NOT WRITABLE

Problem: C-library function freopen() is not atomic. It closes the
stream given as argument before trying (and thereby checking if it is
possible) to open the file which the stream is to be associated with. So
in the event that the open() fails, the user is left without a working
stream where the error message for the failed open can go. Likewise, any
buffered messages not yet written to the stream are also lost.

Solution: Stop using freopen() and re-implement my_freopen() using
fopen(), fileno() and dup2(). That way any error from open() (or
fileno()) is caught before doing anything which affects the stream.
According to the man-page, dup2() is atomic.