function pointers and pointer arguments in C

Question

Complete noob to C, just getting started with some goofing around, wondering how (read "if") the following is possible.

Trying to create a struct, with a member that is a function pointer, and the function pointer points to a function that takes an argument with the same type as the aforementioned struct. For example (mind the syntax, just getting familiar here):

typedef struct{
    void (*myStructFunc)(void);
} MyStructType;

void myFunc(void){
    printf("Hello world!");
}

// ...

MyStructType myStruct;
myStruct.myStructFunc = &myFunc;
myStruct.myStructFunc(); // <= Hello world!

This works fine, but when I try to introduce arguments of the MyStructType type to the function:

typedef struct{
    void (*myStructFunc)(*MyStructType); // <= parse error
} MyStructType;

void myFunc(MyStructType *myStruct){
    printf("Hello world!");
}

// ...

MyStructType myStruct;
myStruct.myStructFunc = &myFunc;
myStruct.myStructFunc(&myStruct);

These examples are brief for obvious reasons, but they illustrate my intentions. Again, just getting my feet wet with C so please forgive any syntactical ignorance.

Anyways, how can I achieve this? Clearly I'm doing something incorrect syntactically, or perhaps I'm trying to do something that's plain not possible.

Also note, that the reason for this is purely academic.

P.S. Have fun implementing OO. ;)

Chris Lutz
– Chris Lutz

2011-04-25 03:36:22 +00:00
Commented Apr 25, 2011 at 3:36 — Chris Lutz
– Chris Lutz, Commented Apr 25, 2011 at 3:36

Kevin Lacquement · Accepted Answer · 2011-04-25 03:28:04Z

5

In your second example, the name MyStructType isn't defined when you're declaring your function. Also, you have the * in the wrong spot. You need to do something similar to the following:

typedef struct MyStruct {
    void (*myStructFunc)(struct MyStruct *);
} MyStructType;

answered Apr 25, 2011 at 3:28

Kevin Lacquement

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2 Comments

Chris Lutz Over a year ago

You might take out the // <= parse error because your example works.

Dan Lugg Over a year ago

Thanks lacqui; Also spot on; gave you the +1 but went with Chris for the extra info.

Chris Lutz · Accepted Answer · 2011-04-25 03:28:42Z

2

The parse error is coming from *MyStructType, which appears to be a typo as you've correctly declared the struct pointer in the actual function definition.

Fixing that is still an error because, at the time of the struct definition, the typedef hasn't taken effect. The same problem can be illustrated (more clearly IMHO) with a linked list:

typedef struct {
  node *next; // error - what is a node?
  void *data;
} node; // this is where node is defined

To solve it, you need to use a named struct:

struct node {
  struct node *next;
  void *data;
};

Or put the typedef first, then define the (again named) struct:

typedef struct node node;

struct node {
  node *next;
  void *data;
};

Or (if you're feeling obnoxious) just use a void * pointer:

typedef struct {
  void *next;
  void *data;
} node;

The same applies to your situation.

answered Apr 25, 2011 at 3:28

Chris Lutz

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1 Comment

Dan Lugg Over a year ago

Thanks Chris Lutz; Much appreciated, as I was just reading into linked list implementations in C, so your answer was spot on for now and later :) Gotta say StackOverflow + C Programming: A Modern Approach has gotta be the best way to learn C.

Paul J. Lucas · Accepted Answer · 2011-04-25 03:29:29Z

2

Try:

struct MyStructType {
  void (*myStructFunc)(struct MyStructType*);
};

instead. In C, structs are in their own namespace, so if you want to refer to a struct, you need to prefix the name by struct. Your typedef isn't seen until after you reference, which is an error. If you still want the typedef, you can do:

typedef MyStructType MyStructType_t;

answered Apr 25, 2011 at 3:29

Paul J. Lucas

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