how to hook a Variable argument function?

[Ingenious-FanBlade]

how to hook a Variable argument function such as printf with MinHook?

[m417z]

Generally that's not supported, but there are workarounds you can try.

If the function later calls a va_list-style function (e.g. if printf calls vprintf), you can hook that function instead.

If not, the following workarounds should work for both x86 and x64:

• If you know the maximum amount of arguments, you can just define a function with that amount of arguments. For example:

  int printf(const char * format, void* p1, void* p2, void* p3, void* p4, void* p5, void* p6, void* p7, void* p8, void* p9);

  Unused arguments will have garbage values, but it's OK to pass them to the original function which will ignore them.
• You can force a tail call if you don't need post-processing and if your compiler supports that. The first four arguments must be specified since they're passed via registers in x64. For example:

  int printf_hook(const char* format, void* p1, void* p2, void* p3) {
      // Pre-hook logic...
  
      [[clang::musttail]] return printf_original(format, p1, p2, p3);
      // For other compilers: [[gnu::musttail]], [[msvc::musttail]]
  }

Note that if some of the arguments are floating point values or other non-standard types, that might not be enough.