Question

I’m currently writing a file loader for an ipad program and I’m getting strange EXC_BAD_ACCESS exceptions. Here is a short snipped of code that I think is the reason for the error:

float testFloat() {
    char mem[32];
    char *charPtr = &mem[0];
    float *floatPtr = (float*)(charPtr + 1);
    float f = *floatPtr; //EXC_BAD_ACCESS
    return f;
}

The error happens only if the offset of charPtr is not divisible by 4, so I guess it could have something to do with pointer alignment on ARM CPUs.

Answer 1

You are correct, this is due to pointer alignment. On many RISC systems, the alignment needs to be at least as large as the data-type itself. (ARM falls into this category.)

In this case, float is 4 bytes, so the address needs to be aligned to 4 bytes. (divisible by 4)

Furthermore, this type of type-punning violates strict-aliasing.

On x86 systems, memory accesses do not always have to be aligned – but there will usually be a performance penalty on a misaligned access.