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readv, writev - read or write data into multiple buffers




#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);


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 buffers are filled.

The writev() function works just like write(2) except that multiple buffers are written out.


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. -->
EINVAL The sum of the iov_len values overflows an ssize_t value. Or, the vector count count is zero or greater than MAX_IOVEC.


4.4BSD (the readv and writev functions first appeared in BSD 4.2), Unix98, POSIX 1003.1-2001. Linux libc5 used size_t as the type of the count parameter, and int as return type for these functions.


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.


read(2), write(2)

Created by Blin Media, 2008-2013