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

How a struct is passed to or from a function depends on the application binary interface (ABI) and the procedure call standard (PCS, sometimes included in the ABI) for your target platform (CPU/OS, for some platforms there may be more than one version).

If the PCS actually allows to pass a struct in registers, this not only depends on its size, but also on its position in the argument list and the types of preceeding arguments. ARM-PCS (AAPCS) for instance packs arguments into the first 4 registers until they are full and passes further data onto the stack, even if that means an argument is split (all simplified, if interested: the documents are free for download from ARM).

For structs returned, if they are not passed through registers, most PCS allocate the space on the stack by the caller and pass a pointer to the struct to the callee (implicit variant). This is identical to a local variable in the caller and passing the pointer explicitly - for the callee. However, for the implicit variant, the result has to be copied to another struct, as there is no way to get a reference to the implicitly allocated struct.

Some PCS might do the same for argument structs, others just use the same mechanisms as for scalars. In any way, you defer such optimizations until you really know you need them. Also read the PCS of your target platform. Remember, that your code might perform even worse on a different platform.

Note: passing a struct through a global temp is not used by modern PCS, as it is not thread-safe. For some small microcontroller architectures, this might be different, however. Mostly if they only have a small stack (S08) or restricted features (PIC). But for these most times structs are not passed in registers, either, and pass-by-pointer is strongly recommended.

If it is just for immutability of the original: pass a const mystruct *ptr. Unless you cast away the const that will give a warning at least when writing to the struct. The pointer itself can also be constant: const mystruct * const ptr.

So: No rule of thumb; it depends on too many factors.