View Compilation Error

0_0_15289828_4792.cpp: In function 'int main()':
0_0_15289828_4792.cpp:22:18: error: 'strcat' was not declared in this scope
   strcat(str, "_");
                  ^
0_0_15289828_4792.cpp:25:28: error: 'strlen' was not declared in this scope
   for (int i=0;i<strlen(str);++i)
                            ^