// dynamically generate code for the identity function
#include <assert.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>

/*
  using 
	objdump --disassemble a.out
  gives:

 080483c4 <g>:
 80483c4:       55                      push   %ebp
 80483c5:       89 e5                   mov    %esp,%ebp
 80483c7:       8b 45 08                mov    0x8(%ebp),%eax
 80483ca:       5d                      pop    %ebp
 80483cb:       c3                      ret    
*/
int g(int x) {
	return x;
}


/* simple program to demonstrate dynamic code generation
   on the x86 architecture (note: very very machine dependent) */
int main() {
        int (*f)(int);

	// ugh.  linux tries to be clever and prevent you from executing heap stuff.
	int pagesize = sysconf(_SC_PAGE_SIZE);
	char *buf = memalign(pagesize, pagesize);
   	if(mprotect(buf, pagesize, PROT_READ | PROT_WRITE | PROT_EXEC) == -1) {
		perror("failed");
		assert(0);
	}

        /* dynamically generate code equivalent to
                int f(int x) { return x; }
           into buf */
        buf[0] = 0x55;
        buf[1] = 0x89; buf[2] = 0xe5;
        buf[3] = 0x8b; buf[4] = 0x45; buf[5] = 0x8;
        buf[6] = 0x5d;
        buf[7] = 0xc3;
        /* cast buf's address to a function pointer. */
        f = (int (*)(int))buf;

        /* check that generated code returns right answer */
        printf("%d = %d\n", 10, f(10));
        printf("%d = %d\n", 20, f(20));
        return 0;
}