View Compilation Error

0_0_24569611_561.cpp: In function 'int main()':
0_0_24569611_561.cpp:31:28: error: 'strlen' was not declared in this scope
  while (gets(s) && strlen(s)) {
                            ^
0_0_24569611_561.cpp:34:28: error: 'strlen' was not declared in this scope
  while (gets(s) && strlen(s)) {
                            ^