View Compilation Error

0_0_36655613_1071.cpp: In function 'int index(char*, char*, int)':
0_0_36655613_1071.cpp:6:21: error: 'strlen' was not declared in this scope
  while(i <= strlen(s) && j <= strlen(t))
                     ^
0_0_36655613_1071.cpp:19:17: error: 'strlen' was not declared in this scope
  if(j > strlen(t))
                 ^