View Compilation Error

0_0_22150435_8852.cpp: In constructor 'BigNum::BigNum(const char*)':
0_0_22150435_8852.cpp:58:12: error: 'strlen' was not declared in this scope
  l=strlen(s);   
            ^
0_0_22150435_8852.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_22150435_8852.cpp:95:17: error: 'strlen' was not declared in this scope
  int l=strlen(ch);
                 ^