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x86 JIT Assembler, call C functions

Alexandre Bencz asked:

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Hi :)
I'm developing a simple JIT Assembly system in C++, but, I whant to call C functions in this jit system, so, what I have thinked... I need the pointer of the command... but, I don't know how I can get this...

That is my code

#include <cstdio>
#include <vector>
#include <windows.h>

int Execute(std::vector<unsigned char> code)
{
	int eaxRegister;

	unsigned char* func = (unsigned char*)VirtualAlloc( 0, code.size() + 1, 0x1000, 0x40 );

	memcpy( func, code.data(), code.size() );
	func[code.size()] = 0xC3; // add the ret to the final of code final

	CallWindowProc( (WNDPROC)func, 0, 0, 0, 0 );

	_asm mov eaxRegister, eax;

	VirtualFree( func, code.size() + 1, 0x4000 );

	return eaxRegister;
}

int main()
{
	std::vector<unsigned char> code;

	//mov eax, 10
	code.push_back( 0xc7 );
	code.push_back( 0xc0 );
	code.push_back( 0xa );
	code.push_back( 0x0 );
	code.push_back( 0x0 );
	code.push_back( 0x0 );

	//mov ecx, 10
	code.push_back( 0xc7 );
	code.push_back( 0xc1 );
	code.push_back( 0xa );
	code.push_back( 0x0 );
	code.push_back( 0x0 );
	code.push_back( 0x0 );

	//add eax, ecx
	code.push_back( 0x3 );
	code.push_back( 0xc1 );

	// push MESSAGE
	const char* ohi = "HI";
	code.push_back( 0x69 );
	code.push_back( *ohi );

	// call prinf ?????
	code.push_back( 0xe8 );
	code.push_back( 0xfff/* offset of printf */ ) ;

	// add esp, 4
	code.push_back( 0x83 );
	code.push_back( 0xc4 );
	code.push_back( 0x04 );
	code.push_back( 0x0 );
	code.push_back( 0x0 );
	code.push_back( 0x0 );

	int exec = Execute( code );
	printf("SUM = %d", exec);

	return 0;
}


So, my problem is, how I can get the offset of printf command to use in JIT, or, how I can use the C function using the JIT ???

Thanks
Alexandre
Tags: C, ASM

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