System-wide global variable / semaphore / mutex in C++/Linux?

You can use a named semaphore if you can get all the processes to agree on a common name.

A named semaphore is identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX-4 (i.e., 251) characters consisting of an initial slash, followed by one or more characters, none of which are slashes. Two processes can operate on the same named semaphore by passing the same name to sem_open(3).

For interprocess mutual exclusion, you can use file locking. With linux, the code is as simple as protecting the critical section with a call to flock.

int fd_lock = open(LOCK_FILE, O_CREAT);flock(fd_lock, LOCK_EX);// do stuffflock(fd_lock, LOCK_UN);

If you need POSIX compatibility, you can use fcntl.

You can make C++ mutexes work across process boundaries on Linux. However, there's some black magic involved which makes it less appropriate for production code.

Explanation:

The standard library's std::mutex and std::shared_mutex use pthread's struct pthread_mutex_s and pthread_rwlock_t under the hood. The native_handle() method returns a pointer to one of these structures.

The drawback is that certain details are abstracted out of the standard library and defaulted in the implementation. For example, std::shared_mutex creates its underlying pthread_rwlock_t structure by passing NULL as the second parameter to pthread_rwlock_init(). This is supposed to be a pointer to a pthread_rwlockattr_t structure containing an attribute which determines sharing policy.

public:    __shared_mutex_pthread()    {        int __ret = pthread_rwlock_init(&_M_rwlock, NULL);        ...

In theory, it should receive default attributes. According to the man pages for pthread_rwlockattr_getpshared():

The default value of the process-shared attribute is PTHREAD_PROCESS_PRIVATE.

That said, both std::shared_mutex and std::mutex work across processes anyway. I'm using Clang 6.0.1 (x86_64-unknown-linux-gnu / POSIX thread model). Here's a description of what I did to check:

• Create a shared memory region with shm_open.

• Check the size of the region with fstat to determine ownership. If .st_size is zero, then ftruncate() it and the caller knows that it is the region's creating process.

• Call mmap on it.

  • The creator process uses placement-new to construct a std::mutex or std::shared_mutex object within the shared region.
  • Later processes use reinterpret_cast<>() to obtain a typed pointer to the same object.

• The processes now loop on calling trylock() and unlock() at intervals. You can see them blocking one another using printf() before and after trylock() and before unlock().

Extra detail: I was interested in whether the c++ headers or the pthreads implementation were at fault, so I dug into pthread_rwlock_arch_t. You'll find a __shared attribute which is zero and a __flags attribute which is also zero for the field denoted by __PTHREAD_RWLOCK_INT_FLAGS_SHARED. So it seems that by default this structure is not intended to be shared, though it seems to provide this facility anyway (as of July 2019).

Summary

It seems to work, though somewhat by chance. I would advise caution in writing production software that works contrary to documentation.