#include <stdio.h>
#include <stdlib.h>
#include <string.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;
}

static void print(void)
{
  int n = pos_null(environ);
  int i;
  char **p;

  printf("  %08p = &environ, %d elements\n", &environ, n);
  for (p = environ, i = 0; *p != NULL; p++, i++)
    {
      printf("  %08p = &environ[%d] --> %08p --> \"%s\"\n",
	&environ[i], i, environ[i], environ[i]);
    }
  printf("\n");
}

int main(int argc, char *argv[], char *envp[])
{
  char *a, *b, *c;

  printf("initial conditions\n");
  print();

  putenv("A=value_a");
  putenv("B=value_b");
  putenv("C=value_c");

  printf("initial conditions\n");
  print();

  printf("proper use of getenv()\n");

  a = getenv("A");
  printf("  value of A at %08p is \"%s\"\n", a, a);

  b = getenv("B");
  printf("  value of B at %08p is \"%s\"\n", b, b);

  c = getenv("C");
  printf("  value of C at %08p is \"%s\"\n", c, c);

  printf("\n");

  printf("this might be an improper use of getenv()\n");

  a = getenv("A");
  b = getenv("B");
  c = getenv("C");

  printf("  value of A at %08p is \"%s\"\n", a, a);
  printf("  value of B at %08p is \"%s\"\n", b, b);
  printf("  value of C at %08p is \"%s\"\n", c, c);
  printf("\n");

  printf("this is not acceptable, but possibly not dangerous\n");

  a = getenv("A");

  /* The C Standard does not allow for direct changes to an environment
   * variable.  The change via strcpy(a, "new_A") might actually work,
   * since the new string is shorter than the old string.  However,
   * the GCC implementation of putenv() leaves a pointer to a string
   * constant, which is write-protected.  Consequently we get a
   * Segmentation fault when attempting either of the following.
   */
  strcpy(a, "new_A");
  *a = 'x';

  printf("  value of A at %08p is \"%s\"\n", a, a);
  printf("  value of B at %08p is \"%s\"\n", b, b);
  printf("  value of C at %08p is \"%s\"\n", c, c);
  printf("\n");

  printf("this is not acceptable, and dangerous\n");

  a = getenv("A");
  strcpy(a, "very_long_replacement_for_the_value_of_A");

  printf("  value of A at %08p is \"%s\"\n", a, a);
  printf("  value of B at %08p is \"%s\"\n", b, b);
  printf("  value of C at %08p is \"%s\"\n", c, c);
  printf("\n");

  return 0;
}