Re: mmap for I/O access

Robert Scott wrote:
My call to mmap is causing a Segmentation fault. Is this the right way to do
it?

It should work more or less as you have guessed, but there are a few things to fix :

- you need to map to an offset which is a multiple of the page size, usually 4K. Get the page size with a call to getpagesize(2).

- The size you're mapping to also needs to be a multiple of the page size.

I usually write a little wrapper function which allows the user to map arbitrary addresses, computing the offset and alignment for you behind the scenes.

- check that you're mapping to the physical address as actually presented on the CPU's bus, and not the mapped or translated address supported by the CPU. I don't know what way this is set up on ARM. Ask your hardware engineer or check the hardware guide for the board.

- I would strongly recommend that you ass the O_SYNC flag to the open(2) call. This will tell the OS to use an uncached mapping. Not using O_SYNC may cause weird effects.
.

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