I have some data stored in a FLASH memory that I need to access with C pointers to be able to make a non-Linux graphics driver work (I think this requirement is DMA related, not sure). Calling read works, but I don’t want to have intermediate RAM buffers between the FLASH and the non-Linux driver.
However, just creating a pointer and storing the address that I want on it is making Linux emit an exception about invalid access on me.
void *ptr = 0xdeadbeef;
int a = *ptr; // invalid access!
What am I missing here? And could someone point me to a material to make this concepts clear for me?
I’m reading about mmap but I’m not sure that this is what I need.
The problem you have is that linux runs your program in a virtual address space. So every address you use directly in the code (like 0xdeadbeef) is a virtual address that gets translated by the memory management unit into a physical address which is not necessarily the same as your virtual address. This allows easy separation of multiple independent processes and other stuff like paging, etc.
The problem is now, that in your case no physical address is mapped to the virtual address 0xdeadbeef causing the kernel to abort execution.
The call mmap you already found asks the kernel to assign a specific file (from a specific offset) to a virtual address of your process. Note that the returning address of mmap could be a completely different address. So don’t make any assumptions about the virtual address you get.
Therefore there are examples with mmap and /dev/mem out there where the offset for the memory device is the physical address. After the kernel was able to assign the file from the offset you gave to a virtual address of your process you can access the memory area asif it were a direct access.
After you don’t need the area anymore don’t forget to munmap the area. Otherwise you’ll cause something similar to a memory leak.
One problem with the /dev/mem method is that the user running the process needs access to this device. This could introduce a security issue (e.g. Samsung recently introduced such a security hole in their hand held devices)
A more secure way is the way described in a article i found (The Userspace I/O HOWTO) as you still have control about the memory areas accessable by the user’s process.