View Compilation Error

0_0_18188573_25924.cpp: In function 'void str_plus(char*, char*, char*)':
0_0_18188573_25924.cpp:8:22: error: 'strlen' was not declared in this scope
     int la = strlen(a), lb = strlen(b);
                      ^
0_0_18188573_25924.cpp:10:18: error: 'lb' was not declared in this scope
     while (la || lb || carry)
                  ^
0_0_18188573_25924.cpp:19:22: error: 'reverse' was not declared in this scope
     reverse(c, c + lc);
                      ^