When is the release system call called?

I am writing a device driver in which I have the mmap system call refer to a function of my driver, which is registered as a file operation. Then I have the release system call refer to another function, registered as vm_operations_struct. When my user level program which uses those system calls exits or is killed while running I see that the release function is called automatically, with no code written. My question is whether the release system call is called actually automatically when the program returns or crashes and is there a way to call the release system call myself in the code?

Yes, whenever a process terminates, either normally (via the _exit system call) or by the action of a signal, the kernel automatically releases all resources held by that process. For Linux I believe you can look at the code of do_exit to see how this is done.

There is no "release" system call, but assuming you have wired everything up correctly, I would expect that your driver's release function will also be called when the program uses munmap to tear down memory mappings backed by your driver, and/or when it uses close to close file descriptors backed by your driver.

It is true that the release function in the vm_operations_struct is automatically called by the kernel whenever the last user-level process or thread that has a reference to the memory region returned by mmap exits or the memory region is specifically unmapped using the munmap system call.

A resource that was allocated during an mmap call, such as physical memory or device registers, must be released using the release function. Before the memory region is unmapped, the kernel calls this function to give the driver time to defragment its resources.

The vm_area_struct structure, which is passed as an argument to the release function, can be used to explicitly call the function from within your driver code. The release function is designed to only be called by the kernel when it is safe to release resources related to the memory region, so doing so is typically not advised. Self-releasing could result in unexpected behavior and even a system crash.

Consider installing a SIGINT or SIGTERM signal handler or registering an exit handler using the atexit function if you need to perform cleanup tasks when your user-level program crashes or exits. These mechanisms enable you to carry out cleanup actions prior to the user killing or terminating your program.