man-pages/man2/readv.2

.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\" Modified Sat Jul 24 18:34:44 1993 by Rik Faith (faith@cs.unc.edu)
.\" Merged readv.[23], 2002-10-17, aeb
.\"
.TH READV 2  2002-10-17 "" "Linux Programmer's Manual"
.SH NAME
readv, writev \- read or write data into multiple buffers
.SH SYNOPSIS
.nf
.B #include <sys/uio.h>
.sp
.BI "ssize_t readv(int " fd ", const struct iovec *" vector ", int " count );
.sp
.BI "ssize_t writev(int " fd ", const struct iovec *" vector ", int " count );
.fi
.SH DESCRIPTION
The
.BR readv ()
function reads
.I count
blocks from the file associated with the file descriptor
.I fd
into the multiple buffers described by
.IR vector .
.PP
The
.BR writev ()
function writes at most
.I count
blocks described by
.I vector
to the file associated with the file descriptor
.IR fd .
.PP
The pointer
.I vector
points to a
.I struct iovec
defined in
.I <sys/uio.h>
as
.PP
.br
.in +0.25in
.nf
struct iovec {
    void *iov_base;   /* Starting address */
    size_t iov_len;   /* Number of bytes */
};
.fi
.in 0.25in
.PP
Buffers are processed in the order specified.
.PP
The
.BR readv ()
function works just like
.BR read (2)
except that multiple buffers are filled.
.PP
The
.BR writev ()
function works just like
.BR write (2)
except that multiple buffers are written out.
.SH "RETURN VALUE"
On success, the
.BR readv ()
function returns the number of bytes read; the
.BR writev ()
function returns the number of bytes written.
On error, \-1 is returned, and \fIerrno\fP is set appropriately.
.SH ERRORS
The errors are as given for
.BR read (2)
and
.BR write (2).
Additionally the following error is defined:
.TP
.B EINVAL
The sum of the
.I iov_len
values overflows an
.I ssize_t
value.
Or, the vector count \fIcount\fR is less than zero or greater than the
permitted maximum.
.SH "CONFORMING TO"
4.4BSD (the
.BR readv ()
and
.BR writev ()
functions first appeared in 4.2BSD), POSIX.1-2001.
Linux libc5 used \fIsize_t\fR as the type of the \fIcount\fR parameter,
and \fIint\fP as return type for these functions.
.\" The readv/writev system calls were buggy before Linux 1.3.40.
.\" (Says release.libc.)
.SH "LINUX NOTES"
POSIX.1-2001 allows an implementation to place a limit on
the number of items that can be passed in
.IR vector .
An implementation can advertise its limit by defining
.B IOV_MAX
in
.IR <limits.h>
or at run time via the return value from
.IR 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
.BR readv ()
the wrapper function allocates a temporary buffer large enough
for all of the items specified by
.IR vector ,
passes that buffer in a call to
.BR read (),
copies data from the buffer to the locations specified by the
.I iov_base
fields of the elements of
.IR vector ,
and then frees the buffer.
The wrapper function for
.BR writev ()
performs the analogous task using a temporary buffer and a call to
.BR write ().
.SH BUGS
It is not advisable to mix calls to functions like
.BR readv ()
or
.BR writev (),
which operate on file descriptors, with the functions from the stdio
library; the results will be undefined and probably not what you want.
.SH "SEE ALSO"
.BR read (2),
.BR write (2)