readv, writev - 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);
DESCRIPTION
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; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
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)).
RETURN VALUE
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.
ERRORS
The errors are as given for
read(2)
and
write(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.
CONFORMING TO
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 argument,
and int as return type for these functions.
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 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.
EXAMPLE
The following code sample demonstrates the use of
writev():
This page is part of release 3.14 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/.
