Disabling Debug Process I/O Multiplexing

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Wing alters the I/O environment in order to make it possible to keep the debug process responsive while waiting for I/O. This code mimics the environment found outside of the debugger, so any code that uses only Python-level I/O does not need to worry about this change of environment.

There are however several cases that can affect users that bypass Python-level I/O by doing C/C++ level I/O from within an extension module:

  • Any C/C++ extension module code that does standard I/O calls using the C-level stdin or stdout will bypass Wing's I/O environment (which affects only Python-level stdin and stdout). This means that waiting on stdin in C or C++ code will make the debug process unresponsive to Wing, causing time out and termination of the debug session if you attempt to Pause or alter breakpoints at that time. In this case, redirection of I/O to the debugger I/O tool and Debug Probe (in Wing Pro only) will also not work.
  • On all platforms, calling C-level stdin from multiple threads in a multi-threaded program may result in altered character read order when running under the Wing debugger.
  • When debugging on win32, calling C-level stdin, even in a single-threaded program, can result in a race condition with Wing's I/O multiplexer that leads to out-of-order character reads. This is an unavoidable result of limitations on multiplexing keyboard and socket I/O on this platform.

If you run into a problem with keyboard I/O in Wing's debugger, you should:

  • Turn off Wing's I/O multiplexer by setting the Use sys.stdin Wrapper preference to False.
  • Turn on the Use External Console preference (for details see External I/O Consoles)

Once that is done, I/O should work properly in the external console, but the debug process will remain unresponsive to Pause or breakpoint commands from Wing whenever it is waiting for input, either at the C/C++ or Python level.

Also, in this case keyboard input invoked as a side effect of using the Debug Probe in Wing Pro will happen through unmodified stdin instead of within the Debug Probe, even though command output will still appear there.