jit trick in c

jit.c

// from http://dginasa.blogspot.com/2012/05/jit-compilation-without-much-assembler.html
// gcc -std=gnu99 jit.c -o jit
// still some warnings, need refinement, and put into some bigger works.
#include <stdio.h>  
#include <stdlib.h>  

#include <sys/mman.h>  
#include <unistd.h>

#define COPY_SECTION(sec_s, sec_e, dest) ({                \
    unsigned char *buff = NULL;                            \
    for (buff = &&sec_s; buff < &&sec_e; buff++, (dest)++) \
        *(dest) = *buff;                                   \
})  

int main(void) { 
    unsigned char *buff, *d;  
    int v = 0;  
    
    printf ("Address: \n"  
       "\tinc_v = %p\n"  
       "\tdec_v = %p\n"  
       "\tjump_end = %p\n"  
       "\tend = %p\n", &&inc_v, &&dec_v, &&jump_end, &&end);  

    /* Allocate. */  
    void *dyn = mmap (0, 1024, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);  

    /* Here's our sample program, `inc, inc, inc, dec, done` */  
    d = dyn;  
    COPY_SECTION (inc_v, dec_v, d);  
    COPY_SECTION (inc_v, dec_v, d);  
    COPY_SECTION (inc_v, dec_v, d);  
    COPY_SECTION (dec_v, jump_end, d);  
    COPY_SECTION (jump_end, end, d);  

    /* Now call the code. */  
    goto *dyn;  
 
 inc_v:  
    v++;  
 dec_v:  
    v--;  

 jump_end:  
    __asm__ volatile ("jmp *%0" :: "r" (&&end));  
 
 end:
    printf ("Value of v: %d\n", v);  
    return 0; 
 }