C Function Prototype With Struct Argument

Question

I want to write a function prototype for a function, whose argument is a pointer to a struct.

int mult(struct Numbers *n)

However, the struct Numbers, which is defined as

struct Numbers {
    int a;
    int b;
    int c;
};

is not defined yet. How should I write a suitable prototype for mult?

forward declarations only work with pointers, so if you ever want to actually pass in a struct by value, you'll need to actually fully define the struct before you use it. This is because the system knows how big a pointer is, but doesn't know how big an undefined structure is. — xaxxon
– xaxxon, Commented Aug 4, 2013 at 23:20

ouah · Accepted Answer · 2013-08-04 23:09:48Z

5

Just declare struct Numbers as an incomplete type before your function declaration:

struct Numbers;

int mult(struct Numbers *n);

answered Aug 4, 2013 at 23:09

ouah

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Comments

Jack · Accepted Answer · 2013-08-04 23:11:34Z

4

You must forward the declaration of the structure to tell the compiler that a struct with that name will be defined:

struct Numbers;

int mult(struct Numbers *n) {

}

struct Numbers {
    int a;
    int b;
    int c;
};

Mind that the compiler is not able to determine the size in memory of the structure so you can't pass it by value.

answered Aug 4, 2013 at 23:11

Jack

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9 Comments

Dave Over a year ago

can you ever pass a struct by value in C?

xaxxon Over a year ago

@Dave absolutely. It's not normally a good idea, and you have to have the structure fully defined before you use it as a parameter, but there's nothing stopping you if you really want to.

Dave Over a year ago

Huh, I never knew that. I thought passing by value was C++, not C. But yeah I agree that 99% of the time it would be a really bad idea.

andy256 Over a year ago

The thing to remember about C (and C++) is that you can do what you want, even if it's a bad idea - you just have to tell the compiler you really want to do it. Passing small, frequently created and destroyed, structs by value can reduce dynamic memory allocations, improving performance and reducing heap fragmentation.

Dave Over a year ago

@andy256 in that case you'd still be better off passing a pointer to a local struct. I think it would only be an advantage if the copy costs less than the dereference, which probably means the object is very very small. Maybe a complex number would be a use-case.

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