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

My experience, nearly 40 years of real-time embedded, last 20 using C; is that the best way is to pass a pointer.

In either case the address of the struct needs to be loaded, then the offset for the field of interest needs to be calculated...

When passing the whole struct, if it is not passed by reference, then

1. it is not placed on the stack
2. it is copied, usually by a hidden call to memcpy()
3. it is copied to a section of memory that is now 'reserved' and unavailable to any other part of the program.

Similar considerations exist for when a struct is returned by value.

However, "small" structs, that can be completely held in a working register to two are passed in those registers especially if certain levels of optimization are used in the compile statement.

The details of what is considered 'small' depend on the compiler and the underlying hardware architecture.