View Compilation Error

0_0_27583248_6977.cpp: In function 'int main()':
0_0_27583248_6977.cpp:11:18: error: 'strlen' was not declared in this scope
        k=strlen(a);
                  ^
0_0_27583248_6977.cpp:12:18: error: 'strcpy' was not declared in this scope
        strcpy(b,a);
                  ^
0_0_27583248_6977.cpp:19:21: error: 'strcmp' was not declared in this scope
        if(strcmp(a,b)==0)
                     ^