What does an “alignment trap” error means?

Question

One of my friends is having a big problem trying to debug a code that started showing \"alignment trap\" errors. The problem happens when a global structure is accessed by a

Answer 1

An alignment trap is triggered by ARM whenever an unaligned access is made. What is an unaligned access? It's when a multibyte value is accessed where its pointer is not a multiple of its alignment, e.g. when a uint32_t is accessed by dereferencing a pointer that isn't a multiple of 4.

You can get them if you have __attribute__((packed)) data structures like this:

struct foo {
    uint8_t a;
    uint32_t b;
} __attribute__((packed));

Accesses to b will be unaligned and therefore will cause an alignment trap. You have to memcpy the data into an aligned value, and then access it.