An example closure in C that requires MAP_32BIT to work with 64 bit

thunk.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

#ifndef MAP_32BIT
#error "MAP_32BIT not defined"
#endif

struct scope
{
    void *args;
};

struct thunk
{
    unsigned char push_op;  /* push rdi */
    unsigned char mov_op[2]; /* mov $env, rdi */
    struct scope *scope; 
    unsigned char call_op; /* callq */
    int call_offset; /*$offset => function - address_of_call_offset*/
    unsigned char pop_op; /* popq rdi */
    unsigned char ret_op; /* ret */
} __attribute__((packed));

static struct thunk initialize_thunk = { .push_op =  0x57, .mov_op = {0x48, 0xbf},
                                      .call_op = 0xe8, .call_offset = 0,
                                      .pop_op = 0x5f, .ret_op = 0xc3
};

typedef void (*closure_t)(void);

static void closure_block(struct scope *scope)
{
    int *acc = scope->args;
    ++*acc;
}

/*
 * Take a scope and the function block to be bound to the scope.
 */
static closure_t make_closure(void *arg, void *block)
{
    struct thunk *thunk = (struct thunk *)mmap(0, sizeof(*thunk), PROT_WRITE | PROT_EXEC, 
                                               MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT,
                                               -1, 0);
    if((char*)thunk == MAP_FAILED)
        return NULL;
    *thunk = initialize_thunk;
    thunk->scope = calloc(1, sizeof(*thunk->scope));
    if(!thunk->scope)
    {
        goto out_free;
    }
    thunk->scope->args = arg;
    thunk->call_offset = (long)block - (long)&thunk->pop_op;
    mprotect((void*)thunk, sizeof(*thunk), PROT_EXEC);
    return (closure_t)thunk;

    out_free:
    munmap(thunk, sizeof(*thunk));
    return (closure_t)NULL;
}

static closure_t closure;
static void update(int n)
{
    int i;
    for(i = 0; i < n; ++i) closure();
}

int main(int argc, char **argv)
{
    int acc = 0, c = 10;
    closure = make_closure((void*)&acc, (void*)&closure_block);
    update(argc > 1 ? (c=atoi(argv[1])) : c);
    printf("Accumulator at [%d]\n", acc);
    return 0;
}

[1] 2763 segmentation fault ./a.out