realloc() invalid old size

Question

I am doing an exercise for fun from KandR C programming book. The program is for finding the longest line from a set of lines entered by the user and then prints it.

Her

Answer 1

You are trying to realloc() memory that was not allocated dynamically with malloc(). You cannot do that.

Also, if realloc() fails, the original memory is still allocated, and thus still needs to be freed with free(). So do not assign the return value of realloc() to the original pointer unless it is not NULL, otherwise you are leaking the original memory. Assign the return value of realloc() to a temp variable first, check its value, and then assign to the original pointer only if realloc() was successful.