Run dynamic code on Windows (x86)

dynamicCodeWin.c

// Run dynamic code on Windows (x86)
// Compile with: gcc -o dynamicCodeWin dynamicCodeWin.c
// Tutorial: https://nicolashollmann.de/blog/run-dynamic-code-windows/

#include <stdio.h>
#include <stdbool.h>
#include <Windows.h>

/**
* Helper function to execute dynamic code.
* 
* @param code a pointer to the code to execute
* @param length the length of the code
* @return true if the code run successfully, otherwise false
*/
bool run_code(const void* code, size_t length)
{
	// Allocate new memory for code execution.
	// We don't set the execute flag because we won't want to
	// open doors to malicious software.
	// dynMemory must be declared as volatile, as some Release
	// optimizations cause problems on VirtualFree. Maybe some
	// sort of reordering.
	volatile LPVOID dynMemory = VirtualAlloc(NULL, length, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
	if (dynMemory == NULL)
	{
		printf("Alloc Error: %ld\n", GetLastError());
		return false;
	}

	// Copy code to new memory.
	memcpy(dynMemory, code, length);

	// Change protection.
	// We remove the write flag and set the execute flag on that page.
	DWORD oldProtection;
	if (!VirtualProtect(dynMemory, length, PAGE_EXECUTE, &oldProtection))
	{
		printf("Protect Error: %ld\n", GetLastError());
		return false;
	}

	// Call the new function in memory.
	int result = ((int (*)(void))dynMemory)();
	printf("Result: %d\n", result);

	// At the end we free the memory.
	if (!VirtualFree(dynMemory, 0, MEM_RELEASE))
	{
		printf("Free Error: %ld\n", GetLastError());
		return false;
	}

	return true;
}

/**
* Entrypoint of the program.
* 
* @return exit code of the program, 0 if successful, 
*		  -1 in case of an error
*/
int main(void)
{
	// Define the code we want to execute
	const char code[] = {
		// Direct set
		0xb8, 0x05, 0x00, 0x00, 0x00,		// mov eax, 0x05
		// Addition
		0x05, 0x06, 0x00, 0x00, 0x00,		// add eax, 0x06
		// Subtraction
		0x2d, 0x01, 0x00, 0x00, 0x00,		// sub eax, 0x01
		// Return (result in EAX)
		0xc3								// ret
	};

	// Run the code
	if (!run_code(code, sizeof code)) {
		return -1;
	}
	return 0;
}