// 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;
}


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

  printf("initial conditions\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", &argc);
  printf("\n");

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

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


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

  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", &argc);
  printf("\n");

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

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


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

  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", &argc);
  printf("\n");

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

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


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

  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", &argc);
  printf("\n");

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

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


  return 0;
}