K&R exercise 1-19: reversing each line of input
getlinea does too much: it reads the line and reverses it. A Single Responsibility Principle mandates restructuring the code:

while((len = getlinea(...)) &gt; 0) {
    reverse();
    print();
}

Avoid naked loops. Each loop represent an important action, and deserves a name. The code:

reverse(char turnaround[])
{
    char aux[...];
    int len = length_of_string(turnaround);
    copy_backwards(aux, turnaround, len);
    copy(turnaround, aux, len);
}


is much easier to reason about.
The length of the input is known to the caller. Consider passing it as parameter:

reverse(char turnaround[], int len)
{
    char aux[...];
    copy_backwards(aux, turnaround, len);
    copy(turnaround, aux, len);
}

Finally, a string can be reversed in place. There is no need for aux. I hope you can figure a solution yourself.