How to resolve the algorithm Closures/Value capture step by step in the C programming language

Published on 7 June 2024 03:52 AM
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How to resolve the algorithm Closures/Value capture step by step in the C programming language

Table of Contents

Problem Statement

Create a list of ten functions, in the simplest manner possible   (anonymous functions are encouraged),   such that the function at index   i   (you may choose to start   i   from either   0   or   1),   when run, should return the square of the index,   that is,   i 2. Display the result of running any but the last function, to demonstrate that the function indeed remembers its value.

Demonstrate how to create a series of independent closures based on the same template but maintain separate copies of the variable closed over. In imperative languages, one would generally use a loop with a mutable counter variable. For each function to maintain the correct number, it has to capture the value of the variable at the time it was created, rather than just a reference to the variable, which would have a different value by the time the function was run. See also: Multiple distinct objects

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Closures/Value capture step by step in the C programming language

  1. Function Declarations:

    • _tmpl() is an internal function that returns a fixed value (TAG * TAG).
    • dupf() takes an integer v and returns a pointer to a function that is a copy of _tmpl() but with the TAG replaced with v.

  2. Function Definition of dupf():

    • It calculates the size of the function code to be copied (len) by subtracting the address of _tmpl from the address of dupf.
    • It creates a new memory mapping of size len, with read-write-execute permissions, and private and anonymous flags.
    • It copies the code from _tmpl to the new memory mapping.
    • It searches for all occurrences of TAG in the copied code and replaces them with v.
    • Finally, it returns the address of the new function.

  3. main() Function:

    • It creates an array of function pointers funcs.
    • It iterates through the array and calls dupf() to create copies of _tmpl() with different values of v.
    • It prints the results of calling each function, which are the squares of the values v.

Additional Notes:

  • This code essentially creates multiple runtime-generated functions that return squared values for different input values.
  • It demonstrates the use of memory mapping to create executable code at runtime.
  • The code is platform-specific and may only work on certain operating systems (e.g., Linux).

Source code in the c programming language

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

typedef int (*f_int)();
 
#define TAG 0xdeadbeef
int _tmpl() { 
	volatile int x = TAG;
	return x * x;
}

#define PROT (PROT_EXEC | PROT_WRITE)
#define FLAGS (MAP_PRIVATE | MAP_ANONYMOUS) 
f_int dupf(int v)
{
	size_t len = (void*)dupf - (void*)_tmpl;
	f_int ret = mmap(NULL, len, PROT, FLAGS, 0, 0);
	char *p;
	if(ret == MAP_FAILED) {
		perror("mmap");
		exit(-1);
	}
	memcpy(ret, _tmpl, len);
	for (p = (char*)ret; p < (char*)ret + len - sizeof(int); p++)
		if (*(int *)p == TAG) *(int *)p = v;
	return ret;
}
 
int main()
{
	f_int funcs[10];
	int i;
	for (i = 0; i < 10; i++) funcs[i] = dupf(i);
 
	for (i = 0; i < 9; i++)
		printf("func[%d]: %d\n", i, funcs[i]());
 
	return 0;
}


func[0]: 0
func[1]: 1
func[2]: 4
func[3]: 9
func[4]: 16
func[5]: 25
func[6]: 36
func[7]: 49
func[8]: 64


void init(void)
{
  t = intern(lit("t"));
  x = intern(lit("x"));
}

val square(val env)
{
  val xbind = assoc(env, x); /* look up binding of variable x in env */
  val xval = cdr(xbind);     /* value is the cdr of the binding cell */
  return num(cnum(xval) * cnum(xval));
}

int main(void)
{
  int i;
  val funlist = nil, iter;

  init();

  for (i = 0; i < 10; i++) {
    val closure_env = cons(cons(x, num(i)), nil);
    funlist = cons(func_f0(closure_env, square), funlist);
  }

  for (iter = funlist; iter != nil; iter = cdr(iter)) {
    val fun = car(iter);
    val square = funcall(fun, nao);

    printf("%d\n", cnum(square));
  }
  return 0;
}

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