#include <stdio.h>
#include <stdint.h>
#include <sys/mman.h>
#include <unistd.h>
 
void bar()
{
        printf("function bar\n");
}
 
void foo()
{
        printf("function foo\n");
}
 
void monkey_patch(void* old_function, void* new_function)
{
        void* page;
 
        page = (void*)((uint64_t)old_function & ~(getpagesize()-1));
        if(mprotect(page, getpagesize(), PROT_WRITE|PROT_READ|PROT_EXEC)) {
                perror("mprotect");
        }
        ((uint8_t*)old_function)[0] = 0xeb; /* jmp */
        /*
         * Number of bytes to jump relative to the eip which is 2 bytes past
         * the address of this instruction. Cannot jump more than 128 bytes.
         */
        ((uint8_t*)old_function)[1] = new_function - old_function - 2;
}
 
int main()
{
	foo();
	monkey_patch(foo, bar);
	foo();
	return 0;
}