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pread.html
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<!-- Creator : groff version 1.22.4 -->
<!-- CreationDate: Wed Jan 29 11:27:16 2020 -->
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta name="generator" content="groff -Thtml, see www.gnu.org">
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>PREAD</title>
</head>
<body>
<h1 align="center">PREAD</h1>
<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#RETURN VALUE">RETURN VALUE</a><br>
<a href="#ERRORS">ERRORS</a><br>
<a href="#VERSIONS">VERSIONS</a><br>
<a href="#CONFORMING TO">CONFORMING TO</a><br>
<a href="#NOTES">NOTES</a><br>
<a href="#BUGS">BUGS</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<a href="#COLOPHON">COLOPHON</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">pread, pwrite -
read from or write to a file descriptor at a given
offset</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>#include
<unistd.h></b></p>
<p style="margin-left:11%; margin-top: 1em"><b>ssize_t
pread(int</b> <i>fd</i><b>, void *</b><i>buf</i><b>,
size_t</b> <i>count</i><b>, off_t</b>
<i>offset</i><b>);</b></p>
<p style="margin-left:11%; margin-top: 1em"><b>ssize_t
pwrite(int</b> <i>fd</i><b>, const void *</b><i>buf</i><b>,
size_t</b> <i>count</i><b>, off_t</b>
<i>offset</i><b>);</b></p>
<p style="margin-left:5%; margin-top: 1em">Feature Test
Macro Requirements for glibc (see
<b>feature_test_macros</b>(7)):</p>
<p style="margin-left:11%; margin-top: 1em"><b>pread</b>(),
<b>pwrite</b>():</p>
<p style="margin-left:17%;">_XOPEN_SOURCE >= 500
<br>
|| /* Since glibc 2.12: */
_POSIX_C_SOURCE >= 200809L</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>pread</b>()
reads up to <i>count</i> bytes from file descriptor
<i>fd</i> at offset <i>offset</i> (from the start of the
file) into the buffer starting at <i>buf</i>. The file
offset is not changed.</p>
<p style="margin-left:11%; margin-top: 1em"><b>pwrite</b>()
writes up to <i>count</i> bytes from the buffer starting at
<i>buf</i> to the file descriptor <i>fd</i> at offset
<i>offset</i>. The file offset is not changed.</p>
<p style="margin-left:11%; margin-top: 1em">The file
referenced by <i>fd</i> must be capable of seeking.</p>
<h2>RETURN VALUE
<a name="RETURN VALUE"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">On success,
<b>pread</b>() returns the number of bytes read (a return of
zero indicates end of file) and <b>pwrite</b>() returns the
number of bytes written.</p>
<p style="margin-left:11%; margin-top: 1em">Note that it is
not an error for a successful call to transfer fewer bytes
than requested (see <b>read</b>(2) and <b>write</b>(2)).</p>
<p style="margin-left:11%; margin-top: 1em">On error, -1 is
returned and <i>errno</i> is set to indicate the cause of
the error.</p>
<h2>ERRORS
<a name="ERRORS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>pread</b>()
can fail and set <i>errno</i> to any error specified for
<b>read</b>(2) or <b>lseek</b>(2). <b>pwrite</b>() can fail
and set <i>errno</i> to any error specified for
<b>write</b>(2) or <b>lseek</b>(2).</p>
<h2>VERSIONS
<a name="VERSIONS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The
<b>pread</b>() and <b>pwrite</b>() system calls were added
to Linux in version 2.1.60; the entries in the i386 system
call table were added in 2.1.69. C library support
(including emulation using <b>lseek</b>(2) on older kernels
without the system calls) was added in glibc 2.1.</p>
<h2>CONFORMING TO
<a name="CONFORMING TO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">POSIX.1-2001,
POSIX.1-2008.</p>
<h2>NOTES
<a name="NOTES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The
<b>pread</b>() and <b>pwrite</b>() system calls are
especially useful in multithreaded applications. They allow
multiple threads to perform I/O on the same file descriptor
without being affected by changes to the file offset by
other threads.</p>
<p style="margin-left:11%; margin-top: 1em"><b>C
library/kernel differences</b> <br>
On Linux, the underlying system calls were renamed in kernel
2.6: <b>pread</b>() became <b>pread64</b>(), and
<b>pwrite</b>() became <b>pwrite64</b>(). The system call
numbers remained the same. The glibc <b>pread</b>() and
<b>pwrite</b>() wrapper functions transparently deal with
the change.</p>
<p style="margin-left:11%; margin-top: 1em">On some 32-bit
architectures, the calling signature for these system calls
differ, for the reasons described in <b>syscall</b>(2).</p>
<h2>BUGS
<a name="BUGS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">POSIX requires
that opening a file with the <b>O_APPEND</b> flag should
have no effect on the location at which <b>pwrite</b>()
writes data. However, on Linux, if a file is opened with
<b>O_APPEND</b>, <b>pwrite</b>() appends data to the end of
the file, regardless of the value of <i>offset</i>.</p>
<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>lseek</b>(2),
<b>read</b>(2), <b>readv</b>(2), <b>write</b>(2)</p>
<h2>COLOPHON
<a name="COLOPHON"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">This page is
part of release 5.02 of the Linux <i>man-pages</i> project.
A description of the project, information about reporting
bugs, and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.</p>
<hr>
</body>
</html>