Date: Tue, 9 Jul 2019 07:21:34 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-11@freebsd.org Subject: svn commit: r349855 - stable/11/lib/libc/sys Message-ID: <201907090721.x697LYE8044739@repo.freebsd.org>
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Author: kib Date: Tue Jul 9 07:21:33 2019 New Revision: 349855 URL: https://svnweb.freebsd.org/changeset/base/349855 Log: MFC r349794: Document atomicity for read(2) and write(2). Modified: stable/11/lib/libc/sys/read.2 Directory Properties: stable/11/ (props changed) Modified: stable/11/lib/libc/sys/read.2 ============================================================================== --- stable/11/lib/libc/sys/read.2 Tue Jul 9 07:19:36 2019 (r349854) +++ stable/11/lib/libc/sys/read.2 Tue Jul 9 07:21:33 2019 (r349855) @@ -28,7 +28,7 @@ .\" @(#)read.2 8.4 (Berkeley) 2/26/94 .\" $FreeBSD$ .\" -.Dd December 15, 2015 +.Dd July 6, 2019 .Dt READ 2 .Os .Sh NAME @@ -128,6 +128,25 @@ return the number of bytes actually read and placed in The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case. +.Pp +In accordance with +.St -p1003.1-2004 , +both +.Xr read 2 +and +.Xr write 2 +syscalls are atomic with respect to each other in the effects on file +content, when they operate on regular files. +If two threads each call one of the +.Xr read 2 +or +.Xr write 2 , +syscalls, each call will see either all of the changes of the other call, +or none of them. +The +.Fx +kernel implements this guarantee by locking the file ranges affected by +the calls. .Sh RETURN VALUES If successful, the number of bytes actually read is returned.
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