I have a cplusplus shared library, with ac interface, that writes log entries in stdout. I'm using it in a python application using the ctypes
library. The python application uses logging
library to write log entries.
What I need to do is to capture the stdout entries of the shared library to write the log entries with the logging
module. In other words, I want to redirect the stdout entries of the c library to the logging
module, so I can use logging
to write to files and console using its handlers.
I found that is possible to capture the stdout ( see this SO question ), but I can access it only when the c module call ends, and hence it is useless for logging. I want a none-blocking way to access the stdout entries.
A minimum example is as follows.
module.cpp (compiled with g++ -fPIC -shared module.cpp -o module.so
)
#include <unistd.h>
#include <iostream>
using namespace std;
extern "C" int callme()
{
cout<<"Hello world\n";
sleep(2);
cout<<"Some words\n";
sleep(2);
cout<<"Goodby world\n";
return 0;
}
The python app that calls it:
import ctypes as ct
import logging
format='%(asctime)s - %(levelname)s - %(message)s', level=logging.DEBUG
logging.basicConfig(format=format)
logging.debug('This logging modules works like a charm!')
mymodule = ct.CDLL('./module.so')
mymodule.callme()
logging.info('I want to capture the shared library log entries')
logging.warning('Can I?')
This produces:
2016-02-04 16:16:35,976 - DEBUG - This logging modules works like a charm!
Hello world
Some words
Goodby world
2016-02-04 16:16:39,979 - INFO - I want to capture the shared library log entries
2016-02-04 16:16:39,979 - WARNING - Can I?
I have access to the c++ library, so a solution that needs modifications in the library are also welcome.
You should be able to modify the code from the linked answer by reading from the pipe in a thread while the C module call is running. The following should work, although I haven't tested it with a long-running module call:
def redirected_printed_output(module_call):
# the pipe would fail for some reason if I didn't write to stdout at some point
# so I write a space, then backspace (will show as empty in a normal terminal)
sys.stdout.write(' \b')
pipe_out, pipe_in = os.pipe()
# save a copy of stdout
stdout = os.dup(1)
# replace stdout with our write pipe
os.dup2(pipe_in, 1)
# check if we have more to read from the pipe
def more_data():
r, _, _ = select.select([pipe_out], [], [], 0)
return bool(r)
# read the pipe, writing to (former) stdout
def write_pipe_to_stdout():
while more_data():
os.write(stdout, os.read(pipe_out, 1024))
done = False
def read_loop():
# rewrite the pipe out to stdout in a loop while the call is running
while not done:
write_pipe_to_stdout()
# Finish the remnants
write_pipe_to_stdout()
t = threading.Thread(target=read_loop)
t.start()
module_call()
done = True
t.join() # wait for the thread to finish
# put stdout back in place
os.dup2(stdout, 1)
I tested it as follows (OSX):
import ctypes
libc = ctypes.CDLL('libc.dylib')
def zomg():
for i in xrange(5):
libc.printf('libc stdout: %d\n', i)
time.sleep(1)
redirected_printed_output(zomg)
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