readv(2) System Calls Manual readv(2)
NAME
readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data
into multiple buffers
LIBRARY
Standard C library (libc, -lc)
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, described in
iovec(3type).
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 inter-
mingled with output from writes in other processes; analogously, readv()
is guaranteed to read a contiguous block of data from the file, regard-
less 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 argu-
ment, offset, which specifies the file offset at which the input opera-
tion 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 ar-
gument, offset, which specifies the file offset at which the output op-
eration 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 ba-
sis.
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 in-
stantly if it would have to read data from the backing storage or
wait for a lock. If some data was successfully read, it will re-
turn the number of bytes read. If no bytes were read, it will
return -1 and set errno to EAGAIN (but see BUGS). Currently,
this flag is meaningful only for preadv2().
RWF_APPEND (since Linux 4.16)
Provide a per-write equivalent of the O_APPEND open(2) flag.
This flag is meaningful only for pwritev2(), and its effect ap-
plies only to the data range written by the system call. The
offset argument does not affect the write operation; the data is
always appended to the end of the file. However, if the offset
argument is -1, the current file offset is updated.
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 to indicate the error.
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 de-
fined:
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.
EOPNOTSUPP
An unknown flag is specified in flags.
VERSIONS
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 un-
packed 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.
STANDARDS
readv()
writev()
POSIX.1-2008.
preadv()
pwritev()
BSD.
preadv2()
pwritev2()
Linux.
HISTORY
readv()
writev()
POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD).
preadv(), pwritev(): Linux 2.6.30, glibc 2.10.
preadv2(), pwritev2(): Linux 4.6, glibc 2.26.
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 ex-
tra 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 be-
havior until glibc 2.10. Starting with glibc 2.9, the wrapper functions
provide this behavior only if the library detects that the system is
running a Linux kernel older than Linux 2.6.18 (an arbitrarily selected
kernel version). And since glibc 2.20 (which requires a minimum of
Linux 2.6.32), the glibc wrapper functions always just directly invoke
the system calls.
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.
BUGS
Linux 5.9 and Linux 5.10 have a bug where preadv2() with the RWF_NOWAIT
flag may return 0 even when not at end of file.
EXAMPLES
The following code sample demonstrates the use of writev():
char *str0 = "hello ";
char *str1 = "world\n";
ssize_t nwritten;
struct iovec iov[2];
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)
Linux man-pages 6.9.1 2024-06-15 readv(2)
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