Comment by astrobe_

The program abort()s if the reallocation fails. But indeed, for an educational example, it's not good to be too smart.

I believe the test if(!num_lines) is unnecessary, because reallocating a NULL pointer is equivalent to malloc(). This is also a bit "smart", but I think it is also more correct because you don't use the value of one variable (num_lines is 0) to infer the value of another (lines is NULL).

To go further, an opened-ended structure like:

    struct
    {
      unsigned count;
      char* lines[];
    };

... could also be preferable in practice. But actually writing good C is not the topic of TFA.