View Compilation Error

0_0_16754134_2841.cpp: In constructor 'BigNum::BigNum()':
0_0_16754134_2841.cpp:17:43: error: 'memset' was not declared in this scope
     BigNum(){ len = 1;memset(a,0,sizeof(a)); }   //构造函数
                                           ^
0_0_16754134_2841.cpp: In constructor 'BigNum::BigNum(int)':
0_0_16754134_2841.cpp:42:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_16754134_2841.cpp: In constructor 'BigNum::BigNum(const char*)':
0_0_16754134_2841.cpp:54:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_16754134_2841.cpp:55:15: error: 'strlen' was not declared in this scope
     l=strlen(s);   
               ^
0_0_16754134_2841.cpp: In copy constructor 'BigNum::BigNum(const BigNum&)':
0_0_16754134_2841.cpp:74:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a)); 
                         ^
0_0_16754134_2841.cpp: In member function 'BigNum& BigNum::operator=(const BigNum&)':
0_0_16754134_2841.cpp:82:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a)); 
                         ^
0_0_16754134_2841.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_16754134_2841.cpp:92:20: error: 'strlen' was not declared in this scope
     int l=strlen(ch);
                    ^