x86/x64 Simple JMP (Relative Offset) function hooking.

hook.c

#include <windows.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <stdbool.h>

#if _WIN32 || _WIN64
  #if _WIN64
    typedef uint64_t uint_t;
  #else
    typedef uint32_t uint_t;
  #endif
#endif

#if __GNUC__
  #if __x86_64__ || __ppc64__
    typedef uint64_t uint_t;
  #else
    typedef uint32_t uint_t;
  #endif
#endif

bool install_hook(char * module, char * function, void * rfcn)
{
        void *p_module = NULL, *p_function = NULL;
        uint_t jmp_offset;
        uint32_t old;

        char patch[1 + sizeof(uint_t)] = {
                0xE9
        };

        p_module = LoadLibraryA(module);

        if ( !p_module )
                return FALSE;

        p_function = GetProcAddress(p_module, function);

        if ( !p_function )
                return FALSE;

        if ( !VirtualProtect(p_function, sizeof(patch), PAGE_READWRITE, (LPDWORD)&old) )
                return FALSE;

        jmp_offset = (uint_t)rfcn - (uint_t)p_function - sizeof(uint_t) - 1;

        memcpy(((void *)&patch) + 1, &jmp_offset, sizeof(uint_t));

        memcpy(p_function, &patch, sizeof(patch));

        if ( !VirtualProtect(p_function, sizeof(uint_t), old, (LPDWORD)&old) )
                return FALSE;

        return TRUE;
}

int hook(HWND window, char *text, char *box, UINT lol)
{
        printf("AHA! NOT TODAY\n");
        return 0;
}

void main(void)
{
        install_hook("user32.dll", "MessageBoxA", (void *)hook);

        MessageBoxA(NULL, "test", "test", MB_OK);
}

jmp_offset = (uint_t)rfcn - (uint_t)p_function - sizeof(uint_t) - 1;

This is correct only in 32 bit not on 64 bit. E9 jump is rel32.