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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。
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/* 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 << 1) + 1;
return 0;
}
int sem_wait(sem_t * sem)
{
long oldstatus, newstatus;
volatile pthread_t self = thread_self();
pthread_t * th;
while (1) {
do {
// oldstatus可能是线程或者资源数
oldstatus = sem->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。
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