How to run machine code as a function in c++ - c++

Question

system: Windows 10
compiler: MinGW
error: Segmentation fault
I'm trying to run machine code as a function in c++. Here is my code:
#include <iostream>
int main()
{
int(*fun_ptr)(void) = ((int(*)())("\xB8\x0C\x00\x00\x00\xC3"));
std::cout << fun_ptr();
return 0;
}
In online compilers like ideone.com program succesfully print 12 and exits. In my computer I receive "Segmentation fault" error. Can anyone help me?

Answer 1

A string literal such as "\xB8\x0C\x00\x00\x00\xC3" is an object of static storage duration [lex.string]/15. A compiler will typically place such string literal objects in the .rdata section of your binary, i.e., into read-only, non-executable memory. As a consequence, trying to execute the bytes of a string literal will result in an access violation. If you want to execute machine code bytes contained in a global array object, you have to make sure your object is allocated in a section that is executable. For example (targeting Windows with Visual C++):
#include <iostream>
#pragma section("runstuff", read, execute)
__declspec(allocate("runstuff"))
const unsigned char code[] = {
0xB8, 0x0C, 0x0, 0x0, 0x0, 0xC3
};
int main()
{
auto fun_ptr = reinterpret_cast<int(*)()>(&code[0]);
std::cout << fun_ptr();
return 0;
}
Note that stuff like that is inherently not portable and has implementation-defined behavior at best. If you know at build time what machine code you want to run, consider using an assembler and just linking the resulting object file to your executable. If you want to dynamically generate machine code on Windows, you will have to allocate executable memory. To do so, either create a large-enough array in executable (and also writeable) memory (e.g., analogously to my example above) into which you can place your code, or dynamically allocate executable memory, e.g. using VirtualAlloc or using HeapAlloc from a Heap with the executable flag set. You will also want to be aware of the FlushInstructionCache API…

Answer 2

You can do that by using an inline assembler:
#include <iostream>
int code() {
__asm (
".byte 0xB8, 0x0C, 0x00, 0x00, 0x00"
);
}
int main() {
std::cout << code() << std::endl;
return 0;
}

Answer 3

I found a method:
#include <iostream>
#include <windows.h>
using namespace std;
int main() {
unsigned char bytes[] = "\xB8\x0C\x00\x00\x00\xC3";
HANDLE mem_handle = CreateFileMappingA(INVALID_HANDLE_VALUE, NULL, PAGE_EXECUTE_READWRITE, 0, sizeof(bytes), NULL);
void *mem_map = MapViewOfFile(mem_handle, FILE_MAP_ALL_ACCESS | FILE_MAP_EXECUTE, 0x0, 0x0, sizeof(bytes));
memcpy(mem_map, bytes, sizeof(bytes));
int result = ((int (*)(void))mem_map)();
cout << "argument:\n" << result << '\n';
return 0;
}