How can I call (not define) a function with a variable number of arguments in C?

Question

Is there any way to make this code shorter?

long call_f(int argc, long *argv) {
  switch (argc) {
    case 0:
      return f();
      break;
    case 1:
             


        

Answer 1

There's actually a method to call a function at run-time if you know its calling convention and which parameters it receives. This however lies out of the standard C/C++ language scope.

For x86 assembler:

Assuming the following:

1. You know to prepare all the parameters for your function in a solid buffer, exactly in the manner they'd be packed on the stack.
2. Your function doesn't take/return C++ objects by value.

You may use then the following function:

int CallAnyFunc(PVOID pfn, PVOID pParams, size_t nSizeParams)
{
    // Reserve the space on the stack
    // This is equivalent (in some sense) to 'push' all the parameters into the stack.
    // NOTE: Don't just subtract the stack pointer, better to call _alloca, because it also takes
    // care of ensuring all the consumed memory pages are accessible
    _alloca(nSizeParams);

    // Obtain the stack top pointer
    char* pStack;
    _asm {
        mov pStack, esp
    };

    // Copy all the parameters into the stack
    // NOTE: Don't use the memcpy function. Because the call to it
    // will overwrite the stack (which we're currently building)
    for (size_t i = 0; i < nSizeParams; i++)
        pStack[i] = ((char*) pParams)[i];

    // Call your function
    int retVal;
    _asm {
        call pfn
        // Most of the calling conventions return the value of the function (if anything is returned)
        // in EAX register
        mov retVal, eax
    };

    return retVal;
}

You may need to adjust this function, depending on the calling convention used