READV(2) Linux Programmer's Manual READV(2)NAME
readv, writev - read or write data into multiple buffers
SYNOPSIS
#include <sys/uio.h>
ssize_t readv(int fd, const struct iovec *vector, int count);
ssize_t writev(int fd, const struct iovec *vector, int count);
DESCRIPTION
The readv() function reads count blocks from the file associated with
the file descriptor fd into the multiple buffers described by vector.
The writev() function writes at most count blocks described by vector
to the file associated with the file descriptor fd.
The pointer vector points to a struct iovec defined in <sys/uio.h> as
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes */
};
Buffers are processed in the order specified.
The readv() function works just like read(2) except that multiple buf‐
fers are filled.
The writev() function works just like write(2) except that multiple
buffers are written out.
RETURN VALUE
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.
ERRORS
The errors are as given for read(2) and write(2). Additionally the
following error is defined:
EINVAL The sum of the iov_len values overflows an ssize_t value. Or,
the vector count count is less than zero or greater than the
permitted maximum.
CONFORMING TO
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 count parame‐
ter, and int as return type for these functions.
LINUX NOTES
POSIX.1-2001 allows an implementation to place a limit on the number of
items that can be passed in vector. 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 buf‐
fer large enough for all of the items specified by vector, passes that
buffer in a call to read(), copies data from the buffer to the loca‐
tions specified by the iov_base fields of the elements of vector, and
then frees the buffer. The wrapper function for writev() performs the
analogous task using a temporary buffer and a call to write().
BUGS
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.
SEE ALSOread(2), write(2)
2002-10-17 READV(2)
