TxBuffer problem on serial port

[Eddy Van Esch]

hi all,
i have a (small?) problem concerning the serial com ports.
this problem was already mentioned once by somebody else in the following post: http://www.powerbasic.com/support/pb...ead.php?t=1691
but no replies...
thing is that the comm(#hcomm, txbuffer) statement returns 0 as the
txbuffer length although it has been set to a certain value before.
i tried the "c:\pbdll60\samples\comm\comm.bas" example.
this works fine until you try to send a file. then comm.bas tries
to retrieve the txbuffer length using comm(#hcomm, txbuffer).
this statement returns 0 as a value. this causes a program crash later
on in the program because of a division by zero error.
you can try it in the sample program on your drive by adding 1 line to
the following routine:

function startcomms as long
    comm open $comport as #hcomm
    if errclear then exit function          ' exit if port cannot be opened

    comm set #hcomm, baud     = 9600        'baud
    comm set #hcomm, byte     = 8           ' 8 bits
    comm set #hcomm, parity   = %true       ' parity
    comm set #hcomm, paritytype   = 2         '0 = none, 1 = odd, 2 = even, 3 = mark, 4 = space
    comm set #hcomm, stop     = 2           ' 2 stop bit
    comm set #hcomm, txbuffer = 4096        ' 4k transmit buffer
    comm set #hcomm, rxbuffer = 4096        ' 4k receive buffer

'add the following line to display buffer size:
msgbox str$(comm(#hcomm, txbuffer)),,"read size of txbuffer"

    function = %true
end function

on my windows 2000 pc the messagebox shows a returned value of 0...
does anybody have any suggestion?
kind regards
eddy


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[this message has been edited by eddy van esch (edited june 12, 2001).]

[Guest]

Check the GetCommProperties API. The input and output buffer sizes may not be available from windows.

[Tom Hanlin]

The design of the 32-bit Windows communications APIs is very flexible, but
noticeably demented. There are a number of little gotcha's, documented or
otherwise. In particular, retrieving the number of characters in a comm tx
buffer may return zero in all cases (the issue you've discovered). Another
fine problem is, setting the size of a buffer may fail under different
versions of Windows for various reasons: buffer too large, buffer too small,
buffer not a power of two, buffer not a power of two minus one, or it's
Wednesday and the moon is full.

So, if possible, avoid setting the size of comm buffers, and don't expect a
useful result from TXBUFFER. Or consider writing a 16-bit Windows program:
the 16-bit comm APIs also had problems, but were less dramatically screwy.


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Tom Hanlin
PowerBASIC Staff

[Eddy Van Esch]

Thanks guys for your answer.
It is clear to me now.. forget the txbuffer...
Eddy


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