How to use function pointers in a struct in C?

Question

It's pretty simple what I'm trying to do, and I'm merely having trouble figuring out the right syntax.

I want my struct to look like this:

struct myStruct
{
   functionPointer myPointer;
}

Then I have another function somewhere else that passes a function to a function and returns an instance of my struct. Here's what it does:

struct myStruct myFunction(int (*foo) (void *))
{
   myStruct c;
   c.myPointer = foo;
   return c;
}

How can I make this actually work? What's the correct syntax for:

Declaring a function pointer in my struct (obviously functionPointer myPointer; is incorrect)
Assigning a function's address to that function pointer (pretty sure c.myPointer = foo is incorrect)?

If you return a myStruct *, you should allocate (eg, use malloc) and return the malloced value. If you return a myStruct, you should declare your function as such. (Right now, you declare that the function returns a pointer, but you attempt to return a struct.) — William Pursell
– William Pursell, Commented Jan 14, 2012 at 2:14
possible duplicate of Function Pointer in C. See also Function pointer as a member of a C struct, Struct with pointer to a function, typedef is your friend. — outis
– outis, Commented Jan 14, 2012 at 2:19

Oliver Charlesworth · Accepted Answer · 2012-01-14 02:16:34Z

4

It's really no different to any other instance of a function pointer:

struct myStruct
{
   int (*myPointer)(void *);
};

Although often one would use a typedef to tidy this up a little.

answered Jan 14, 2012 at 2:16

Oliver Charlesworth

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Jonathan Leffler · Accepted Answer · 2012-01-14 02:19:26Z

0

More or less:

struct myStruct
{
    struct myStruct (*myPointer)(int (*foo)(void *));
};
typedef struct myStruct myStruct;

The c.myPointer should be fine, but you need to return a copy of the structure, not a pointer to the now vanished local variable c.

struct myStruct
{
    struct myStruct (*myPointer)(int (*foo)(void *));
};
typedef struct myStruct myStruct;

struct myStruct myFunction(int (*foo) (void *))
{
    myStruct c;
    c.myPointer = foo;
    return c;
}

This compiles but the compiler (reasonably) complains that foo is not really the correct type. So, the issue is - what is the correct type for the function pointer in the structure? And that depends on what you want it to do.

answered Jan 14, 2012 at 2:19

Jonathan Leffler

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1 Comment

Dave Over a year ago

Your struct contains a pointer to a function returning your struct taking a pointer to another function returning an int taking a pointer to void? My head hurts.

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