/in/foo.cc: In function 'void build()':
/in/foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int x=0;x<strlen(s1);x++){
              ~^~~~~~~~~~~
/in/foo.cc:25:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int x=0;x<strlen(s2);x++){
              ~^~~~~~~~~~~