View Compilation Error

0_0_18188577_23180.cpp: In function 'void str_plus(char*, char*, char*)':
0_0_18188577_23180.cpp:9:22: error: 'strlen' was not declared in this scope
     int la = strlen(a), lb = strlen(b);
                      ^
0_0_18188577_23180.cpp:11:18: error: 'lb' was not declared in this scope
     while (la || lb || carry)
                  ^