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 applied to small structs without penalty in most cases. See Is there any case for which returning a structure directly is good practice? and Are there any downsides to passing structs by value in C, rather than passing a pointer?

And that avoiding a dereference can be beneficial from both a speed and clarity perspective. But what counts as small? I think we can all agree that this is a small struct:

struct Point { int x, y; };

That we can pass by value with relative impunity:

struct Point sum(struct Point a, struct Point b) {
  return struct Point { .x = a.x + b.x, .y = a.y + b.y };
}

And that Linux's task_struct is a large struct:

https://github.com/torvalds/linux/blob/b953c0d234bc72e8489d3bf51a276c5c4ec85345/include/linux/sched.h#L1292-1727

That we'd want to avoid putting on the stack at all costs (especially with those 8K kernel mode stacks!). But what's about middling ones? I assume structs smaller than a register are fine. But what about these?

typedef struct _mx_node_t mx_node_t;
typedef struct _mx_edge_t mx_edge_t;

struct _mx_edge_t {
  char symbol;
  size_t next;
};

struct _mx_node_t {
  size_t id;
  mx_edge_t edge[2];
  int action;
};

What is the best rule of thumb for determining whether a struct is small enough that it's safe to pass it around by value (short of extenuating circumstances such as some deep recursion)?

Lastly please don't tell me that I need to profile. I'm asking for a heuristic to use when I'm too lazy/it's not worth it to investigate further.

EDIT: I have two followup questions based on the answers so far:

1. What if the struct is actually smaller than a pointer to it?

2. What if a shallow copy is the desired behavior (the called function will perform a shallow copy anyway)?

EDIT: Not sure why this got marked as a possible duplicate as I actually link the other question in my question. I'm asking for clarification on what constitutes a small struct and am well aware that most of the time structs should be passed by reference.

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.