关于sem_unlink什么时候删除信号量

最新推荐文章于 2024-08-31 22:38:19 发布

mrwangwang

最新推荐文章于 2024-08-31 22:38:19 发布

阅读量1.7k

点赞数 1

分类专栏： linux相关

linux相关专栏收录该内容

179 篇文章

订阅专栏

本文探讨了sem_unlink函数的行为特点，即它会立即删除信号量名称，但信号量本身需等所有进程关闭后才销毁。通过示例代码展示了信号量在调用sem_unlink后的状态变化及对后续创建同名信号量的影响。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

sem_unlink在man手册里有这么一段话：

sem_unlink() removes the named semaphore referred to by name.  
The semaphore name is removed immediately.  The semaphore  is
destroyed  once all other processes that have the semaphore open close it.

这段话的意思是说，sem_unlink会马上删除指定的信号量名，但要等到所有打开该信号量的进程关闭该信号量后才删除该信号。详细地说，当进程创建一个有名信号量，会在/dev/shm下生成一个sem.xxx的文件，所有打开该信号量的进程（包括创建它的进程）都会增加该文件的引用计数，并且这个计数由内核管理。当调用sem_unlink时，/dev/shm下的sem.xxx文件会马上被删除，但是信号量本身并没有被删除，所有已打开该信号量的进程仍能正常使用它。直到所有打开该信号量的进程关闭该信号量后，内核才真正删除信号量。

　　因此，信号量跟sem.xxx并不是共存亡的。下面来测试下：

/* sem_create.cpp */

#include <iostream>
#include <fcntl.h>           /* For O_* constants */
#include <sys/stat.h>        /* For mode constants */
#include <semaphore.h>
#include <unistd.h>
#include <cerrno>
#include <cstring>

int main()
{
    sem_t *psem = sem_open( "/sem_test",O_CREAT,S_IRWXU,1 );
    if ( SEM_FAILED == psem )
    {
        std::cout << "sem_open fail:" << strerror(errno) << std::endl;
        return 0;
    }
    
    int sval = 0;
    sem_getvalue( psem,&sval );
    
    std::cout << "sleep now ....value is " << sval << std::endl;
    sleep( 30 );
    
    sem_getvalue( psem,&sval );
    std::cout << "wake up ....value is " << sval << std::endl;
    
    sem_post( psem );
    sem_getvalue( psem,&sval );
    std::cout << "raise value and try again ....value is " << sval << std::endl;
    
    sem_close( psem );
}

/**
 *
 * sem_destruct.cpp
 */

#include <iostream>
#include <fcntl.h>           /* For O_* constants */
#include <sys/stat.h>        /* For mode constants */
#include <semaphore.h>
#include <cerrno>
#include <cstring>

int main()
{
    sem_t *psem = sem_open( "/sem_test",O_CREAT,S_IRWXU,1 );
    if ( SEM_FAILED == psem )
    {
        std::cout << "sem_open fail:" << strerror(errno) << std::endl;
        return 0;
    }
    
    std::cout << "decrease value and remove sem now..." << std::endl;
    sem_wait ( psem );
    
    sem_close( psem );
    sem_unlink( "/sem_test" );
    
    std::cout << "sem should remove from /dev/shm,but not really remove" << std::endl;
}

分别编译上面的文件：

g++ -pthread -o sem_create sem_create.cpp
g++ -pthread -o sem_destruct sem_destruct.cpp

先运行sem_create，再运行sem_destruct这时个可以看到输出为：

xzc@xzc-HP-ProBook-4446s:~/code/sem$ ./sem_create
sleep now ....value is 1
xzc@xzc-HP-ProBook-4446s:~/code/sem$ ./sem_destruct
decrease value and remove sem now...
sem should remove from /dev/shm,but not really remove
wake up ....value is 0
raise value and try again ....value is 1

可以看到，即使sem_destruct从/dev/shm把信号量删除了，ls /dev/shm也确认对应的文件删除了。但是经过sleep的sem_create进程在wake up后仍能对信号量进行操作。

　　可以看到，有名信号量一旦打开后，跟名字并没有多大关系。下面再来测试有名信号量调用sem_unlink但仍有进程在使用，这时另外一个进程又要创建同名信号量的情况

/**
  *
  * sem_again.cpp
  */

#include <iostream>
#include <fcntl.h>           /* For O_* constants */
#include <sys/stat.h>        /* For mode constants */
#include <semaphore.h>
#include <unistd.h>
#include <cerrno>
#include <cstring>

int main()
{
    sem_t *psem = sem_open( "/sem_test",O_CREAT|O_EXCL,S_IRWXU,1 );
    if ( SEM_FAILED == psem )
    {
        std::cout << "sem_open fail:" << strerror(errno) << std::endl;
        return 0;
    }
    
    int sval = 0;
    sem_getvalue( psem,&sval );
    
    //std::cout << "sleep now ....value is " << sval << std::endl;
    //sleep( 30 );
    
    sem_getvalue( psem,&sval );
    std::cout << "value is " << sval << std::endl;
    
    sem_post( psem );
    sem_getvalue( psem,&sval );
    std::cout << "raise value and try again ....value is " << sval << std::endl;
    
    sem_close( psem );
    sem_unlink( "/sem_test" );
}

编译一下：g++ -pthread -o sem_again sem_again.cpp，然后依次运行sem_create、sem_destruct、sem_again

xzc@xzc-HP-ProBook-4446s:~/code/sem$ ./sem_create
sleep now ....value is 1

xzc@xzc-HP-ProBook-4446s:~/code/sem$ ./sem_destruct
decrease value and remove sem now...
sem should remove from /dev/shm,but not really remove

xzc@xzc-HP-ProBook-4446s:~/code/sem$ ./sem_again
value is 1
raise value and try again ....value is 2

wake up ....value is 0
raise value and try again ....value is 1