foo.cc: In function 'void xx(char*)':
foo.cc:8:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<strlen(s);i++){
               ~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:35:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
      for (j=0;j<strlen(s1);j++){
               ~^~~~~~~~~~~
foo.cc:33:16: warning: 'i' may be used uninitialized in this function [-Wmaybe-uninitialized]
     while (s2[i]!='\0'){
            ~~~~^

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