ARM Hacking: EXC_ARM_DA_ALIGN exception

http://www.galloway.me.uk/2010/10/arm-hacking-exc_arm_da_align-exception/

I came across a problem today that I’d seen before but couldn’t remember when. Then I stumbled across Peter Bakhirev writing up his findings at Byte Club about a problem I’d helped him with. So I thought I’d quickly write up my summary here incase it helps anyone else.

It started back when I was browsing the Apple developer forums once – I came across someone having a problem where the EXC_ARM_DA_ALIGN exception was thrown. It turned out that this was a problem with setting the value of a variable by dereferencing a pointer, like so:

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char *mem = malloc(16); // alloc 16 bytes of data double *dbl = mem + 2; double set = 10.0; *dbl = set;

The compiler was emitting a STMIA instruction that was using a memory location that isn’t word (32-bit) aligned (because of the +2 into the ‘mem’ memory location – assuming mem is word aligned). This causes the processor to throw the exception that bubbles up as an EXC_ARM_DA_ALIGN.

The solution above would be to memcpy instead of dereferencing like so:

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char *mem = malloc(16); // alloc 16 bytes of data double *dbl = mem + 2; double set = 10.0; memcpy(dbl, &set, sizeof(set));

This is just a simple example of the problems that can occur when you’re not careful with reading/writing arbitrary memory that you have allocated. Granted, it’s probably a compiler bug in this case, but it illustrates the point.