Assigning when returning a value

Question

I'm not understanding a function used by my co-worker. He cannot He is using strange return blocks.

block_t *get_block(void *p)
{
    char *tmp;
    tmp = p;
    return (p = tmp -= 16);
}

He says it is supposed to return form and the address.

Need some help to understand the return ! Thanks a lot :)

Answer 1

All C operators compute values (except a cast operator with void as the target type or a function call to a function returning void). In particular, assignment and op/assignment operations evaluate to the value assigned. Therefore, this statement:

    return (p = tmp -= 16);

is equivalent to

    tmp -= 16;
    p = tmp;
    return p;

And the latter is exactly how I would write it if that's what I wanted to do. Compacting it all into one expression makes the code harder for a human to read and understand, and conveys no offsetting advantage whatsoever.

HOWEVER, your particular case is even worse. In your case, p and tmp are local variables of the function, so assignments to them have no effect on the caller. There is therefore no point in assigning a new value to p if it will not afterward be read back (from p). This may or may not reflect a programming error. If the function's actual behavior is correct as-is, then I would write the overall function much differently:

block_t *get_block(void *p) {
    return (block_t *) (((char *) p) - 16);
}