When should I pass or return a struct by value?

Question

A struct can be either passed/returned by value or passed/returned by reference (via a pointer) in C.

The general consensus seems to be that the former can be applie

Answer 1

On small embedded architectures (8/16-bitters) -- always pass by pointer, as non-trivial structures don't fit into such tiny registers, and those machines are generally register-starved as well.

On PC-like architectures (32 and 64 bit processors) -- passing a structure by value is OK provided sizeof(mystruct_t) <= 2*sizeof(mystruct_t*) and the function does not have many (usually more than 3 machine words' worth of) other arguments. Under these circumstances, a typical optimizing compiler will pass/return the structure in a register or register pair. However, on x86-32, this advice should be taken with a hefty grain of salt, due to the extraordinary register pressure a x86-32 compiler must deal with -- passing a pointer may still be faster due to reduced register spilling and filling.

Returning a structure by value on PC-likes, on the other hand, follows the same rule, save for the fact that when a structure is returned by pointer, the structure to be filled out should be passed in by pointer as well -- otherwise, the callee and the caller are stuck having to agree on how to manage the memory for that structure.