View Compilation Error

0_0_29656271_31472.cpp: In function 'int main()':
0_0_29656271_31472.cpp:13:18: error: 'strlen' was not declared in this scope
   int at=strlen(a);
                  ^
0_0_29656271_31472.cpp:15:22: error: 'strstr' was not declared in this scope
   while((p=strstr(p,b))!=NULL){
                      ^