Pärserk Pärserk - 3 months ago 14
C Question

How to demonstrate a memory misalignment error in C on a macbook pro (Intel 64 bit processor)

In an attempt to understand C memory alignment or whatever the term is (data structure alignment?), i'm trying to write code that results in a alignment error. The original reason that brought me to learning about this is that i'm writing data parsing code that reads binary data received over the network. The data contains some uint32s, uint64s, floats, and doubles, and i'd like to make sure they are never corrupted due to errors in my parsing code.

An unsuccessful attempt at causing some problem due to misalignment:

uint32_t integer = 1027;
uint8_t * pointer = (uint8_t *)&integer;
uint8_t * bytes = malloc(5);
bytes[0] = 23; // extra byte to misalign uint32_t data
bytes[1] = pointer[0];
bytes[2] = pointer[1];
bytes[3] = pointer[2];
bytes[4] = pointer[3];
uint32_t integer2 = *(uint32_t *)(bytes + 1);
printf("integer: %u\ninteger2: %u\n", integer, integer2);


On my machine both integers print out the same. (macbook pro with Intel 64 bit processor, not sure what exactly determines alignment behaviour, is it the architecture? or exact CPU model? or compiler maybe? i use Xcode so clang)

I guess my processor/machine/setup supports unaligned reads so it takes the above code without any problems.

What would a case where parsing of say an uint32_t would fail because of code not taking alignment in account? Is there a way to make it fail on a modern Intel 64 bit system? Or am i safe from alignment errors when using simple datatypes like integers and floats (no structs)?

Edit: If anyone's reading this later, i found a similar question with interesting info: Mis-aligned pointers on x86

Answer

Normally, the x86 architecture doesn't have alignment requirements [except for some SIMD instructions like movdqa).

However, since you're trying to write code to cause such an exception ...

There is an alignment check exception bit that can be set into the x86 flags register. If you turn in on, an unaligned access will generate an exception which will show up [under linux at least] as a bus error (i.e. SIGBUS)

See my answer here: any way to stop unaligned access from c++ standard library on x86_64? for details and some sample programs to generate an exception.

Comments