// Environment Variables, demonstration program

#include <stdio.h>
#include <stdlib.h>

extern char **environ;


/* find the NULL pointer that indicates the end of the array v */

static int pos_null(char *v[])
{
  if (v == NULL) return -1;

  int i = 0;

  while (*v != NULL) { v++; i++; }

  return i;
}


// Exercise.  Why did we require the extra level of indirection?

static void print(int *pargc, char **pargv[], char **penvp[])
{
  int n;

  n = pos_null(environ);
  printf("  %08p  &environ, %d elements\n", &environ, n);
  printf("  %08p  &environ[0]\n", &environ[0]);
  printf("  %08p  &environ[%d]\n", &environ[n], n);
  printf("  %08p  &environ[0][0]\n", &environ[0][0]);
  printf("\n");

  printf("  %08p  &argc\n", pargc);
  printf("\n");

  n = pos_null(*pargv);
  printf("  %08p  &argv, %d elements\n", pargv, n);
  printf("  %08p  &argv[0]\n", &(*pargv)[0]);
  printf("  %08p  &argv[%d]\n", &(*pargv)[n], n);
  printf("  %08p  &argv[0][0]\n", &(*pargv)[0][0]);
  printf("\n");

  n = pos_null(*penvp);
  printf("  %08p  &envp, %d elements\n", penvp, n);
  printf("  %08p  &envp[0]\n", &(*penvp)[0]);
  printf("  %08p  &envp[%d]\n", &(*penvp)[n], n);
  printf("  %08p  &envp[0][0]\n", &(*penvp)[0][0]);
  printf("\n");
}


int main(int argc, char *argv[], char *envp[])
{
  int n;

  printf("initial conditions\n");

  print(&argc, &argv, &envp);


  printf("after putenv(\"A=B\");\n");
  putenv("A=B");

  print(&argc, &argv, &envp);


  printf("after setenv(\"C\", \"D\", 1);\n");
  setenv("C", "D", 1);

  print(&argc, &argv, &envp);


  printf("after unsetenv(\"A\");\n");
  unsetenv("A=B");

  print(&argc, &argv, &envp);


  return 0;
}