How can i share memory between parent-child process that free the memory automatically when they die

Question

I know about "mmap", but as far as i know if i want to share memory allocated by a parent process and accessed it by a client process i need to create a temporary file.

But this file will continue to exist if the processes die.

I was educated to never leave garbage behind. Both in real life and in programming.

The solution should work on Linux, FreeBSD and Solaris.

Answer 1

This article is a very good starting point for shared memory.

However, I'd recommend that you use a pipe instead generally, so as to avoid race conditions and all the mental overhead of concurrency. When you open a child process, its stdin and stdout are file descriptors that you can read and write to from the parent process.

Answer 2

Allocate Memory in Parent. Use this memory both in parent and Child. Give the responsibility of freeing the memory to the parent. Make the parent wait (wait system call) on the child. Parent frees the memory before exiting.

Alternatively to be on safer side, before the child exits check if the parent is alive or not. If not, free the memory in child itself.But this won't work if there can be multiple child.

You can use first few bits of memory to track the number of process using this memory. Whenever a process starts using memory it increment this count and before exiting it decrements the count.Plus if the count becomes 0 , Free the memory.

There is yet another method.Write a function ownMalloc which sits on top of system malloc (or whichever function you uses). This keeps track of all allocated memory and also the processes which uses it. It periodically goes through the the different chunks allocated and frees the chunk not in use.

Answer 3

Mmap usually supports "anonymous" mode which does not create any garbage. It works on LInux, according to man page works on Solaris, I am not sure about FreeBSD - look at the man page and search for MAP_ANON or MAP_ANONYMOUS.

Answer 4

Use POSIX shm_open(3) and related functions to map memory not backed by a file. The file descriptor returned by shm_open() should be automatically closed if parent and child cease to exist, but I'm not 100% sure if that's always the case. Maybe someone else can shed more light on that?

Answer 5

Named Pipes should do for you as mentioned above. You can use two pipes (if possible) :

1. parentpid_childpid -- Parent writes and child reads
2. childpid_parentpid -- Child writes and parebt reads.

This works for me. Please mention if you have any special senarios to consider.

Answer 6

Allocate the memory in Parent process, keep on wait till the child process executes or let him do some other tasks. Once the child process is over then it will return to the parent process, Deallocate the memory there.

If the parent process needs to be stopped before child(Child is said to be orphaned in this case) then use the exec() which will start child process as a new process.