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Document atomicity for read(2) and write(2).

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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
This commit is contained in:
Konstantin Belousov 2019-07-06 20:31:37 +00:00
parent ec38f4f941
commit 8c95181495
1 changed files with 20 additions and 1 deletions
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21
lib/libc/sys/read.2
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@ -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 buffer.
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.