Capturing print output from shared library called from python with ctypes module

We can use os.dup2() and os.pipe() to replace the entire stdout file descriptor (fd 1) with a pipe we can read from ourselves. You can do the same thing with stderr (fd 2).

This example uses select.select() to see if the pipe (our fake stdout) has data waiting to be written, so we can print it safely without blocking execution of our script.

As we are completely replacing the stdout file descriptor for this process and any subprocesses, this example can even capture output from child processes.

import os, sys, select

# 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 whole pipe
def read_pipe():
        out = ''
        while more_data():
                out += os.read(pipe_out, 1024)

        return out

# testing print methods
import ctypes
libc = ctypes.CDLL('libc.so.6')

print 'This text gets captured by myStdOut'
libc.printf('This text fails to be captured by myStdOut\n')

# put stdout back in place 
os.dup2(stdout, 1)
print 'Contents of our stdout pipe:'
print read_pipe()

Simplest example, because this question in google top.

import os
from ctypes import CDLL

libc = CDLL(None)
stdout = os.dup(1)
silent = os.open(os.devnull, os.O_WRONLY)
os.dup2(silent, 1)
libc.printf(b"Hate this text")
os.dup2(stdout, 1)