readv.2: Document preadv2() and pwritev2()

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/readv.2

@@ -45,6 +45,12 @@ readv, writev, preadv, pwritev \- read or write data into multiple buffers
 .sp
 .BI "ssize_t pwritev(int " fd ", const struct iovec *" iov ", int " iovcnt ,
 .BI "                off_t " offset );
+.sp
+.BI "ssize_t preadv2(int " fd ", const struct iovec *" iov ", int " iovcnt ,
+.BI "                off_t " offset ", int " flags );
+.sp
+.BI "ssize_t pwritev2(int " fd ", const struct iovec *" iov ", int " iovcnt ,
+.BI "                 off_t " offset ", int " flags );
 .fi
 .sp
 .in -4n
@@ -166,9 +172,9 @@ The
 system call combines the functionality of
 .BR writev ()
 and
-.BR pwrite (2).
+.BR pwrite (2) "."
 It performs the same task as
-.BR writev (),
+.BR writev () ","
 but adds a fourth argument,
 .IR offset ,
 which specifies the file offset at which the output operation
@@ -178,15 +184,43 @@ The file offset is not changed by these system calls.
 The file referred to by
 .I fd
 must be capable of seeking.
-.SH RETURN VALUE
-On success,
-.BR readv ()
-and
+.SS preadv2() and pwritev2()
+
+This pair of system calls has similar functionality to the
 .BR preadv ()
-return the number of bytes read;
-.BR writev ()
 and
 .BR pwritev ()
+calls, but adds a fifth argument, \fIflags\fP, which modifies the behavior on a per call basis.
+
+Like the
+.BR preadv ()
+and
+.BR pwritev ()
+calls, they accept an \fIoffset\fP argument. Unlike those calls, if the \fIoffset\fP argument is set to -1 then the current file offset is used and updated.
+
+The \fIflags\fP arguments to
+.BR preadv2 ()
+and
+.BR pwritev2 ()
+contains a bitwise OR of one or more of the following flags:
+.TP
+.BR RWF_HIPRI " (since Linux 4.6)"
+High priority read/write.  Allows block based filesystems to use polling of the
+device, which provides lower latency, but may use additional ressources.  (Currently
+only usable on a file descriptor opened using the
+.BR O_DIRECT " flag)."
+
+.SH RETURN VALUE
+On success,
+.BR readv () ","
+.BR preadv ()
+and
+.BR preadv2 ()
+return the number of bytes read;
+.BR writev () ","
+.BR pwritev ()
+and
+.BR pwritev2 ()
 return the number of bytes written.
 
 Note that is not an error for a successful call to transfer fewer bytes
@@ -202,9 +236,11 @@ The errors are as given for
 and
 .BR write (2).
 Furthermore,
-.BR preadv ()
-and
+.BR preadv () ","
+.BR preadv2 () ","
 .BR pwritev ()
+and
+.BR pwritev2 ()
 can also fail for the same reasons as
 .BR lseek (2).
 Additionally, the following error is defined:
@@ -218,12 +254,17 @@ value.
 .TP
 .B EINVAL
 The vector count \fIiovcnt\fP is less than zero or greater than the
-permitted maximum.
+permitted maximum. Or, an unknown flag is specified in \fIflags\fP.
 .SH VERSIONS
 .BR preadv ()
 and
 .BR pwritev ()
 first appeared in Linux 2.6.30; library support was added in glibc 2.10.
+.sp
+.BR preadv2 ()
+and
+.BR pwritev2 ()
+first appeared in Linux 4.6
 .SH CONFORMING TO
 .BR readv (),
 .BR writev ():
@@ -237,6 +278,10 @@ POSIX.1-2001, POSIX.1-2008,
 .BR preadv (),
 .BR pwritev ():
 nonstandard, but present also on the modern BSDs.
+.sp
+.BR preadv2 (),
+.BR pwritev2 ():
+nonstandard, Linux extension.
 .SH NOTES
 POSIX.1 allows an implementation to place a limit on
 the number of items that can be passed in