Fault when casting from char* to uint64_t*

[Ellaine]

In my code, I write many casting from char* to uint64_t*. Someone success, and someone get failure. I thought I used the unallocated memory, but I rewrote by casting to uint32_t, and then no error happened. Is this normal?

My code:

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 uint32_t sz_t, sz_h;

 sz_t = ((uint32_t*)bptr)[0];

 bptr += sizeof(sz_t);

 sz_h = ((uint32_t*)bptr)[0];

 bptr += sizeof(sz_h);

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[Tesla DeLorean · Accepted solution]

>>Is this normal?

Yes, LDRD/STRD instructions will fault on unaligned (32-bit) reads, so can't randomly cast pointers safely. Problem for uint64_t and doubles, most frequently.

Typical method in ARM's old and new, is to memcpy() to an aligned buffer and read out of that.

The Cortex-M0(+) is even more unforgiving 32-bit reads can't be misaligned.

[Ellaine]

Ok, thanks.