Question

Not sure what I’m doing wrong here. I have a struct that is used heavily through my program.

typedef struct _MyStruct {
  // ... handful of non-trivial fields ...
} MyStruct;

I expect (read, intend) for lots of parts of the program to return one of these structs, but many of them should be able to return a “null” struct, which is a singleton/global. The exact use case is for the implementing function to say “I can’t find what you asked me to return”.

I assumed this would be a simple case of defining a variable in a header file, and initializing it in the .c file.

// MyStruct.h

// ... Snip ...

MyStruct NotFoundStruct;

–

// MyStruct.c

NotFoundStruct.x = 0;
NotFoundStruct.y = 0;
// etc etc

But the compiler complains that the initialization is not constant.

Since I don’t care about what this global actually references in memory, I only care that everything uses the same global, I tried just removing the initialization and simply leaving the definition in the header.

But when I do this:

MyStruct thing = give_me_a_struct(some_input);
if (thing == NotFoundStruct) {
  // ... do something special
}

Th compiler complains that the operands to the binary operator “==” (or “!=”) are invalid.

How does one define such as globally re-usable (always the same memory address) struct?

Answer 1

// MyStruct.h

typedef struct _MyStruct {
  // fields
} MyStruct;

extern MyStruct NotFoundStruct;

// MyStruct.c

#include "my_struct.h"
MyStruct NotFoundStruct = {0};

But since you can’t use the == operator, you will have to find another way to distinguish it. One (not ideal) way is to have a bool flag reserved to indicate validity. That way, only that must be checked to determine if it’s a valid instance.

But I think you should consider James’s proposed solution instead