View Compilation Error

0_0_32245695_342.cpp: In function 'void add(char*, char*)':
0_0_32245695_342.cpp:19:19: error: 'strlen' was not declared in this scope
  int l1 = strlen(a);
                   ^
0_0_32245695_342.cpp: In function 'void multiply(char*, char*, char*)':
0_0_32245695_342.cpp:73:26: error: 'strlen' was not declared in this scope
  for (i = 0; i < strlen(b); i++)
                          ^
0_0_32245695_342.cpp: In function 'int main()':
0_0_32245695_342.cpp:110:28: error: 'strlen' was not declared in this scope
    for (j = 0; j < strlen(c); j++)
                            ^
0_0_32245695_342.cpp:116:24: error: 'strlen' was not declared in this scope
   for (int i = strlen(b) - 1; i >= 0; i--)
                        ^