CODEWITHSUNDEEP
cfunction
piratos
piratos

Reputation: 29

Can anyone explain a misunderstanding with functions?

I want to understand why we write this DWORD MyExceptionHandler(void); and this int foo(char *buf);, two times in this example.

Why we just write those functions without writing the definition:

DWORD MyExceptionHandler(void);                    
int foo(char *buf);

Example:

#include <windows.h>
#include <stdio.h>

DWORD MyExceptionHandler(void);
int foo(char *buf);

int main(int argc, char *argv[])
{
        HMODULE l;
        l = LoadLibrary("msvcrt.dll");
        l = LoadLibrary("netapi32.dll");
        printf("\n\nHeapoverflow program.\n");
        if(argc != 2)
                return printf("ARGS!");
        foo(argv[1]);
        return 0;
}

DWORD MyExceptionHandler(void)
{
        printf("In exception handler....");
        ExitProcess(1);
        return 0;
}

int foo(char *buf)
{
        HLOCAL h1 = 0, h2 = 0;
        HANDLE hp;

        __try{
                hp = HeapCreate(0,0x1000,0x10000);
                if(!hp){
                        return printf("Failed to create heap.\n");
    }
                h1 = HeapAlloc(hp,HEAP_ZERO_MEMORY,260);

                printf("HEAP: %.8X %.8X\n",h1,&h1);

                // Heap Overflow occurs here:
                strcpy(h1,buf);

                // This second call to HeapAlloc() is when we gain control
                h2 = HeapAlloc(hp,HEAP_ZERO_MEMORY,260);
                printf("hello");
        }
        __except(MyExceptionHandler())
        {
                printf("oops...");
        }
        return 0;
}

Upvotes: 1

Views: 51

Answers (1)

Barmar
Barmar

Reputation: 780851

A function has to be declared before you can call it. There are two ways to do it:

  1. You can put the entire function definition before the definitions of any functions that call it. The definition serves as a declaration as well.

  2. You can put a prototype of the function before the definitions of any functions that call it. This simply declares the function's parameter and return types. The definition can be put later, or even in another compilation unit that you link with later.

Many programmers like to put prototypes of all their functions at the beginning of the file. This allows them to put the definitions in any order, rather than keeping track of which calls which so you can get all the dependencies right. In particular, it allows you to put the main() function first, which can make it easier to follow the logic of the program.

Upvotes: 3

Related Questions