View Compilation Error

0_0_35264881_14883.cpp: In function 'double calculator(char*)':
0_0_35264881_14883.cpp:19:25: error: 'strlen' was not declared in this scope
     for(i=0;i<strlen(str);i++){
                         ^
0_0_35264881_14883.cpp: In function 'int main()':
0_0_35264881_14883.cpp:68:26: error: 'strcmp' was not declared in this scope
         if(strcmp(buf,"0")==0) break;
                          ^