readv, writev — read or write data into multiple buffers
#include <sys/uio.h>
ssize_t readv( |
int | fd, |
| const struct iovec * | iov, | |
| int | iovcnt); |
ssize_t writev( |
int | fd, |
| const struct iovec * | iov, | |
| int | iovcnt); |
The readv() function reads
iovcnt buffers from
the file associated with the file descriptor fd into the buffers described
by iov ("scatter
input").
The writev() function writes
iovcnt buffers of
data described by iov
to the file associated with the file descriptor fd ("gather output").
The pointer iov
points to an array of iovec
structures, defined in <sys/uio.h>
as:
struct iovec { void * iov_base;size_t iov_len;};
The readv() function works
just like read(2) except that
multiple buffers are filled.
The writev() function works
just like write(2) except that
multiple buffers are written out.
Buffers are processed in array order. This means that
readv() completely fills
iov[0] before
proceeding to iov[1],
and so on. (If there is insufficient data, then not all
buffers pointed to by iov may be filled.) Similarly,
writev() writes out the entire
contents of iov[0]
before proceeding to iov[1], and so on.
The data transfers performed by readv() and writev() are atomic: the data written by
writev() is written as a single
block that is not intermingled with output from writes in
other processes (but see pipe(7) for an exception);
analogously, 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 open(2)).
On success, the readv()
function returns the number of bytes read; the writev() function returns the number of
bytes written. On error, −1 is returned, and
errno is set appropriately.
The errors are as given for read(2) and write(2). Additionally the following error is defined:
The sum of the iov_len values overflows
an ssize_t value. Or, the
vector count iovcnt is less than zero
or greater than the permitted maximum.
4.4BSD (the readv() and
writev() functions first
appeared in 4.2BSD), POSIX.1-2001. Linux libc5 used
size_t as the type of the
iovcnt parameter, and
int as return type for these
functions.
POSIX.1-2001 allows an implementation to place a limit
on the number of items that can be passed in iov. An implementation can
advertise its limit by defining IOV_MAX in <limits.h>
or at run time via the return value from sysconf(_SC_IOV_MAX). On
Linux, the limit advertised by these mechanisms is 1024,
which is the true kernel limit. However, the glibc wrapper
functions do some extra work if they detect that the
underlying kernel system call failed because this limit was
exceeded. In the case of readv() the wrapper function allocates a
temporary buffer large enough for all of the items
specified by iov,
passes that buffer in a call to read(2), copies data from
the buffer to the locations specified by the iov_base fields of the
elements of iov,
and then frees the buffer. The wrapper function for
writev() performs the
analogous task using a temporary buffer and a call to
write(2).
It is not advisable to mix calls to functions like
readv() or writev(), which operate on file
descriptors, with the functions from the stdio library; the
results will be undefined and probably not what you want.
The following code sample demonstrates the use of
writev():
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);
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com> and (c) 1993 by Thomas Koenig (ig25rz.uni-karlsruhe.de) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. License. Modified Sat Jul 24 18:34:44 1993 by Rik Faith (faithcs.unc.edu) Merged readv.[23], 2002-10-17, aeb 2007-04-30 mtk, A fairly major rewrite to fix errors and add more details. |