22.6.3 Accessing Environment Variables

You can optionally include a third parameter to main, another array of strings, to capture the environment variables available to the program. Unlike what happens with argv, there is no additional parameter for the count of environment variables; rather, the array of environment variables concludes with a null pointer.

#include <stdio.h>   /* Declares printf. */

int
main (int argc, char *argv[], char *envp[])
{
  /* Print out all environment variables. */
  int i = 0;
  while (envp[i])
    {
      printf ("%s\n", envp[i]);
      i++;
    }
}

Another method of retrieving environment variables is to use the library function getenv, which is defined in stdlib.h. Using getenv does not require defining main to accept the envp pointer. For example, here is a program that fetches and prints the user’s home directory (if defined):

#include <stdlib.h>  /* Declares getenv. */
#include <stdio.h>   /* Declares printf. */

int
main (void)
{
  char *home_directory = getenv ("HOME");
  if (home_directory)
    printf ("My home directory is: %s\n", home_directory);
  else
    printf ("My home directory is not defined!\n");
}