getenv

    #include <stdlib.h>
    char *getenv(const char *name);

getenv matches the string pointed to by name

returns a pointer to the value in the environment, or NULL if there is no match.

/* 
 * getenv example code
 * http://code-reference.com/c/stdlib.h/getenv
 */
#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <stdlib.h>
 
int main ( void )
{
  char *env;
  env = getenv ("USER");
  if (env!=NULL) {
    printf ("Your Username is: %s\n", env);
  }
  return 0;
}

  user@host:~$ ./getenv 
  Your Username is: code-reference.com