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gh-93180: Update documentation of os.copy_file_range #93182

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Merged
merged 10 commits into from
Jun 8, 2022
Merged

gh-93180: Update documentation of os.copy_file_range #93182

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c57e76f
Update docs about an old constraint of `os.copy_file_range`
illia-v May 24, 2022
23bbe3a
Add a note about a known issue of `os.copy_file_range`
illia-v May 24, 2022
1e38126
Document some of optimizations `os.copy_file_range` can use implicitly
illia-v May 24, 2022
caeb624
Add a news entry
illia-v May 24, 2022
59561bf
Update the note
illia-v May 30, 2022
bcb339d
Merge branch 'main' into fix-issue-93180
illia-v May 30, 2022
069651d
Explain what copying as if files are opened as binary means
illia-v Jun 6, 2022
f6152e4
Merge branch 'main' into fix-issue-93180
illia-v Jun 7, 2022
94ef9cc
Remove the news entry
illia-v Jun 7, 2022
7ab57e1
Apply Victor's suggestions
illia-v Jun 7, 2022
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20 changes: 17 additions & 3 deletions Doc/library/os.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -798,18 +798,32 @@ as internal buffering of data.
Copy *count* bytes from file descriptor *src*, starting from offset
*offset_src*, to file descriptor *dst*, starting from offset *offset_dst*.
If *offset_src* is None, then *src* is read from the current position;
respectively for *offset_dst*. The files pointed by *src* and *dst*
respectively for *offset_dst*.

In Linux kernel older than 5.3, the files pointed by *src* and *dst*
must reside in the same filesystem, otherwise an :exc:`OSError` is
raised with :attr:`~OSError.errno` set to :data:`errno.EXDEV`.

This copy is done without the additional cost of transferring data
from the kernel to user space and then back into the kernel. Additionally,
some filesystems could implement extra optimizations. The copy is done as if
both files are opened as binary.
some filesystems could implement extra optimizations, such as the use of
reflinks (i.e., two or more inodes that share pointers to the same
copy-on-write disk blocks; supported file systems include btrfs and XFS)
and server-side copy (in the case of NFS).

The function copies bytes between two file descriptors. Text options, like
the encoding and the line ending, are ignored.

The return value is the amount of bytes copied. This could be less than the
amount requested.

.. note::

On Linux, :func:`os.copy_file_range` should not be used for copying a
range of a pseudo file from a special filesystem like procfs and sysfs.
It will always copy no bytes and return 0 as if the file was empty
because of a known Linux kernel issue.

.. availability:: Linux kernel >= 4.5 or glibc >= 2.27.

.. versionadded:: 3.8
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