View Compilation Error

0_0_21867598_21837.cpp: In function 'int main()':
0_0_21867598_21837.cpp:16:17: error: 'strlen' was not declared in this scope
     len=strlen(a);
                 ^
0_0_21867598_21837.cpp:50:21: error: 'strcpy' was not declared in this scope
        strcpy(b[j],c);
                     ^
0_0_21867598_21837.cpp:62:26: error: 'strcmp' was not declared in this scope
       if(strcmp(b[i],b[k])==0 && b[k][0]!='#')
                          ^