Virtual COM port on Linux

Stuart Jansen sjansen at buscaluz.org
Mon Mar 15 12:44:09 MDT 2010


On Mon, 2010-03-15 at 12:32 -0600, Dave Smith wrote:
> Dave Smith wrote:
> > That's what I was looking for. I didn't escape the 'x' when I tried 
> > initially.
> 
> Actually, that appears to not work as I expected. The first problem is 
> that echo needs -n to *not* print a newline character, which would not 
> normally be a problem (but for purity's sake I added -n). The second 
> problem is that the last byte (0x00) does not get written to the device, 
> I assume because echo interprets it as a null-termination character. 
> After it failed to turn on the relay, here's what I did to investigate:
> 
> echo -n $'\xff\x01\x00' > /tmp/foo.txt
> hexdump -C /tmp/foo.txt
> 0000000 ff 01
> 
> And ls -l confirms that the file is only 2 bytes in size (not 3 as I 
> expected). Is it not possible to write a 0x00 byte as the final byte 
> using echo?
> 
> Any ideas why this happens?

Because echo is merely processing its argv and the '\0' is seen as
terminating the arg, not part of the arg.

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