/in/foo.cc: In function 'int main()':
/in/foo.cc:21:21: error: 'strlen' was not declared in this scope
     if (s1[strlen(s1) - 1] == '\n') {
                     ^
/in/foo.cc:24:21: error: 'strlen' was not declared in this scope
     if (s2[strlen(s2) - 1] == '\n') {
                     ^