View Compilation Error

0_0_20799530_31329.cpp: In function 'void GetNextval()':
0_0_20799530_31329.cpp:12:25: error: 'strlen' was not declared in this scope
     int w_len = strlen(W) - 1;  
                         ^
0_0_20799530_31329.cpp: In function 'int KMP()':
0_0_20799530_31329.cpp:34:25: error: 'strlen' was not declared in this scope
     int t_len = strlen(T) - 1;  
                         ^