View Compilation Error

0_0_13615276_16256.cpp: In constructor 'BigNum::BigNum()':
0_0_13615276_16256.cpp:13:43: error: 'memset' was not declared in this scope
     BigNum(){ len = 1;memset(a,0,sizeof(a)); }   //构造函数
                                           ^
0_0_13615276_16256.cpp: In constructor 'BigNum::BigNum(int)':
0_0_13615276_16256.cpp:38:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_13615276_16256.cpp: In constructor 'BigNum::BigNum(const char*)':
0_0_13615276_16256.cpp:50:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_13615276_16256.cpp:51:15: error: 'strlen' was not declared in this scope
     l=strlen(s);
               ^
0_0_13615276_16256.cpp: In copy constructor 'BigNum::BigNum(const BigNum&)':
0_0_13615276_16256.cpp:70:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_13615276_16256.cpp: In member function 'BigNum& BigNum::operator=(const BigNum&)':
0_0_13615276_16256.cpp:78:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));
                         ^
0_0_13615276_16256.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_13615276_16256.cpp:88:20: error: 'strlen' was not declared in this scope
     int l=strlen(ch);
                    ^