#include <stdlib.h>
#include <unistd.h>

int main(void) {
  char *dir;

  dir = getenv("FOODIR");
  if (dir != 0)
    dir = dir + strlen("FOODIR=");
  else
    dir = "/var/foo";
  if (chdir(dir) != 0)
    exit(1);
  /* ... */

  return 0;
}