请教使用sem_unlink删除信号量的问题

[yueyixing]

两个进程之间使用同一个命名信号量进行通讯（使用sem_open创建），文档上说sem_unlink会维护一个引用计数只有当此计数为零时才真正删除此信号量，但是现在我发现只要一调用sem_unlink函数信号量就会删除，不知道为什么？（开发环境：Redhat.Enterprise.Linux5.U3.i386）

[yueyixing]

sem_getvalue取得应该是信号量的值，而不是信号量的引用计数值吧。

[yueyixing]

没有人遇到过这种情况？

[rzsheng]

我不懂
我想知道如何计数呢？
你怎么知道没有计数完成就删除了呢？

[yueyixing]

就是在创建的时候判断是否已经存在指定名称的信号量，如果没有就创建，有就返回已创建的并把引用计数加一。
因为Linux是通过文件来实现的，所以创建成功后会在/dev/shm路径下生成对应名称的文件（针对Posix信号量），所以信号量删除的话文件也会删除，所以就会知道何时删除的。不知道我说清楚没有？

[猫已经找不回了]

The sem_unlink() function removes the specified named semaphore. If other process are currently referencing the specified semaphore, sem_unlink() has no effect on the state of the semaphore. If one or more processes have the specified semaphore open when you call sem_unlink(), the semaphore is not removed until all references to the semaphore has been destroyed by sem_close(), _exit(), or exec() calls. Calls to sem_open() to re-create or re-connect to the specified semaphore refer to a new semaphore after sem_unlink() is called. The sem_unlink() call does not block until all references have been destroyed; it returns immediately.



你是怎么确定被删除的时候，引用计数不为0.

[yueyixing]

因为我另一个程序还正在运行阿，另外我也想看看他的引用计数值，但是不知道怎么得到，有人告诉一下吗？

[yueyixing]

还有我写了两个测试程序，一个创建完以后就用pause()挂起，然后另一个也创建同名的信号量，创建成功后就调用sem_close和sem_unlink删除，发现只要第二个程序一调用sem_unlink，信号量就会删除。RedHat 4和5都是这样，不知道为什么。

[pottichu]

sem_getvalue - get the value of a semaphore

SYNOPSIS
#include <semaphore.h>

int sem_getvalue(sem_t *sem, int *sval);

DESCRIPTION
sem_getvalue() places the current value of the semaphore pointed to sem into the integer pointed to by sval.

If one or more processes or threads are blocked waiting to lock the semaphore with sem_wait(3), POSIX.1-2001 permits
two possibilities for the value returned in sval: either 0 is returned; or a negative number whose absolute value is
the count of the number of processes and threads currently blocked in sem_wait(3). Linux adopts the former behavior.

RETURN VALUE
sem_getvalue() returns 0 on success; on error, -1 is returned and errno is set to indicate the error.

ERRORS
EINVAL sem is not a valid semaphore.

CONFORMING TO
POSIX.1-2001.

NOTES
The value of the semaphore may already have changed by the time sem_getvalue() returns.

[yueyixing]

ipcs命令是看不到用posix系列函数创建的信号量的

[jiangfeng999]

[Quote=引用 6 楼 yueyixing 的回复:]
还有我写了两个测试程序，一个创建完以后就用pause()挂起，然后另一个也创建同名的信号量，创建成功后就调用sem_close和sem_unlink删除，发现只要第二个程序一调用sem_unlink，信号量就会删除。RedHat 4和5都是这样，不知道为什么。
[/Quote]
你是先调用 sem_close还是先调用sem_unlink呢?
最好把你写的两个测试程序发出来看看

[threeleafzerg007]

用 ipcs 看看状态先

[Joan201401]

guanzhu

[yueyixing]

楼上MM头像好可爱阿，呵呵！再过两天结帖