Initializing a structure with a function pointer

[Daugaard]

As the title says, I'm trying to put a function pointer in a structure at initialization time. The code below demonstrates this and also includes code for testing whether or not it works. It's fairly self-explanatory with the comments.

#include <stdlib.h>
#include <stdio.h>
 
// The structure containing the function pointer
typedef struct {
    int (*MyFunctionPointer)(int);
} MyStructure;
 
// A function to take a function pointer, initialize it with a MyStructure, and return it
MyStructure *put_function_pointer_into_structure(int (*FunctionPointer)(int))
{
    MyStructure *StructureInstance = {FunctionPointer};
    return StructureInstance;
}
 
// A trivial function for testing purposes
int divide_by_two(int number)
{
    return number/2;
}
 
// Entry-point. Performs the testing
int main(int argc, char *argv[])
{
    MyStructure *StructureWithFunctionPointer = put_function_pointer_into_structure(divide_by_two);
 
    printf("The number is %i, half of it is %i\n", 9000,
           StructureWithFunctionPointer->MyFunctionPointer(9000));
    return EXIT_SUCCESS;
}
 

When I try to compile this I get this warning:

func_ptr_test.c:12: warning: initialization from incompatible pointer type
(This refers to the line MyStructure *StructureInstance = {FunctionPointer};)

Of course that is just a warning and I can run the program regardless, but it gives a segfault immediately (or a "general protection error" for you Windows people out there).

I can't for the life of me figure out what is wrong with my code, any suggestions will be highly appreciated.

I'm using GCC (version 4.1.3).

[weaknessforcats]

You have several errors:
1) you have not created a MyStructure variable so there is no member to assign to. All you did was create a pointer to a MyStrucure.
2) The name of a function is the address of the function. Therefore, the function argument is used without parentheses. It's a variable. Not a function.

The code below compiles and links. You will need to free() the StructureInstance yourself.

// A function to take a function pointer, initialize it with a MyStructure, and return it
MyStructure *put_function_pointer_into_structure(int (*FunctionPointer)(int))
{
    MyStructure* StructureInstance =
    (MyStructure*)malloc(sizeof(MyStructure));
    StructureInstance->MyFunctionPointer = FunctionPointer;
    return StructureInstance;
}
 

[Daugaard]

Hey thanks man... What was I thinking :)