Is it possible to use mutex in multiprocessing case on Linux/UNIX ?

Question

This is an interview question.

Is it possible to use mutex in multiprocessing case on Linux/UNIX ?

My idea: No, different processes have separate memory space.

mutex is only used for multithreading.

semaphore is used for multiprocessing to do synchronization.

right ?

Any comments are welcome.

thanks

Answer 1

 Mutual exclusion locks (mutexes)  prevent  multiple  threads
 from simultaneously executing critical sections of code that
 access shared data (that is, mutexes are used  to  serialize
 the  execution  of  threads).  All mutexes must be global. A
 successful call for a mutex lock  by  way  of   mutex_lock()
 will  cause  another  thread that is also trying to lock the
 same mutex to block until the owner thread unlocks it by way
 of   mutex_unlock().  Threads  within  the  same  process or
 within other processes can share mutexes.

 Mutexes can synchronize threads within the **same  process**  or
 in  ***other   processes***.  Mutexes  can  be used to synchronize
 threads between processes if the mutexes  are  allocated  in
 writable  memory  and shared among the cooperating processes
 (see mmap(2)), and have been initialized for this task.

Initialize Mutexes are either intra-process or inter-process, depending upon the argument passed implicitly or explicitly to the initialization of that mutex. A statically allocated mutex does not need to be explicitly initialized; by default, a statically allocated mutex is initialized with all zeros and its scope is set to be within the calling process.

 For inter-process synchronization, a mutex needs to be allo-
 cated   in  memory shared between these processes. Since the
 memory for such a mutex must be allocated dynamically,   the
 mutex needs to be explicitly initialized using mutex_init().

Answer 2

It is quite possible to use a process-shared mutex.

In fact, modern applications prefer using a process shared mutex along with process shared condition variable over a semaphore because the latter is less flexible.

I remember using Red Hat Linux in 2004 and at that time it supported both process shared mutexes and condition variables.

Answer 3

Not quite. POSIX threads has a concept of a process-shared attribute which can be used to create mutexes that can be operated on by multiple processes.

You can put such a mutex in shared memory so that multiple processes can all get at it.

Whether LINUX implements this., I'm not sure, I've never had a need to use it since it seems unnecessarily complex.

For a useful precis of attributes, see my answer to this question.

Answer 4

I was looking for a named mutex so that I could ensure mutual exclusion for the lifetime of a process (making sure only one process running per some set of properties). I didn't find one (looks like I might have not looked hard enough) and so I implemented my own pseudo named mutex in linux by using an abstract UNIX domain socket. Only a single bind() to that socket will succeed. The other nice thing is that the OS will cleanup the abstract UNIX domain socket if the process dies and thus doesn't cleanup the socket itself. Unfortunately I'm not sure of any way for you to "wait" on this pseudo mutex to become available.

An abstract UNIX domain socket is a UNIX domain socket whose name begins with a null byte. Beware though, I believe that the entire buffer is used as the name and thus you want to ensure that you don't just memcpy or strcpy a partial string into it, or if you do make sure you first fill the entire buffer with some character.

All but the first bind() will fail with an errno of EADDRINUSE.

// Create an abstract socket to use as a mutex.                             

int err;
int mutex_sock = socket(AF_UNIX, SOCK_STREAM, 0);
if (mutex_sock == -1)
    {
    err = errno;
    printf("main, failed creating mutex socket: %s\n",
            get_error_string(errno, error_string, sizeof(error_string)));
    log_event(LOG_LEVEL_ERROR, "main, failed creating mutex socket: "
            "%s", get_error_string(errno, error_string,
            sizeof(error_string)));
    errno = err;
    goto done;
    }

// Bind to abstract socket.  We use this as a sort of named mutex.          

struct sockaddr_un addr;
memset(&addr, 0, sizeof(addr));
addr.sun_family = AF_UNIX;
strncpy(addr.sun_path + 1, socket_name, sizeof(addr.sun_path) - 2);
result = bind(mutex_sock, (struct sockaddr*) &addr, sizeof(addr));
if (result == -1)
    {
    err = errno;
    if (errno == EADDRINUSE)
        {
        printf("main, failed bind to mutex socket: %s.  "
                "Another instance must be running.\n",
                get_error_string(errno,
                error_string, sizeof(error_string)));
        log_event(LOG_LEVEL_ERROR, "main, failed bind to mutex socket: "
                "%s.  "
                "Another instance must be running.",
                get_error_string(errno,
                error_string, sizeof(error_string)));
        }
    else
        {
        printf("main, failed bind to mutex socket: %s\n",
                get_error_string(errno, error_string,
                sizeof(error_string)));
        log_event(LOG_LEVEL_ERROR, "main, failed bind to mutex socket: %s",
                get_error_string(errno, error_string,
                sizeof(error_string)));
        }
    errno = err;
    goto done;
    }

Thanks, Nick

Answer 5

Yes, in general in Linux we have only unnamed mutexes due to which they cannot operate between processes. We need a semaphore to get over that.

In windows, they have a concept of named mutexes which lets us use mutexes across processes.