printf goes to serial port?

Larry Hall (Cygwin)
Fri Dec 19 21:32:00 GMT 2008

Alex Martin wrote:
> Gary R. Van Sickle wrote:


>> A few gaps that need filling before anything better than SWAGs can 
>> come your
>> way:
>> - Is this app a Cygwin app, 
> No, it is a windows app.
> I am using cygwin primarily because I get the /dev/ttyS* devices. 

If you're using /dev/ttyS*, then you're using cygwin1.dll.  That makes
it a Cygwin app.  You're not going to get POSIX/Linux support from a
vanilla Win32 environment.  I feel obliged to mention that because of
you're using the Cygwin DLL, whatever you're working on needs to comply
with the Cygwin licensing scheme if you're distributing it in any way.

> or is Cygwin involved only perhiperally (e.g.
>> you're using Cygwin make to run the MinGW toolchain), 
> No mingw. Using all of cygwin's make, g++, etc.
> or is Cygwin not
>> directly involved at all and you're saying "My non-Cygwin-related app was
>> working fine, I changed a bunch of stuff in the app and also upgraded
>> Cygwin, and now the app is broke and I'm thinking Cygwin could be to 
>> blame"?
> I do not think the app is broken. Somehow stdout is going to the serial 
> port, my code relating to file descriptors (a serialio class that 
> provides functions like open, close, read, write, flush), opening and 
> closing the devices, etc has never changed (that is, the code is the 
> same code since back when printf worked). Other code has changed, that 
> is, the gui layout, function flow, etc.

Given the description so far, this is starting to sound like a case of
mixed CRTs, Cygwin's and MS's.  Only one can be in charge of stdin/stdout/
stderr if you expect to get sane results.  And calls to CRT I/O functions
have to occur entirely in one or the other to see consistent results.

>> - Since your app is a GUI app, where did you expect your printf()s to go?
> Well, someone taught me to use cygwin in this manner, and I have been 
> doing so for a long time, that is, I can use a cygwin window to invoke 
> my windows app, and the cygwin console would catch printfs (stdout 
> chatter), so you can use it as a debug console.

That's not guaranteed.  It really sounds to me like you just want a
way to look at some messages when they are triggered.  You'd be
better off with something like "OutputDebugString()" and DebugView
if the application isn't a console app.  And even if it is, this
could go a long way towards unraveling any mixing and matching
that you may have inadvertently done with I/O and CRTs.

Larry Hall                    
RFK Partners, Inc.                      (508) 893-9779 - RFK Office
216 Dalton Rd.                          (508) 893-9889 - FAX
Holliston, MA 01746


A: Yes.
 > Q: Are you sure?
 >> A: Because it reverses the logical flow of conversation.
 >>> Q: Why is top posting annoying in email?

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