D20867.id59479.diff
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	Index: lib/libc/sys/read.2
	===================================================================
	--- lib/libc/sys/read.2
	+++ lib/libc/sys/read.2
	@@ -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 @@
	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 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.