I’m trying to figure out a crash that only happens in a 32-bit ARM build of my
code (well, in a C library my code uses.)
The crash is an unaligned memory access:
EXC_BAD_ACCESS (code=EXC_ARM_DA_ALIGN, address=0x18083b34)
during a 64-bit load from a non-8-byte-aligned address. The instruction that
crashed is
ldrexd r1, r2, [r0]
where r0= 0x18083b34. (This is part of code generated by the compiler intrinsic
__sync_add_and_fetch.)
The part I don’t understand is why the address isn’t aligned — it’s declared as
a uint64_t:
typedef struct {
union {
volatile uint64_t val_64; // <— this is what r0 points to
volatile uint32_t val_32;
volatile uint16_t val_16;
volatile uint8_t val_8;
} value;
} atomic_val_t;
The atomic_val_t in question is itself a member of a larger structure that was
allocated by malloc.
Shouldn’t C alignment rules dictate that the ‘value’ union and the surrounding
‘atomic_val_t’ type be 8-byte-aligned, since they contain a 64-bit integer? And
if they don’t, what’s the workaround to ensure safe alignment?
—Jens _______________________________________________
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