Windows C++ / Inline Hooking

Code

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

// Typedef for the target function
typedef int (*target_func_t)(int);

// Trampoline function that will be used to execute the original code from the target function
int trampoline(int arg) {
  printf("Trampoline function called with argument: %d\n", arg);
  return 0;
}

// Hook function that will be used to intercept calls to the target function
int hook(int arg) {
  printf("Hook function called with argument: %d\n", arg);

  // Perform desired processing

  // Transfer control back to the trampoline function
  return trampoline(arg);
}

int main() {
  // Determine the address of the target function and the code that needs to be replaced
  target_func_t target_func = (target_func_t)GetProcAddress(GetModuleHandle(NULL), "target_func");
  unsigned char* target_code = (unsigned char*)target_func;

  // Calculate the relative address of the trampoline function and store it in a jump instruction
  unsigned char jump_instruction[] = {0xE9, 0x00, 0x00, 0x00, 0x00};
  int trampoline_offset = (int)trampoline - (int)target_func - 5;
  *(int*)(jump_instruction + 1) = trampoline_offset;

  // Overwrite the first few bytes of the target function with the jump instruction
  DWORD old_protect;
  VirtualProtect(target_code, 5, PAGE_EXECUTE_READWRITE, &old_protect);
  memcpy(target_code, jump_instruction, 5);
  VirtualProtect(target_code, 5, old_protect, &old_protect);
}