Protecting HW resources with many processes

Question

I have created a driver for Linux that exists in the user space. I have created a memory map shared file that maps down to some custom HW registers.

The driver will do the usual read-modify-write and so I need some kind of lock to ensure synchronization.

The driver I am making can be used by many processes and of course threads and so I was wondering which is the best way to handle this.

Is it so that std::mutex would be enough to handle this kind of situation? I have read that std::mutex does not work for multi-process applications, is this true?

I cannot use the Boost libraries.

Answer 1

Assuming that you're running bespoke software in combination with the hardware, the easiest solution is just flock. A more fine-grained approach is fcntl(F_SET_LK) but that's broken-by-design (Linux may release the lock when you close an unrelated file descriptor!).

Answer 2

You could create a daemon (a normal program), which actually mmap() and access the HW registers, and enable some kind of interface to communicate with other programs (for example dbus, or whatever IPC message queue mechanism you chose).

This way, you do not have to worry if multiple threads or processes are going to access the same register at the same time, avoiding race conditions.

Of course, this daemon would have to run, before someone tries to access your hardware. Something like bluetooth's daemon (man bluetoothd).