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:
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
parameter, 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);
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