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Use fcntl(fd, F_GETFD) to check whether an fd is valid #90073
Description
| BPO | 45915 |
|---|---|
| Nosy | @vstinner, @tiran, @eryksun, @corona10 |
| PRs |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2021-11-28.10:13:06.935>
labels = ['expert-C-API', 'type-bug', '3.11']
title = 'Use fcntl(fd, F_GETFD) to check whether an fd is valid'
updated_at = <Date 2021-12-14.03:31:17.636>
user = 'https://github.com/tiran'bugs.python.org fields:
activity = <Date 2021-12-14.03:31:17.636>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2021-11-28.10:13:06.935>
creator = 'christian.heimes'
dependencies = []
files = []
hgrepos = []
issue_num = 45915
keywords = ['patch']
message_count = 5.0
messages = ['407197', '407217', '407222', '407224', '408514']
nosy_count = 4.0
nosy_names = ['vstinner', 'christian.heimes', 'eryksun', 'corona10']
pr_nums = ['29821', '30082']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45915'
versions = ['Python 3.11']
Activity
tiran commentedon Nov 28, 2021
is_valid_fd() uses dup() or fstat() to check whether an fd is valid. Both operations are costly.
fcntl() with F_GETFD returns the file descriptor flags (e.g. CLOEXEC) and -1 with errno set to EBADF when fd is not an open file descriptor. It's faster than duplicating + closing a fd or calling fstat(). The idea to use fcntl(fd, F_GETFD) is inspired by the patch [1]. According to Stackoverflow [2]:
[1] singlestore-labs@0364554
[2] https://stackoverflow.com/questions/12340695/how-to-check-if-a-given-file-descriptor-stored-in-a-variable-is-still-valid
tiran commentedon Nov 28, 2021
New changeset f87ea03 by Christian Heimes in branch 'main':
bpo-45915: use fcntl(fd, F_GETFD) in is_valid_fd() (GH-29821)
f87ea03
eryksun commentedon Nov 28, 2021
I've created bpo-45919 with a suggested enhancement to use GetFileType() in Windows, since the Windows C runtime does not provide fcntl().
tiran commentedon Nov 28, 2021
Thank you!
eryksun commentedon Dec 14, 2021
PR 29821 adds __APPLE__ to the platforms that use fcntl(fd, F_GETFD). Is this okay on macOS, given bpo-30225? Apparently fstat() fails if the other end of a pipe is closed.
kumaraditya303 commentedon May 9, 2022
Fixed by #29821