linux下如何实现进程间互斥锁？高手来。

zc5524528

问题：
一个动态库，其中有两个函数都会创建一个文件，是同样的文件，并可对文件进行操作。
这个库可以供多个进程调用，此时是否可以通过某种机制，实现进程A上锁后，进程B就不可以操作文件，当A解锁后进程B才可以继续访问。

我通过sem_init()/sem_wait()/sem_post()可以实现在同一个进程内的多个线程之间的互斥。
但是进程之间就有点想不通了。

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

man sem_init
sem_init() initialises the unnamed semaphore at the address pointed to by sem.  The value argument speci-
       fies the initial value for the semaphore.

       The pshared argument indicates whether this semaphore is to be shared between the threads of  a  process,
       or between processes.

       If  pshared has the value 0, then the semaphore is shared between the threads of a process, and should be
       located at some address that is visible to all threads (e.g., a global variable, or a variable  allocated
       dynamically on the heap).

       If pshared is non-zero, then the semaphore is shared between processes, and should be located in a region
       of shared memory (see shm_open(3), mmap(2), and shmget(2)).  (Since a child created by  fork(2)  inherits
       its  parent’s memory mappings, it can also access the semaphore.)  Any process that can access the shared
       memory region can operate on the semaphore using sem_post(3), sem_wait(3), etc.

       Initialising a semaphore that has already been initialised results in undefined behaviour.


其中第四段是重点，我试着去按照提示实现，但是google后，有的说这个暂时还没实现。

疑惑。。。。
求高手帮忙。

heartless_cu

#include <pthread.h>

       int pthread_mutexattr_getpshared(const pthread_mutexattr_t *
              restrict attr, int *restrict pshared);
       int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr,
              int pshared);

DESCRIPTION
       The pthread_mutexattr_getpshared() function shall obtain the value of the process-shared attribute from the attributes  object  referenced  by  attr.  The
       pthread_mutexattr_setpshared() function shall set the process-shared attribute in an initialized attributes object referenced by attr.

       The  process-shared attribute is set to PTHREAD_PROCESS_SHARED to permit a mutex to be operated upon by any thread that has access to the memory where the
       mutex is allocated, even if the mutex is allocated in memory that is shared by multiple processes. If the process-shared attribute is PTHREAD_PROCESS_PRI-
       VATE,  the  mutex shall only be operated upon by threads created within the same process as the thread that initialized the mutex; if threads of differing
       processes attempt to operate on such a mutex, the behavior is undefined. The default value of the attribute shall be PTHREAD_PROCESS_PRIVATE.

MMMIX

回复 1# zc5524528


    试下文件锁。

bzcat

1) 得到共享内存shm_open,mmap/shmget,shmat
2) pthread_mutexattr_setpshared

zc5524528

最开始用的就是文件锁，但操作文件太慢了，所以想换掉。已经OK了回复 3# MMMIX


   

cxytz01

unp卷一第二版中用的就是文件锁，锁的是/dev/zero。27.8章

qiankunhuanying

回复 2# heartless_cu

请问楼主最后是怎么实现的?我现在也要实现一个相似功能，但是做不到进程间的数据同步，因此想咨询一下你的方法。
多谢了


   

cutrain2010

pthread_mutex_t

cutrain2010

看错鸟，是进程的，使用文件锁flock，或者fctrl

qqrilxk

使用有名信号量即可。
有名信号量：可以用于不同进程间或多线程间的互斥与同步
  创建打开有名信号量
  sem_t *sem_open(const char *name, int oflag);
  sem_t *sem_open(const char *name, int oflag, mode_t mode, unsigned int value);
  成功返回信号量指针；失败返回SEM_FAILED，设置errno
  name是文件路径名,但不能写成/tmp/a.sem这样的形式,因为在linux下,sem都是在/dev/shm目录下,可写成"/mysem"或"mysem",创建出来的文件都是"/dev/shm/sem.mysem",mode设置为0666，value设置为信号量的初始值.所需信号灯等已存在条件下指定
  O_CREAT|O_EXCL却是个错误。