View Compilation Error

0_0_12639686_29807.cpp: In function `double Mul(char*, double*, char)':
0_0_12639686_29807.cpp:11: error: `atof' undeclared (first use this function)
0_0_12639686_29807.cpp:11: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_12639686_29807.cpp:16: error: `sscanf' undeclared (first use this function)
0_0_12639686_29807.cpp: In function `double Add(char*, double*, char)':
0_0_12639686_29807.cpp:35: error: `sscanf' undeclared (first use this function)
0_0_12639686_29807.cpp: In function `int main()':
0_0_12639686_29807.cpp:47: error: `gets' undeclared (first use this function)
0_0_12639686_29807.cpp:47: error: `strcmp' undeclared (first use this function)
0_0_12639686_29807.cpp:53: error: `printf' undeclared (first use this function)