View Compilation Error

635516_0_0_1663.cpp: In function `char* sum(char*, char*)':
635516_0_0_1663.cpp:8: error: `strlen' undeclared (first use this function)
635516_0_0_1663.cpp:8: error: (Each undeclared identifier is reported only once for each function it appears in.)
635516_0_0_1663.cpp:10: error: `malloc' undeclared (first use this function)