View Compilation Error

0_0_37100623_29668.cpp: In function 'int main()':
0_0_37100623_29668.cpp:39:22: error: 'strlen' was not declared in this scope
   for (i = strlen(ch1) - 1, j = strlen(ch2) - 1; i >= 0 && j >= 0; i--, j--)
                      ^
0_0_37100623_29668.cpp:44:17: error: 'strlen' was not declared in this scope
   if (strlen(ch1) > strlen(ch2))
                 ^