[Python-Dev] PEP 446: Add new parameters to configure the inherance of files and for non-blocking sockets (original) (raw)

Victor Stinner victor.stinner at gmail.com
Wed Jul 17 13:31:14 CEST 2013

2013/7/7 Cameron Simpson <cs at zip.com.au>:

On 06Jul2013 14:43, Victor Stinner <victor.stinner at gmail.com> wrote: | 2013/7/6 Cameron Simpson <cs at zip.com.au>: | > Yes. Please forget I mentioned fork(); it is only relevant if you | > were offering some facility to undo the addition of cloexec to a | > Popen passed file descriptor. Which you are not. | | Oh... gotcha. I now understood your concern. | | There is a little "trick" here: at fork, file descriptors are | duplicated in the child process and almost all properties (open state, | flags, offset, etc.) are shared. There is one little exception: file | attributes are not shared, and there is only one file attribute: | OCLOEXEC. Setting OCLOEXEC in a child process does not affect the | flag in the parent process ;-) I will add a unit test to check this.

That's news to me. Interesting. I can't find UNIX doco to support this, though I haven't finished looking. None of the POSIX, Linux or MacOSX fork() manual pages seem to describe this. Can you point me at doco for this? I thought file descriptors were a direct index into a per-process table of file handle structures, and did not think the latter got part copies, part shared over a fork.

Sorry, I found this fact my mistake, not in the doc :-) You can verify this behaviour using the follownig script: hg.python.org/peps/file/tip/pep-0466/test_cloexec.py

Output on Linux:

initial state: fd=3, cloexec=False child process after fork, set cloexec: cloexec=True child process after exec: fd=3, cloexec= parent process after fork: cloexec=False parent process after exec: fd=3, cloexec=False

"parent process after fork: cloexec=False" means that the flag was not modified in the parent process.

Also, is OCLOEXEC really the only file attribute? So ONONBLOCK is a flag and not an attribute (guessing the terminology distinguishes these)? Now you mention it, ISTR noticing that there were distinct ioctls to adjust flags and something else.

See the manual page: http://linux.die.net/man/2/fcntl

There are "File descriptor flags " and "File status flags", it looks like they are inherited/shared differently between the parent and the child process.

Your change says:

On Windows, the close-on-exec flag is HANDLEFLAGINHERIT. I feel it would be clearer to say: On Windows, the close-on-exec flag is the inverse of HANDLEFLAGINHERIT.

Ok, fixed in the PEP.

Victor

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