Cactching Stdout

[Massi]

Hi everyone! I'm writing a python script which uses a C-written dll. I
call the functions in the dll using ctypes, but I don't know how to
catch the output of the "printf" which the C functions use. In fact I
don't even know if it is possible! I've heard something about PIPE and
popen...is this what I need? How can I use them? It is very important
for me that I could take the output in real-time.
Thanks for the help!
Massi

[Gabriel Genellina]

Re: Cactching Stdout

At Wednesday 8/11/2006 06:09, Massi wrote:

>Hi everyone! I'm writing a python script which uses a C-written dll. I
>call the functions in the dll using ctypes, but I don't know how to
>catch the output of the "printf" which the C functions use. In fact I
>don't even know if it is possible! I've heard something about PIPE and
>popen...is this what I need? How can I use them? It is very important
>for me that I could take the output in real-time.

Since you are calling a function the same process, popen&co won't help.
This is just an idea; printf is writting to STDOUT; you could replace
STDOUT with the write end of a pipe, and read from the other end.


--
Gabriel Genellina
Softlab SRL

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[Stephan Kuhagen]

Re: Cactching Stdout

Dennis Lee Bieber wrote:

>popen...is this what I need? How can I use them? It is very important
>for me that I could take the output in real-time.
>Thanks for the help!
>Massi

>
That's going to be difficult... popen and pipes work when one
process starts another INDEPENDENT process.

I'm not sure, if it fits the problem, but just a few days ago I had the
problem to catch the stdout and stderr of a module to process it before
writing it to the console. What I did was basically this:

---
import sys
import StringIO

# save the original streams
_stdout_ = sys.stdout
_stderr_ = sys.stderr

# create StringIO string buffers to replace streams
sys.stdout = StringIO.String IO()
sys.stderr = StringIO.String IO()

# Now print something, this goes to the string buffers instead of console
print "Hello World to stdout"
sys.stderr.writ e("Hello World to stderr\n")

# Now process the contents of the buffers...
....
# ...and print them to the real console afterwards
_stdout_.write( sys.stdout)
_stderr_.write( sys.stderr)

# Clean up the string buffers for the next IO
sys.stdout.trun cate(0)
sys.stderr.trun cate(0)
---

Of course you should put all that into defs and create own write/print
functions to encapsulate the whole buffering/processing/cleanup.

This works great for scripting, but I'm pretty sure, that it does not work
for C Libraries in Python. But I do not know, how Python handles its
streams internally, so it might be worth a try.

Regards
Stephan