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I'm following the docs/tutorial here: https://docs.python.org/3/library/ctypes.htmlpython3.8

In section "Calling Functions Continued" it has a snippet for running printf. I played around with it a bit, and I can't understand why the first statement below prints only 5 and not Hello? Why is the newline needed? (update: also why are the orders sometimes different i.e. "5 Hello" vs "Hello 6")

>>> libc = cdll.LoadLibrary("libc.so.6")
>>> libc.printf
<_FuncPtr object at 0x7f68e4d61880>
>>> printf = libc.printf
>>> printf(b"Hello")
5
Hello>>> printf(b"Hello","")
5
Hello>>> printf(b"Hello %S\n","World!")
Hello World!
13
>>> printf(b"Hello\n","")
Hello
6

I suppose it might have something to do with this statement made on the same page:

Note that printf prints to the real standard output channel, not to sys.stdout, so these examples will only work at the console prompt, not from within IDLE or PythonWin:

But I can't really understand this statement, I always thought sys.stdout was the real system output channel. (perhaps these applications set it to something different)?

setup: python 3.8 running inside a linux docker container on a Mac

tjb
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  • https://en.cppreference.com/w/cpp/io/c/fprintf. Also check [\[SO\]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer)](https://stackoverflow.com/a/58611011/4788546). – CristiFati Jan 26 '23 at 15:48

1 Answers1

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The reason it's not printing a newline is because it's Python's native print function that normally adds a newline. The libc printf implementation (what you would call if you were writing in C) does not print a newline at the end. You must explicitly add it.

As for the sys.stdout vs "real standard out", "real standard out" writes to file descriptor 1. sys.stdout ultimately does as well, but it goes through some code that the IDLE/PythonWin likely capture and process. So using printf would circumvent those things, and therefore wouldn't show up. Not sure how it's implemented so that it doesn't, but that's the difference between them.

saquintes
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  • thanks, why does it sometimes happen that "number" is first and the words come second i.e. "5 Hello" vs. "Hello 6" – tjb Jan 26 '23 at 08:10
  • It's just a race condition between the underlying printf and Python's REPL. The REPL (the thing showing you the >>>) is printing the return of ``printf`` which is the number of bytes printed. Then ``printf`` is writing to stdout, which the REPL is also printing. It's not going to be completely serial because you're dealing with IO, which is slow and buffered. – saquintes Jan 26 '23 at 08:15