text: #include "apue.h"
  #include <ctype.h>
  
  int main() {
    int c;
    while ((c = getchar()) != EOF) {
      if (isupper(c)) {
        c = tolower(c);
      }
      if (putchar(c) == EOF) {
        err_sys("output error");
      }
      if (c == '\n') {
        fflush(stdout);
      }
    }
    exit(0);
  }