How do I choose a fixed address for mmap?

Question

mmap() can be optionally supplied with a fixed location to place the map. I would like to mmap a file and then have it available to a few different programs at the same virtual address in each program. I don't care what the address is, just as long as they all use the same address. If need be, the address can be chosen by one of them at run time (and communicated with the others via some other means).

Is there an area of memory that Linux guarantees to be unused (by the application and by the kernel) that I can map to? How can I find one address that is available in several running applications?

Answer 1

Not really, no. With address space randomisation on modern linux systems it is very hard to guarantee anything about what addresses may or may not be used.

Also, if you're thinking of using MAP_FIXED then be aware that you need to be very careful as it will cause mmap to unmap anything that may already be mapped at that address which is generally a very bad thing.

I really think you will need to find another solution to your problem...

Answer 2

Two processes can map a shared memory block to the same virtual address using shm_open() and mmap(); that is, the virtual address returned from mmap() can be the same for both processes. I found that Linux will by default give different virtual addresses to different processes for the same piece of shared memory, but that using mmap() with MAP_FIXED will force Linux to supply the same virtual address to multiple processes.

The process that creates the shared memory block must store the virtual address somewhere, either within the shared memory, in a file, or with some other method so that another process can determine the original virtual address. The known virtual address is then used in the mmap() call, along with the MAP_FIXED flag.

I was able to use the shared memory to do this. When doing so, the "golden" virtual address is stored within the shared memory block; I made a structure that contains a number of items, the address being one of them, and initialized it at the beginning of the block.

A process that wants to map that shared memory must execute the mmap() function twice; once to get the "golden" virtual address, then to map the block to that address using the MAP_FIXED flag.

Interestingly, I'm working with an embedded system running a 2.6 kernel. It will, by default, supply the same virtual address to all mmap() calls to a given file descriptor. Go figure.

Bob Wirka