hello, jit world.

c/jit/README.md

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# Hello, JIT World.
Following along the ["Hello, JIT World: The Joy of Simple JITs"](http://blog.reverberate.org/2012/12/hello-jit-world-joy-of-simple-jits.html) article.

c/jit/hello_jit.c

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
int main(int argc, char *argv[]) {
	// machine code for:
	//   mov eax, 0
	//   ret
	unsigned char code[] = {0xb8, 0x00, 0x00, 0x00, 0x00, 0xc3};
	if (argc < 2) {
		fprintf(stderr, "Usage: hello_jit <integer>\n");
		return 1;
	}
	// copy the given number into the code, e.g:
	//   mov eax, <user's value>
	//   ret
	int num = atoi(argv[1]);
	memcpy(&code[1], &num, 4);
	// allocate writable & executable memory.
	// note: security risk!
	void *mem = mmap(NULL, sizeof(code), PROT_WRITE | PROT_EXEC, MAP_ANON | MAP_PRIVATE, -1, 0);
	memcpy(mem, code, sizeof(code));
	int (*func)() = mem;
	return func();
}