Arguments of Function in a C Structure

Question

We have a structure

typedef struct _disis_thinkgear
{
    t_object x_obj;

    //other members and such

and we need to have a member in the structure be a function like

    void (*handleDataValue)( ????  , unsigned char extendedCodeLevel, 
                               unsigned char code, unsigned char numBytes,
                               const unsigned char *value, void *customData );
} t_disis_thinkgear;

How can the ???? be replaced so that x is the first argument ? To use as

x->handleDataValue =  //another function

Answer 1

You'll need to forward-declare the typedef:

typedef struct _disis_thinkgear t_disis_thinkgear;

Then you can use the type in the definition of the structure:

struct _disis_thinkgear {
    ...

    void (*handleDataValue)(t_disis_thinkgear *x, ...
};

Once you've got an object of this type, you'd call the function as:

t_disis_thinkgear *x = ...
x->handleDataValue(x, ...);

Answer 2

This is a full example that works. You can actually use struct _disis_thinkgear from within the struct.

#include <stdlib.h>
#include <stdio.h>

typedef struct _disis_thinkgear {
        // ... other things ...

        void (*handleDataValue)(struct _disis_thinkgear *);

        // ... other things ...
} t_disis_thinkgear;

void printSomething(t_disis_thinkgear *foo) {
        printf("argument is %p\n", foo);
}

int main()
{
        t_disis_thinkgear *x = malloc(sizeof(x));
        x->handleDataValue = &printSomething;
        x->handleDataValue(x);
        free(x);
        return 0;
}