Date: Sun, 26 Oct 2025 17:36:13 GMT From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 1c8d7bb4cf7a - main - write.2: explain the atomicity guarantees of the writes Message-ID: <202510261736.59QHaDik079513@gitrepo.freebsd.org>
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The branch main has been updated by kib: URL: https://cgit.FreeBSD.org/src/commit/?id=1c8d7bb4cf7a6b19262333b2a9b1dacf3094b938 commit 1c8d7bb4cf7a6b19262333b2a9b1dacf3094b938 Author: Konstantin Belousov <kib@FreeBSD.org> AuthorDate: 2025-10-26 14:57:16 +0000 Commit: Konstantin Belousov <kib@FreeBSD.org> CommitDate: 2025-10-26 17:35:31 +0000 write.2: explain the atomicity guarantees of the writes Also provide the pointer to the latest POSIX standard that justifies the requirement. Reviewed by: emaste Discussed with: rmacklem Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D53361 --- lib/libsys/write.2 | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/lib/libsys/write.2 b/lib/libsys/write.2 index d2ff41ceead9..77067893ab4c 100644 --- a/lib/libsys/write.2 +++ b/lib/libsys/write.2 @@ -132,6 +132,26 @@ and may write fewer bytes than requested; the return value must be noted, and the remainder of the operation should be retried when possible. +.Sh ATOMICITY OF WRITES +When operating on regular files on local file systems, the effects of +.Fn write +are atomic. +As required by the POSIX standard, +the +.Fn read , +.Fn write , +and +.Fn ftruncate +functions and their variations are atomic with respect to +each other on the file data and metadata for regular files. +See for instance +.St -p1003.1-2024 +Volume 2, Section 2.9.7 for more information. +.Pp +.Fx +implements the requirement by taking +a read/write range lock on the file byte range +affected by the corresponding function. .Sh RETURN VALUES Upon successful completion the number of bytes which were written is returned.home | help
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