}

void *d(void *arg) {
    assert(sem_wait(ad)==0);
    printf("Entering D...\n");
    sleep(1);
    printf("Exiting D...\n");
    assert(sem_post(de)==0);
    pthread_exit((void *)99);
}

void *e(void *arg) {
    assert(sem_wait(ce)==0);
    assert(sem_wait(de)==0);
    printf("Entering E...\n");
    sleep(1);
    printf("Exiting E...\n");
    assert(sem_post(ef)==0);
    pthread_exit((void *)99);
}

void *f(void *arg) {
    assert(sem_wait(bf)==0);
    assert(sem_wait(ef)==0);
    printf("Entering F...\n");
    sleep(1);
    printf("Exiting F...\n");
    pthread_exit((void *)99);
}


int main() { 
    pthread_t _a, _b, _c, _d, _e, _f;
    int r1, r2, r3, r4, r5, r6;

    ab=sem_open("foobar", O_CREAT, 0700, 0);
    ac=sem_open("foobar", O_CREAT, 0700, 0);
    ad=sem_open("foobar", O_CREAT, 0700, 0);
    ce=sem_open("foobar", O_CREAT, 0700, 0);
    de=sem_open("foobar", O_CREAT, 0700, 0);
    ef=sem_open("foobar", O_CREAT, 0700, 0);
    bf=sem_open("foobar", O_CREAT, 0700, 0);

    /*sem_init(ab,0,1);
    sem_init(ac,0,1);
    sem_init(ad,0,1);
    sem_init(ce,0,1);
    sem_init(de,0,1);
    sem_init(ef,0,1);
    sem_init(bf,0,1);*/

    assert(pthread_create(&_a, NULL, a, &r1) == 0);
    assert(pthread_create(&_b, NULL, b, &r2) == 0);
    assert(pthread_create(&_c, NULL, c, &r3) == 0);
    assert(pthread_create(&_d, NULL, d, &r4) == 0);
    assert(pthread_create(&_e, NULL, e, &r5) == 0);
    assert(pthread_create(&_f, NULL, f, &r6) == 0);

    assert(pthread_join(_a, NULL) == 0);
    assert(pthread_join(_b, NULL) == 0);
    assert(pthread_join(_c, NULL) == 0);    
    assert(pthread_join(_d, NULL) == 0);
    assert(pthread_join(_e, NULL) == 0);
    assert(pthread_join(_f, NULL) == 0);

    assert( sem_close(ab)==0 ); 
    assert( sem_close(ac)==0 ); 
    assert( sem_close(ad)==0 ); 
    assert( sem_close(ce)==0 );
    assert( sem_close(de)==0 ); 
    assert( sem_close(bf)==0 );
    assert( sem_close(ef)==0 ); 

    return 0; 
}
 
 这很简单，但由于某种原因，它没有按照正确的顺序执行。 输出很不一致，但始终不正确。 以下是一个示例输出： 
 
 输入A ... 
 输入B ... <---- sem_post（ab）还没有被调用 
 退出A ... 
 输入C ... 
 输入D ... 
 退出B ... 
 退出D ... 
 退出C ... 
 进入E ... 
 输入F ... 
 正在退出F ... 
 退出E ... 
 
 它应该遵循以下图表： 
 
 
 任何与此有关的帮助都将不胜感激，但这是一项任务，所以不要开始告诉我以完全不同的方式来完成，并且不要直接给出答案，只需指出正确的方向即可。 

This is an assignment I'm working on. It must use semaphores, not mutex.
 
#include <stdio.h> 
#include <pthread.h> 
#include <assert.h> 
#include <unistd.h> 
#include <semaphore.h> 
#include <fcntl.h>
sem_t *ab, *ac, *ad, *de, *ce, *bf, *ef; 

void *a(void *arg) {
    printf("Entering A...\n");
    sleep(1);
    printf("Exiting A...\n");
    assert(sem_post(ab)==0);
    assert(sem_post(ac)==0);
    assert(sem_post(ad)==0);
    pthread_exit((void *)99);
}

void *b(void *arg) {
    assert(sem_wait(ab)==0);
    printf("Entering B...\n");
    sleep(1);
    printf("Exiting B...\n");
    assert(sem_post(bf)==0);
    pthread_exit((void *)99);
}

void *c(void *arg) {
    assert(sem_wait(ac)==0);
    printf("Entering C...\n");
    sleep(1);
    printf("Exiting C...\n");
    assert(sem_post(ce)==0);
    pthread_exit((void *)99);
}

void *d(void *arg) {
    assert(sem_wait(ad)==0);
    printf("Entering D...\n");
    sleep(1);
    printf("Exiting D...\n");
    assert(sem_post(de)==0);
    pthread_exit((void *)99);
}

void *e(void *arg) {
    assert(sem_wait(ce)==0);
    assert(sem_wait(de)==0);
    printf("Entering E...\n");
    sleep(1);
    printf("Exiting E...\n");
    assert(sem_post(ef)==0);
    pthread_exit((void *)99);
}

void *f(void *arg) {
    assert(sem_wait(bf)==0);
    assert(sem_wait(ef)==0);
    printf("Entering F...\n");
    sleep(1);
    printf("Exiting F...\n");
    pthread_exit((void *)99);
}


int main() { 
    pthread_t _a, _b, _c, _d, _e, _f;
    int r1, r2, r3, r4, r5, r6;

    ab=sem_open("foobar", O_CREAT, 0700, 0);
    ac=sem_open("foobar", O_CREAT, 0700, 0);
    ad=sem_open("foobar", O_CREAT, 0700, 0);
    ce=sem_open("foobar", O_CREAT, 0700, 0);
    de=sem_open("foobar", O_CREAT, 0700, 0);
    ef=sem_open("foobar", O_CREAT, 0700, 0);
    bf=sem_open("foobar", O_CREAT, 0700, 0);

    /*sem_init(ab,0,1);
    sem_init(ac,0,1);
    sem_init(ad,0,1);
    sem_init(ce,0,1);
    sem_init(de,0,1);
    sem_init(ef,0,1);
    sem_init(bf,0,1);*/

    assert(pthread_create(&_a, NULL, a, &r1) == 0);
    assert(pthread_create(&_b, NULL, b, &r2) == 0);
    assert(pthread_create(&_c, NULL, c, &r3) == 0);
    assert(pthread_create(&_d, NULL, d, &r4) == 0);
    assert(pthread_create(&_e, NULL, e, &r5) == 0);
    assert(pthread_create(&_f, NULL, f, &r6) == 0);

    assert(pthread_join(_a, NULL) == 0);
    assert(pthread_join(_b, NULL) == 0);
    assert(pthread_join(_c, NULL) == 0);    
    assert(pthread_join(_d, NULL) == 0);
    assert(pthread_join(_e, NULL) == 0);
    assert(pthread_join(_f, NULL) == 0);

    assert( sem_close(ab)==0 ); 
    assert( sem_close(ac)==0 ); 
    assert( sem_close(ad)==0 ); 
    assert( sem_close(ce)==0 );
    assert( sem_close(de)==0 ); 
    assert( sem_close(bf)==0 );
    assert( sem_close(ef)==0 ); 

    return 0; 
}
 
It's pretty simple but for some reason it's not executing in the right order. The output is far from consistent but always incorrect. Here is one sample output:
 
Entering A...
 Entering B... <----sem_post(ab) has not even been called yet
 Exiting A...
 Entering C...
 Entering D...
 Exiting B...
 Exiting D...
 Exiting C...
 Entering E...
 Entering F...
 Exiting F...
 Exiting E... 
 
It should be following this diagram:
 
 
Any help with this will be greatly appreciated, but it's an assignment so don't start telling me to do it a completely different way and don't give the answer straight up, just point me in the right direction.

原文：https://stackoverflow.com/questions/6405189