readv(2) - Linux manual page (original) (raw)
readv(2) System Calls Manual readv(2)
NAME top
readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write
data into multiple buffersLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_SOURCEDESCRIPTION top
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)](../man3/iovec.3type.html).
The **readv**() system call works just like [read(2)](../man2/read.2.html) except that
multiple buffers are filled.
The **writev**() system call works just like [write(2)](../man2/write.2.html) 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;
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)](../man2/open.2.html)).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)](../man2/pwrite.2.html). 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
following 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](../man3/errno.3.html)_ 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 applies 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.
**RWF_NOAPPEND** (since Linux 6.9)
Do not honor the **O_APPEND open**(2) flag. This flag is
meaningful only for **pwritev2**(). Historically, Linux
honored **O_APPEND** flag if set and ignored the offset
argument, which is a bug. For **pwritev2**(), the _offset_
argument is honored as expected if **RWF_NOAPPEND** flag is
set, the same as if **O_APPEND** flag were not set.
**RWF_ATOMIC** (since Linux 6.11)
Requires that writes to regular files in block-based
filesystems be issued with torn-write protection. Torn-
write protection means that for a power or any other
hardware failure, all or none of the data from the write
will be stored, but never a mix of old and new data. This
flag is meaningful only for **pwritev2**(), and its effect
applies only to the data range written by the system call.
The total write length must be power-of-2 and must be sized
in the range [_stxatomicwriteunitmin_,
_stxatomicwriteunitmax_]. The write must be at a
naturally-aligned offset within the file with respect to
the total write length. For example, a write of length
32KiB at a file offset of 32KiB is permitted, however a
write of length 32KiB at a file offset of 48KiB is not
permitted. The upper limit of _iovcnt_ for **pwritev2**() is
given by the value in _stxatomicwritesegmentsmax._ Torn-
write protection only works with **O_DIRECT** flag, i.e.
buffered writes are not supported. To guarantee
consistency from the write between a file's in-core state
with the storage device, **O_SYNC** or **O_DSYNC** must be
specified for [open(2)](../man2/open.2.html). The same synchronized I/O
guarantees as described in [open(2)](../man2/open.2.html) are provided when these
flags or their equivalent flags and system calls are used
(e.g., if **RWF_SYNC** is specified for **pwritev2**()).RETURN VALUE top
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)](../man2/read.2.html) and [write(2)](../man2/write.2.html)).
On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
The errors are as given for [read(2)](../man2/read.2.html) and [write(2)](../man2/write.2.html). Furthermore,
**preadv**(), **preadv2**(), **pwritev**(), and **pwritev2**() can also fail for
the same reasons as [lseek(2)](../man2/lseek.2.html). Additionally, the following errors
are defined:
**EINVAL** The sum of the _iovlen_ values overflows an _ssizet_ value.
**EINVAL** If **RWF_ATOMIC** is specified, the combination of the sum of
the _iovlen_ values and the _offset_ value does not comply
with the length and offset torn-write protection rules.
**EINVAL** The vector count, _iovcnt_, is less than zero or greater than
the permitted maximum. If **RWF_ATOMIC** is specified, this
maximum is given by the _stxatomicwritesegmentsmax_ value
from _statx._
**EOPNOTSUPP**
An unknown flag is specified in _flags_.VERSIONS top
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** _posl_**, unsigned long** _pos_
These arguments contain, respectively, the low order and high
order 32 bits of _offset_.STANDARDS top
**readv**()
**writev**()
POSIX.1-2008.
**preadv**()
**pwritev**()
BSD.
**preadv2**()
**pwritev2**()
Linux.HISTORY top
**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 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 iovbase 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 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 top
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(SCIOVMAX)_. On modern Linux systems,
the limit is 1024. Back in Linux 2.0 days, this limit was 16.BUGS top
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 top
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 top
[pread(2)](../man2/pread.2.html), [read(2)](../man2/read.2.html), [write(2)](../man2/write.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-04 readv(2)
Pages that refer to this page:strace(1), fcntl(2), fsync(2), io_submit(2), io_uring_enter2(2), io_uring_enter(2), io_uring_setup(2), pread(2), process_vm_readv(2), read(2), recv(2), send(2), statx(2), syscall(2), syscalls(2), write(2), iovec(3type), size_t(3type), io_uring(7), signal(7), socket(7), spufs(7), xfs_io(8)