Java serial comm API - what does inputstream.read() return if a timeout occurs?

Question

Anyone know the exact value returned when the serial driver times out? I'm running Java1.5 on Win XP.

Answer 1

I can't speak for exact values, but I would have thought some sort of IOException would be thrown.

If you're getting the input stream from the comm port and reading from that, the docs state that an IOException will be thrown if "If the first byte cannot be read for any reason other than the end of the file, if the input stream has been closed, or if some other I/O error occurs."

Answer 2

What does the javadoc say?

read()

Reads the next byte of data from the input stream. 
The value byte is returned as an int in the range 
0 to 255. If no byte is available because the end 
of the stream has been reached, the value -1 is 
returned. This method blocks until input data 
is available, the end of the stream is detected, 
or an exception is thrown.

A subclass must provide an implementation of this method. 

---

read(byte[])

Reads some number of bytes from the input stream 
and stores them into the buffer array b. The 
number of bytes actually read is returned as 
an integer. This method blocks until input
data is available, end of file is detected, 
or an exception is thrown. 

If b is null, a NullPointerException is thrown. 
If the length of b is zero, then no bytes 
are read and 0 is returned; otherwise, there is
an attempt to read at least one byte. If no 
byte is available because the stream is at end 
of file, the value -1 is returned; otherwise, 
at least one byte is read and stored into b.

The first byte read is stored into element b[0], 
the next one into b[1], and so on. The number 
of bytes read is, at most, equal to the length 
of b. Let k be the number of bytes actually read;
these bytes will be stored in elements b[0] 
through b[k-1], leaving elements b[k] through 
b[b.length-1] unaffected.

If the first byte cannot be read for any reason 
other than end of file, then an IOException 
is thrown. In particular, an IOException is 
thrown if the input stream has been closed.

The read(b) method for class InputStream 
has the same effect as:

 read(b, 0, b.length) 

---

read(byte[], int, int)

Reads up to len bytes of data from the input 
stream into an array of bytes. An attempt 
is made to read as many as len bytes, but 
a smaller number may be read. The number of bytes 
actually read is returned as an integer.

This method blocks until input data is 
available, end of file is detected, or an exception 
is thrown.

If b is null, a NullPointerException is thrown.

If off is negative, or len is negative, or off+len 
is greater than the length of the array 
b, then an IndexOutOfBoundsException is thrown.

If len is zero, then no bytes are read and 0 is 
returned; otherwise, there is an attempt to 
read at least one byte. If no byte is available 
because the stream is at end of file, the 
value -1 is returned; otherwise, at least one 
byte is read and stored into b.

The first byte read is stored into element b[off], 
the next one into b[off+1], and so on. The number 
of bytes read is, at most, equal to len. Let k be 
the number of bytes actually read; these bytes will
be stored in elements b[off] through b[off+k-1], 
leaving elements b[off+k] through b[off+len-1] unaffected.

In every case, elements b[0] through b[off] and
elements b[off+len] through b[b.length-1] 
are unaffected.

If the first byte cannot be read for any reason other 
than end of file, then an IOException is thrown. In 
particular, an IOException is thrown if the input 
stream has been closed.

The read(b, off, len) method for class InputStream 
simply calls the method read() repeatedly. If the 
first such call results in an IOException, that 
exception is returned from the call to the read(b, off, len) 
method. If any subsequent call to read() results in a 
IOException, the exception is caught and treated as
if it were end of file; the bytes read up to that
point are stored into b and the number of bytes 
read before the exception occurred is returned. 
Subclasses are encouraged to provide a more efficient 
implementation of this method. 

Natively, the InputStream does not have timeouts, so it depends upon how the driver reports the results. If the driver reports EOF, you'll just get a -1. If the driver reports an error, you should get an IOException.