View Compilation Error

0_0_34154338_21609.cpp: In constructor 'BigNum::BigNum()':
0_0_34154338_21609.cpp:29:43: error: 'memset' was not declared in this scope
  BigNum(){ len = 1; memset(a, 0, sizeof(a)); }   //构造函数
                                           ^
0_0_34154338_21609.cpp: In constructor 'BigNum::BigNum(int)':
0_0_34154338_21609.cpp:45:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_34154338_21609.cpp: In copy constructor 'BigNum::BigNum(const BigNum&)':
0_0_34154338_21609.cpp:57:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_34154338_21609.cpp: In member function 'BigNum& BigNum::operator=(const BigNum&)':
0_0_34154338_21609.cpp:65:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_34154338_21609.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_34154338_21609.cpp:75:19: error: 'strlen' was not declared in this scope
  int l = strlen(ch);
                   ^