exebuf.s

	.section .exebuf,"bwx"
	.globl exebuf
exebuf:	.space 1<<12

main.c

// Ref: https://old.reddit.com/r/C_Programming/comments/1pk6oql/
// Ref: https://lowkpro.com/blog/creating-c-closures-from-lua-closures.html
#include <stddef.h>
#include <stdio.h>

typedef struct {
    char *beg;
    char *end;
} Arena;

Arena get_exec_arena()
{
    extern char exebuf[1<<12];
    return (Arena){exebuf, exebuf+sizeof(exebuf)};
}

char *store(char *p, int v)
{
    p[0] = v >>  0;
    p[1] = v >>  8;
    p[2] = v >> 16;
    p[3] = v >> 24;
    return p + 4;
}

char *asm_store(char *p, int *dst, int value)
{
    int rel = (char *)dst - (p + 10);
    *p++ = 0xc7;
    *p++ = 0x05;
    p = store(p, rel);
    p = store(p, value);
    return p;
}

char *asm_jmp(char *p, void *dst)
{
    int rel = (char *)dst - p - 5;
    *p++ = 0xe9;
    p = store(p, rel);
    return p;
}

int findex;

void callback(char *s)
{
    printf("callback-%d(\"%s\")\n", findex, s);
}

void *generate_function(Arena *a, int closed_over_findex)
{
    char *f = a->beg;
    a->beg = asm_store(a->beg, &findex, closed_over_findex);
    a->beg = asm_jmp(a->beg, callback);
    return f;
}

int main()
{
    Arena exe = get_exec_arena();

    void (*funcs[10])(char *);
    for (int i = 0; i < 10; i++) {
        funcs[i] = generate_function(&exe, i);
    }
    funcs[4]("hello");
    funcs[2]("world");
}