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READV(2) Linux Programmer's Manual READV(2)
readv, writev, preadv, pwritev - 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);
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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
preadv(), pwritev(): _BSD_SOURCE
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)).
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.
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.
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.
preadv() and pwritev() first appeared in Linux 2.6.30; library support was
added in glibc 2.10.
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.
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 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);
pread(2), read(2), write(2)
This page is part of release 3.32 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/.
Linux 2010-11-17 READV(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface