View Compilation Error

0_0_35516506_32693.c: In function 'main':
0_0_35516506_32693.c:4:42: error: 'EOF' undeclared (first use in this function)
     while (scanf("%llx %llx", &a, &b) != EOF) {
                                          ^
0_0_35516506_32693.c:4:42: note: each undeclared identifier is reported only once for each function it appears in