View Compilation Error

0_0_37239439_32307.cpp: In function 'int main()':
0_0_37239439_32307.cpp:25:23: error: 'strlen' was not declared in this scope
   int len1 = strlen(s1), len2 = strlen(s2);
                       ^
0_0_37239439_32307.cpp:28:20: error: 'len2' was not declared in this scope
   for (i1=i2=0; i2<len2 && i1<len1; i2++, i1++)
                    ^
0_0_37239439_32307.cpp:32:19: error: 'len2' was not declared in this scope
   make_pi(pi, s2, len2);
                   ^