View Compilation Error

0_0_23257367_10770.cpp: In member function 'void HUGE_INT::Display()':
0_0_23257367_10770.cpp:53:17: error: 'strlen' was not declared in this scope
     n=strlen(num);
                 ^
0_0_23257367_10770.cpp: In constructor 'HUGE_INT::HUGE_INT(char*)':
0_0_23257367_10770.cpp:62:18: error: 'strlen' was not declared in this scope
     len= strlen(n);
                  ^
0_0_23257367_10770.cpp: In function 'int operator>(HUGE_INT, HUGE_INT)':
0_0_23257367_10770.cpp:103:27: error: 'strlen' was not declared in this scope
     len1=(int)strlen(m.num);
                           ^