Shulou(Shulou.com)06/01 Report--

基于linux threads2.0.1线程源码如何分析信号量，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。

/* Semaphores a la POSIX 1003.1b */

#include "pthread.h"

#include "semaphore.h"

#include "internals.h"

#include "restart.h"

#ifndef HAS_COMPARE_AND_SWAP

/* If we have no atomic compare and swap, fake it using an extra spinlock. */

#include "spinlock.h"

// 等于旧值则更新sem_status为新值，如果一样则不更新，更新则返回1

static inline int compare_and_swap(sem_t *sem, long oldval, long newval)

{

int ret;

acquire(&sem->sem_spinlock);

if ((ret = sem->sem_status == oldval) != 0)

sem->sem_status = newval;

release(&sem->sem_spinlock);

return ret;

}

#else

/* But if we do have an atomic compare and swap, use it! */

#define compare_and_swap(sem,old,new) \

__compare_and_swap(&(sem)->sem_status, (old), (new))

#endif

/* The state of a semaphore is represented by a long int encoding

either the semaphore count if >= 0 and no thread is waiting on it,

or the head of the list of threads waiting for the semaphore.

To distinguish the two cases, we encode the semaphore count N

as 2N+1, so that it has the lowest bit set.

A sequence of sem_wait operations on a semaphore initialized to N

result in the following successive states:

2N+1, 2N-1, ..., 3, 1, &first_waiting_thread, &second_waiting_thread, ...

*/

int sem_init(sem_t *sem, int pshared, unsigned int value)

{

if (value > SEM_VALUE_MAX) {

errno = EINVAL;

return -1;

}

// 还没实现

if (pshared) {

errno = ENOSYS;

return -1;

}

// 记录资源数

sem->sem_status = ((long)value sem_status;

// 大于1说明有资源，等于1说着0说明没有资源或没有资源并且有线程阻塞

if ((oldstatus & 1) && (oldstatus != 1))

newstatus = oldstatus - 2;

else {

// 没有可用资源，需要阻塞

newstatus = (long) self;

// 保存这时候的资源数或者上一个被阻塞的线程

self->p_nextwaiting = (pthread_t) oldstatus;

}

}

// sem_status可能指向资源数或者被阻塞的线程链表。赋值成功则返回1，否则返回0

while (! compare_and_swap(sem, oldstatus, newstatus));

// self是按偶数地址对齐的，低位为1说明是还有可用资源

if (newstatus & 1)

/* We got the semaphore. */

return 0;

/* Wait for sem_post or cancellation */

// 等待被restart或者cancel信号唤醒

suspend_with_cancellation(self);

/* This is a cancellation point */

// 判断是否被取消了，即是被cancel信号唤醒的，不是的话重新判断是否有资源，即回到上面的while(1)

if (self->p_canceled && self->p_cancelstate == PTHREAD_CANCEL_ENABLE) {

/* Remove ourselves from the waiting list if we're still on it */

/* First check if we're at the head of the list. */

do {

// 得到被阻塞的第一个线程

oldstatus = sem->sem_status;

// 相等说明当前线程是最后一个被阻塞的线程

if (oldstatus != (long) self) break;

// 得到该线程被阻塞时的资源数或下一个被阻塞的线程

newstatus = (long) self->p_nextwaiting;

}

// sem_status指向资源数或者下一个被阻塞的线程

while (! compare_and_swap(sem, oldstatus, newstatus));

/* Now, check if we're somewhere in the list.

There's a race condition with sem_post here, but it does not matter:

the net result is that at the time pthread_exit is called,

self is no longer reachable from sem->sem_status. */

// 可能是break或者while为true，不是当前线程并且不是资源数，即oldstatus指向一个其他线程

if (oldstatus != (long) self && (oldstatus & 1) == 0) {

th = &(((pthread_t) oldstatus)->p_nextwaiting);

// 不是资源数，即是线程，从等待的线程链表中删除self线程，因为他即将退出

while (*th != (pthread_t) 1 && *th != NULL) {

if (*th == self) {

*th = self->p_nextwaiting;

break;

}

th = &((*th)->p_nextwaiting);

}

}

// 当前线程退出

pthread_exit(PTHREAD_CANCELED);

}

}

}

// 非阻塞获取信号量

int sem_trywait(sem_t * sem)

{

long oldstatus, newstatus;

do {

oldstatus = sem->sem_status;

// oldstatus & 1等于0说明是线程，即有线程在等待，或者等于1，都说明没有可用资源，直接返回

if ((oldstatus & 1) == 0 || (oldstatus == 1)) {

errno = EAGAIN;

return -1;

}

// 更新资源数

newstatus = oldstatus - 2;

}

// 更新资源数，如果失败说明被其他线程拿到锁了，则重新执行do里面的逻辑，因为数据可能被修改了

while (! compare_and_swap(sem, oldstatus, newstatus));

return 0;

}

int sem_post(sem_t * sem)

{

long oldstatus, newstatus;

pthread_t th, next_th;

do {

oldstatus = sem->sem_status;

// 说明原来的资源数是0，并且有线程在等待，则更新为1，2n+1即3

if ((oldstatus & 1) == 0)

newstatus = 3;

else {

if (oldstatus >= SEM_VALUE_MAX) {

/* Overflow */

errno = ERANGE;

return -1;

}

// 否则加2，即资源数加一

newstatus = oldstatus + 2;

}

}

// 更新资源数

while (! compare_and_swap(sem, oldstatus, newstatus));

// 如果之前有线程阻塞，则唤醒所有线程，再次竞争获得信号量

if ((oldstatus & 1) == 0) {

th = (pthread_t) oldstatus;

do {

next_th = th->p_nextwaiting;

th->p_nextwaiting = NULL;

restart(th);

th = next_th;

} while(th != (pthread_t) 1);

}

return 0;

}

// 获取资源数

int sem_getvalue(sem_t * sem, int * sval)

{

long status = sem->sem_status;

// 有资源

if (status & 1)

// 除以2

*sval = (int)((unsigned long) status >> 1);

else

*sval = 0;

return 0;

}

int sem_destroy(sem_t * sem)

{

// 有线程在等待

if ((sem->sem_status & 1) == 0) {

errno = EBUSY;

return -1;

}

return 0;

}

阻塞时的视图。

看完上述内容是否对您有帮助呢？如果还想对相关知识有进一步的了解或阅读更多相关文章，请关注行业资讯频道，感谢您对的支持。

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.