Name

readv, writev, preadv, pwritev — read or write data into multiple buffers

Synopsis

#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);
 
ssize_t preadv( int fd,
  const struct iovec *iov,
  int iovcnt,
  off_t offset);
 
ssize_t pwritev( int fd,
  const struct iovec *iov,
  int iovcnt,
  off_t offset);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
preadv(), pwritev():
_BSD_SOURCE

DESCRIPTION

The readv() system call reads iovcnt buffers from the file associated with the file descriptor fd into the buffers described by iov ("scatter input").

The writev() system call 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;
/* Starting address */
  size_t   iov_len;
/* Number of bytes to transfer */
};

The readv() system call works just like read(2) except that multiple buffers are filled.

The writev() system call 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)).

preadv() and pwritev()

The preadv() system call combines the functionality of readv() and pread(2). It performs the same task as readv(), but adds a fourth argument, offset, which specifies the file offset at which the input operation is to be performed.

The pwritev() system call combines the functionality of writev() and pwrite(2). It performs the same task as writev(), but adds a fourth argument, offset, which specifies the file offset at which the output operation is to be performed.

The file offset is not changed by these system calls. The file referred to by fd must be capable of seeking.

RETURN VALUE

On success, readv() and preadv() return the number of bytes read; writev() and pwritev() return the number of bytes written. On error, −1 is returned, and errno is set appropriately.

ERRORS

The errors are as given for read(2) and write(2). Furthermore, preadv() and pwritev() can also fail for the same reasons as lseek(2). Additionally, the following error is defined:

EINVAL

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.

VERSIONS

preadv() and pwritev() first appeared in Linux 2.6.30; library support was added in glibc 2.10.

CONFORMING TO

readv(), writev(): 4.4BSD (these system calls first appeared in 4.2BSD), POSIX.1-2001. Linux libc5 used size_t as the type of the iovcnt argument, and int as the return type.

preadv(), pwritev(): nonstandard, but present also on the modern BSDs.

NOTES

Linux notes

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).

BUGS

It is not advisable to mix calls to 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.

EXAMPLE

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);

SEE ALSO

pread(2), read(2), write(2)

COLOPHON

This page is part of release 3.52 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, 2010 Michael Kerrisk <mtk.manpagesgmail.com>
and Copyright (c) 1993 by Thomas Koenig (ig25rz.uni-karlsruhe.de)

%%%LICENSE_START(VERBATIM)
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_END

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.
2010-11-16, mtk, Added documentation of preadv() and pwritev()