Attempts to write length bytes of data from the buffer buf to the file filep.
Make sure that filep is open for writing. You might want to check the
number of bytes written (
TSfwrite() returns this value) against the
value of length. If it is less, there might be insufficient space on
disk, for example.
The behavior is undefined if length is greater than SSIZE_MAX.
Returns the number of bytes actually written, or -1 if an error occured.