问题
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
回答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
来源：https://stackoverflow.com/questions/36713356/access-mmap-memory-from-another-process