Access mmap memory from another process

Question

I've started playing with mmap. I'm trying to create an example workspace that will be then extended to the real case.

This is what I want to achieve:

PROCESS 1:

• mmap a file (actually a device, but it's okay to generate an example with a text file)

PROCESS 2: (not foked from process 1; just an independent process)

• read the memory mapped by process 1
• change some bits
• write it to a new file

I've read several examples and documentations, but I still didn't find how to achieve this. What I'm missing is:

• how can process 2 access the memory mapped by process 1, without knowing anything about the opened file?
• how can I put the mmap content in a new file? I suppose I have to ftruncate a new file, mmap this file and memcpy the content of process 1 memory map to process 2 memory map (then msync)

Side info, I have a message queue opened between the two processes, so they can share some messages if needed (ex. the memory address/size, ...).

Any hints?

Thanks in advance!

MIX

Answer 1

This answer considers you are trying to do this stuff on linux/unix.

how can process 2 access the memory mapped by process 1, without knowing anything about the opened file?

Process 1 passes to mmap[1] the flag MAP_SHARED.

You can:

• A) Share the file descriptor using unix domain sockets[2].
• B) Send the name of the file using the queues you mentioned at the end of your message.

Process 2 opens mmap with the flag MAP_SHARED. Modifications to the mmaped memory in Process 1 will be visible for Process 2. If you need fine control of when the changes from process 1 are shown to process 2 you should control it with msync[3]

how can I put the mmap content in a new file? I suppose I have to ftruncate a new file, mmap this file and memcpy the content of process 1 memory map to process 2 memory map (then msync)

Why just don't write the mmaped memory as regular memory with write?

[1] http://man7.org/linux/man-pages/man2/mmap.2.html

[2] Portable way to pass file descriptor between different processes

[3] http://man7.org/linux/man-pages/man2/msync.2.html