Closures in Pure C

closure.c

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

#define FUNC1_PLACEHOLDER 0xDEADBEEFABADF00D

int func1() {
    int *ptr = (int*)FUNC1_PLACEHOLDER;

    return *ptr;
}

int func1_end() {};

typedef struct closure_t {
    int (*call)();
    void **ctx;
    int   length;
} closure_t;

void closure_create(long start, long end, long placeholder, closure_t* c) {
    int   length = end - start;
    int   ptr;

    if (mprotect(c->call, getpagesize(), PROT_READ|PROT_EXEC|PROT_WRITE) == -1) {
        return;
    }

    c->length = length;

    memcpy(c->call, (void*)start, length);

    for (ptr = 0; ptr < length; ptr++) {
        long* memptr = (long*)(c->call + ptr);
        if (*memptr == placeholder) {
            c->ctx = (void*)memptr;
        }

        printf("%2x ", *((unsigned char *)(c->call + ptr)));
    }

    printf("\n");
}

void closure_embed_ctx(closure_t* c, void *ctx) {
    *(c->ctx) = ctx;
}

int main() {
    int      *test_val1 = malloc(sizeof(int));
    int      *test_val2 = malloc(sizeof(int));
    closure_t closure;

    closure.call = valloc(getpagesize());

    closure_create((long)func1, (long)func1_end, FUNC1_PLACEHOLDER, &closure);
    printf("closure->call   = %p\n", closure.call);
    printf("closure->ctx    = %p\n", closure.ctx);
    printf("closure->length = %p\n", closure.length);

    *test_val1 = 1234;
    printf("*[%p] = %d\n", test_val1, *test_val1);
    closure_embed_ctx(&closure, test_val1);
    printf("val1 closure call = %d\n", closure.call());

    *test_val2 = 4321;
    printf("*[%p] = %d\n", test_val2, *test_val2);
    closure_embed_ctx(&closure, test_val2);
    printf("val2 closure call = %d\n", closure.call());

    return 0;
}