Data alignment requirements can make this interpretation difficult. Multi-field keys may contain gaps or padding between fields that don’t align properly otherwise. For example, on some system architectures the struct
struct S {
char c;
int i;
};
contains a three-byte pad between c and i. The contents of pads are
undefined, which means that a byte-by-byte comparison of what should be
identical instances of struct S may fail due to differences in the bits
stored in the pad.
struct S
s1 = { .c = 'a', .i = 10 },
s2 = { .c = 'a', .i = 10 };
assert(0 == memcmp(&s1, &s2, sizeof(struct S))) // may fail.
The problem this causes for hashing is clear: if the hash function includes
the pad bytes as part its computation, s1 and s2 could produce
different hash values, violating the requirement that identical keys produce
identical hash values.
This page last modified on 24 January 2006.