READV(2) Linux Programmer's Manual READV(2)
NAME
readv, writev, preadv, pwritev, preadv2, pwritev2 - 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);
ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
off_t offset, int flags);
ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
off_t offset, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
preadv(), pwritev():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_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() com-
pletely 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.
preadv2() and pwritev2()
These system calls are similar to preadv() and pwritev() calls, but add
a fifth argument, flags, which modifies the behavior on a per-call
basis.
Unlike preadv() and pwritev(), if the offset argument is -1, then the
current file offset is used and updated.
The flags argument contains a bitwise OR of zero or more of the follow-
ing flags:
RWF_DSYNC (since Linux 4.7)
Provide a per-write equivalent of the O_DSYNC open(2) flag.
This flag is meaningful only for pwritev2(), and its effect
applies only to the data range written by the system call.
RWF_HIPRI (since Linux 4.6)
High priority read/write. Allows block-based filesystems to use
polling of the device, which provides lower latency, but may use
additional resources. (Currently, this feature is usable only
on a file descriptor opened using the O_DIRECT flag.)
RWF_SYNC (since Linux 4.7)
Provide a per-write equivalent of the O_SYNC open(2) flag. This
flag is meaningful only for pwritev2(), and its effect applies
only to the data range written by the system call.
RWF_NOWAIT (since Linux 4.14)
Do not wait for data which is not immediately available. If
this flag is specified, the preadv2() system call will return
instantly if it would have to read data from the backing storage
or wait for a lock. If some data was successfully read, it will
return the number of bytes read. If no bytes were read, it will
return -1 and set errno to EAGAIN. Currently, this flag is
meaningful only for preadv2().
RETURN VALUE
On success, readv(), preadv() and preadv2() return the number of bytes
read; writev(), pwritev() and pwritev2() return the number of bytes
written.
Note that it is not an error for a successful call to transfer fewer
bytes than requested (see read(2) and write(2)).
On error, -1 is returned, and errno is set appropriately.
ERRORS
The errors are as given for read(2) and write(2). Furthermore,
preadv(), preadv2(), pwritev(), and pwritev2() can also fail for the
same reasons as lseek(2). Additionally, the following errors are
defined:
EINVAL The sum of the iov_len values overflows an ssize_t value.
EINVAL The vector count, iovcnt, is less than zero or greater than the
permitted maximum.
EINVAL An unknown flag is specified in flags.
VERSIONS
preadv() and pwritev() first appeared in Linux 2.6.30; library support
was added in glibc 2.10.
preadv2() and pwritev2() first appeared in Linux 4.6. Library support
was added in glibc 2.26.
CONFORMING TO
readv(), writev(): POSIX.1-2001, POSIX.1-2008, 4.4BSD (these system
calls first appeared in 4.2BSD).
preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
preadv2(), pwritev2(): nonstandard Linux extension.
NOTES
POSIX.1 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 modern Linux systems, the limit is
1024. Back in Linux 2.0 days, this limit was 16.
C library/kernel differences
The raw preadv() and pwritev() system calls have call signatures that
differ slightly from that of the corresponding GNU C library wrapper
functions shown in the SYNOPSIS. The final argument, offset, is
unpacked by the wrapper functions into two arguments in the system
calls:
unsigned long pos_l, unsigned long pos
These arguments contain, respectively, the low order and high order 32
bits of offset.
Historical C library/kernel differences
To deal with the fact that IOV_MAX was so low on early versions of
Linux, the glibc wrapper functions for readv() and writev() did some
extra work if they detected that the underlying kernel system call
failed because this limit was exceeded. In the case of readv(), the
wrapper function allocated a temporary buffer large enough for all of
the items specified by iov, passed that buffer in a call to read(2),
copied data from the buffer to the locations specified by the iov_base
fields of the elements of iov, and then freed the buffer. The wrapper
function for writev() performed the analogous task using a temporary
buffer and a call to write(2).
The need for this extra effort in the glibc wrapper functions went away
with Linux 2.2 and later. However, glibc continued to provide this
behavior until version 2.10. Starting with glibc version 2.9, the
wrapper functions provide this behavior only if the library detects
that the system is running a Linux kernel older than version 2.6.18 (an
arbitrarily selected kernel version). And since glibc 2.20 (which
requires a minimum Linux kernel version of 2.6.32), the glibc wrapper
functions always just directly invoke the system calls.
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 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 READV(2)
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