Author: kib Date: Sat Jul 6 20:31:37 2019 New Revision: 349794 URL: https://svnweb.freebsd.org/changeset/base/349794
Log: Document atomicity for read(2) and write(2). Take part of the text from POSIX 2018 edition and describe the atomicity requirements for read and write syscalls. See p1003.1-2018, Vol.2, 2.9.7 Threads interaction with Regular File Operations. Reviewed by: asomers Sponsored by: The FreeBSD Foundation MFC after: 3 days Differential revision: https://reviews.freebsd.org/D20867 Modified: head/lib/libc/sys/read.2 Modified: head/lib/libc/sys/read.2 ============================================================================== --- head/lib/libc/sys/read.2 Sat Jul 6 18:02:29 2019 (r349793) +++ head/lib/libc/sys/read.2 Sat Jul 6 20:31:37 2019 (r349794) @@ -28,7 +28,7 @@ .\" @(#)read.2 8.4 (Berkeley) 2/26/94 .\" $FreeBSD$ .\" -.Dd December 1, 2017 +.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. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
