python sys.stdout and C++ iostreams::cout

Nobody nobody at
Thu Jan 17 16:35:27 CET 2013

On Thu, 17 Jan 2013 07:02:24 -0800, Utpal Sarkar wrote:

> I was assuming that sys.stdout would be referencing the same physical
> stream as iostreams::cout running in the same process, but this doesn't
> seem to be the case.

At startup, it refers to the same FILE* as C's stdout. This initially
shares a buffer with C++'s std::cout, but that can be changed via

However ...

> The following code, which makes a call to a C++
> function with a python wrapper called "write", that writes to cout:
> from cStringIO import StringIO
> import sys
> orig_stdout = sys.stdout
> sys.stdout = stringout = StringIO()
> write("cout") # wrapped C++ function that writes to cout print "-" * 40
> print "stdout"
> sys.stdout = orig_stdout
> print stringout.getvalue()

This code changes sys.stdout so that it refers to something other than C's
stdout. C's stdout is still the same FILE*, C++'s std::count is still the
same std::ostream, and the synchronisation between the two hasn't changed.

> immediately writes "cout" to the console, then the separator "---...", and
> finally, as the return value of stringout.getvalue(), the string "stdout".
> My intention was to capture in stringout also the string written to cout
> from C++. Does anyone know what is going on, and if so, how I can capture
> what is written to cout in a python string?

Changing sys.stdout doesn't (and cannot) have any effect upon how C or C++
code behaves. sys.stdout is just a Python variable.

If you want to capture output from C or C++ code, you'll have to do so via
other means, e.g. freopen() or dup2().

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