View Compilation Error

0_0_23166734_30920.cpp: In function 'int main()':
0_0_23166734_30920.cpp:6:38: error: 'strcmp' was not declared in this scope
     while(~scanf("%s",a),strcmp(a,"0"))
                                      ^
0_0_23166734_30920.cpp:8:22: error: 'strlen' was not declared in this scope
    int i,len=strlen(a),sum=0; 
                      ^
0_0_23166734_30920.cpp:10:3: error: 'sum' was not declared in this scope
   sum+=a[i]-'0'; 
   ^
0_0_23166734_30920.cpp:11:19: error: 'sum' was not declared in this scope
   printf("%d \n",(sum)%9);
                   ^