View Compilation Error

0_0_17484829_31534.cpp: In function 'char* bigNumber_add(char*, char*)':
0_0_17484829_31534.cpp:21:27: error: 'strlen' was not declared in this scope
     int strlenc = strlen(c);
                           ^
0_0_17484829_31534.cpp: In function 'void reverse(char*, bool)':
0_0_17484829_31534.cpp:81:31: error: 'strlen' was not declared in this scope
     size_t length = strlen(src);
                               ^
0_0_17484829_31534.cpp: In function 'void Add(const char*, const char*, char*)':
0_0_17484829_31534.cpp:90:33: error: 'strlen' was not declared in this scope
     size_t aLength = strlen(aNum);
                                 ^