NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON
READV(2) Linux Programmer's Manual READV(2)
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; /* 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)).
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:
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.
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.
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);
read(2), write(2)
This page is part of release 3.23 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 2002-10-17 READV(2)