Question

I was trying to understand why structure padding is the reason structures cannot be compared by memcmp.
One small thing i dont understand about structure padding is this…
why should “a short be 2 byte aligned"or"a long be 4 byte aligned". I understand it is with their sizes but why can they not appear at any byte boundary?

Or in other words "why is 0x10004566 not a valid location for a long variable but 0x10004568 is?"

Answer 1

Memory alignment is a very important issue when optimizing a program for speed. C, being a language that – generally – puts strong emphasis on speed, likes to enforce some rules which may make the program faster.

The limitation of aligned and unaligned memory accesses comes directly from the hardware used for fetching the data from the memory, which usually fetches it in chunks which are equal to the machine word in size. Say you want to access a doubleword (4 bytes) stored at location 101. This means that the memory controller would firstly have to (probably) issue a read of a doubleword at location 100, then another read of a doubleword at location 104, and then splice the individual bytes from locations 101, 102, 103, and 104 together. The whole operation takes (hypothetically) two clock cycles.

If you want to access a doubleword at location 100, there’s no such issue, which should be illustrated clearly enough by the example I provided.

In fact, misaligned data access is such a big issue that SSE instructions (the “aligned” versions, there are also “misaligned” versions which don’t do that) will cause a general protection fault if you try to access misaligned data with those.

As a rule of thumb, it never hurts to align 4-byte data on a 4-byte boundary, 8-byte data on a 8-byte boundary, and so forth.