On 07/03/2013 05:26 PM, Flávio J. Saraiva wrote: > In https://github.com/GNS3/dynamips-community/issues/9 I found that > the reason for a crash was the compiler assuming that the pointer was > 4-byte aligned in one of the O3 optimizations. > Using gcc-4.4.7-3.el6.x86_64 in a Cent6.4-x86_64 box. > > I don't understand why it made that assumption, so I'd like to know > how to tell the compiler not to assume an alignment. There is attribute(aligned) and attribute(packed) but I don't think that you need them for this. GCC knows what the integer alignment for a machine is. It is perfectly entitled to assume that a pointer to uint32_t really is aligned, The only thing you are allowed to do in C is convert an int pointer to a void* or a char* and then copy it back again. You can't convert, say, a char* to an int* and us it as you're trying to do. There is a saying: Do not lie to the compiler. It will bite you. So, what's the right way to solve your problem? Like this: /* Byte-swap a memory block */ void mem_bswap32(void *ptr,size_t len) { m_uint32_t *p = ptr; size_t count = len >> 2; int i; m_uint32_t x; for(i=0;i<count;i++,p++) { memcpy(&x, p, sizeof x); x = swap32(x); memcpy(p, &x, sizeof x); } } GCC will generate the best code it can for the memcpy(); this will be the fastest kind of unaligned memory move that your processor supports. Andrew.
