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A fairly substantial rewrite, which among other things fixes the 

reported by Kyle Sluder in http://bugzilla.kernel.org/show_bug.cgi?id=8399
And added some example code.
This commit is contained in:
Michael Kerrisk 2007-04-30 15:32:25 +00:00
parent d6f223a7e3
commit ab44d3d652
1 changed files with 85 additions and 25 deletions
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110
man2/readv.2
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@ -1,4 +1,5 @@
.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\" Copyright (C) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
.\" and (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
@ -22,6 +23,8 @@
.\" License.
.\" Modified Sat Jul 24 18:34:44 1993 by Rik Faith (faith@cs.unc.edu)
.\" Merged readv.[23], 2002-10-17, aeb
.\" 2007-04-30 mtk, A fairly major rewrite to fix errors and
.\" add more details.
.\"
.TH READV 2 2002-10-17 "" "Linux Programmer's Manual"
.SH NAME
@ -30,49 +33,50 @@ readv, writev \- read or write data into multiple buffers
.nf
.B #include <sys/uio.h>
.sp
.BI "ssize_t readv(int " fd ", const struct iovec *" vector ", int " count );
.BI "ssize_t readv(int " fd ", const struct iovec *" iov ", int " iovcnt );
.sp
.BI "ssize_t writev(int " fd ", const struct iovec *" vector ", int " count );
.BI "ssize_t writev(int " fd ", const struct iovec *" iov ", int " iovcnt );
.fi
.SH DESCRIPTION
The
.BR readv ()
function reads
.I count
blocks from the file associated with the file descriptor
.I iovcnt
buffers from the file associated with the file descriptor
.I fd
into the multiple buffers described by
.IR vector .
into the buffers described by
.IR iov
("scatter input").
.PP
The
.BR writev ()
function writes at most
.I count
blocks described by
.I vector
function writes
.I iovcnt
buffers of data described by
.I iov
to the file associated with the file descriptor
.IR fd .
.IR fd
("gather output").
.PP
The pointer
.I vector
points to a
.I struct iovec
.I iov
points to an array of
.I iovec
structures,
defined in
.I <sys/uio.h>
as
as:
.PP
.br
.in +0.25in
.nf
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes */
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
.fi
.in 0.25in
.PP
Buffers are processed in the order specified.
.PP
The
.BR readv ()
function works just like
@ -84,6 +88,43 @@ The
function works just like
.BR write (2)
except that multiple buffers are written out.
.PP
Buffers are processed in array order.
This means that
.BR readv ()
completely fills
.IR iov [0]
before proceeding to
.IR iov [1],
and so on.
(If there is insufficient data, then not all buffers pointed to by
.I iov
may be filled.)
Similarly,
.BR writev ()
writes out the entire contents of
.IR iov [0]
before proceeding to
.IR iov [1],
and so on.
.PP
The data transfers performed by
.BR readv ()
and
.BR writev ()
are atomic: the data written by
.BR writev ()
is written as a single block that is not intermingled with output
from writes in other processes (but see
.BR pipe (7)
for an exception);
analogously,
.BR readv ()
is guaranteed to read a contiguous block of data from the file,
regardless of read operations performed in other threads or processes
that have file descriptors referring to the same open file description
(see
.BR open (2)).
.SH "RETURN VALUE"
On success, the
.BR readv ()
@ -104,7 +145,7 @@ The sum of the
values overflows an
.I ssize_t
value.
Or, the vector count \fIcount\fR is less than zero or greater than the
Or, the vector count \fIiovcnt\fR is less than zero or greater than the
permitted maximum.
.SH "CONFORMING TO"
4.4BSD (the
@ -112,14 +153,14 @@ permitted maximum.
and
.BR writev ()
functions first appeared in 4.2BSD), POSIX.1-2001.
Linux libc5 used \fIsize_t\fR as the type of the \fIcount\fR parameter,
Linux libc5 used \fIsize_t\fR as the type of the \fIiovcnt\fR parameter,
and \fIint\fP as return type for these functions.
.\" The readv/writev system calls were buggy before Linux 1.3.40.
.\" (Says release.libc.)
.SH "LINUX NOTES"
POSIX.1-2001 allows an implementation to place a limit on
the number of items that can be passed in
.IR vector .
.IR iov .
An implementation can advertise its limit by defining
.B IOV_MAX
in
@ -135,13 +176,13 @@ In the case of
.BR readv ()
the wrapper function allocates a temporary buffer large enough
for all of the items specified by
.IR vector ,
.IR iov ,
passes that buffer in a call to
.BR read (),
copies data from the buffer to the locations specified by the
.I iov_base
fields of the elements of
.IR vector ,
.IR iov ,
and then frees the buffer.
The wrapper function for
.BR writev ()
@ -154,6 +195,25 @@ or
.BR writev (),
which operate on file descriptors, with the functions from the stdio
library; the results will be undefined and probably not what you want.
.SH EXAMPLE
The following code sample demonstrates the use of
.BR writev ():
.in +0.5i
.nf
char *str0 = "hello ";
char *str1 = "world\\n";
struct iovec iov[2];
ssize_t nwritten;
iov[0].iov_base = str0;
iov[0].iov_len = strlen(str0);
iov[1].iov_base = str1;
iov[1].iov_len = strlen(str1);
nwritten = writev(STDOUT_FILENO, iov, 2);
.fi
.in
.SH "SEE ALSO"
.BR read (2),
.BR write (2)