Differential D20867 Diff 59481 head/lib/libc/sys/read.2

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head/lib/libc/sys/read.2

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	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT			.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY			.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF			.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	.\" SUCH DAMAGE.			.\" SUCH DAMAGE.
	.\"			.\"
	.\" @(#)read.2 8.4 (Berkeley) 2/26/94			.\" @(#)read.2 8.4 (Berkeley) 2/26/94
	.\" $FreeBSD$			.\" $FreeBSD$
	.\"			.\"
	.Dd December 1, 2017			.Dd July 6, 2019
	.Dt READ 2			.Dt READ 2
	.Os			.Os
	.Sh NAME			.Sh NAME
	.Nm read ,			.Nm read ,
	.Nm readv ,			.Nm readv ,
	.Nm pread ,			.Nm pread ,
	.Nm preadv			.Nm preadv
	.Nd read input			.Nd read input
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	.Fn readv ,			.Fn readv ,
	.Fn pread			.Fn pread
	and			and
	.Fn preadv			.Fn preadv
	return the number of bytes actually read and placed in the buffer.			return the number of bytes actually read and placed in the buffer.
	The system guarantees to read the number of bytes requested if			The system guarantees to read the number of bytes requested if
	the descriptor references a normal file that has that many bytes left			the descriptor references a normal file that has that many bytes left
	before the end-of-file, but in no other case.			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			.Sh RETURN VALUES
	If successful, the			If successful, the
	number of bytes actually read is returned.			number of bytes actually read is returned.
	Upon reading end-of-file,			Upon reading end-of-file,
	zero is returned.			zero is returned.
	Otherwise, a -1 is returned and the global variable			Otherwise, a -1 is returned and the global variable
	.Va errno			.Va errno
	is set to indicate the error.			is set to indicate the error.
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