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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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readv(2) System Calls Manual readv(2)
readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write
data into multiple buffers
Standard C library (libc, -lc)
#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
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()
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)).
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
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 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.
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.
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.
EOPNOTSUPP
An unknown flag is specified in flags.
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.
readv()
writev()
POSIX.1-2008.
preadv()
pwritev()
BSD.
preadv2()
pwritev2()
Linux.
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 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 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.
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.
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.
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);
pread(2), read(2), write(2)
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version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-06-15 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), syscall(2), syscalls(2), write(2), iovec(3type), size_t(3type), io_uring(7), signal(7), socket(7), spufs(7), xfs_io(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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