634 lines
13 KiB
HTML
634 lines
13 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of READV</TITLE>
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</HEAD><BODY>
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<H1>READV</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2018-04-30<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data into multiple buffers
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/sys/uio.h">sys/uio.h</A>></B>
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<B>ssize_t readv(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>);</B>
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<B>ssize_t writev(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>);</B>
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<B>ssize_t preadv(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>,</B>
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<B> off_t </B><I>offset</I><B>);</B>
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<B>ssize_t pwritev(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>,</B>
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<B> off_t </B><I>offset</I><B>);</B>
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<B>ssize_t preadv2(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>,</B>
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<B> off_t </B><I>offset</I><B>, int </B><I>flags</I><B>);</B>
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<B>ssize_t pwritev2(int </B><I>fd</I><B>, const struct iovec *</B><I>iov</I><B>, int </B><I>iovcnt</I><B>,</B>
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<B> off_t </B><I>offset</I><B>, int </B><I>flags</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>preadv</B>(),
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<B>pwritev</B>():
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<BR> Since glibc 2.19:
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<BR> _DEFAULT_SOURCE
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<BR> Glibc 2.19 and earlier:
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<BR> _BSD_SOURCE
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>readv</B>()
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system call reads
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<I>iovcnt</I>
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buffers from the file associated with the file descriptor
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<I>fd</I>
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into the buffers described by
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<I>iov</I>
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("scatter input").
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<P>
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The
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<B>writev</B>()
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system call writes
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<I>iovcnt</I>
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buffers of data described by
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<I>iov</I>
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to the file associated with the file descriptor
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<I>fd</I>
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("gather output").
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<P>
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The pointer
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<I>iov</I>
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points to an array of
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<I>iovec</I>
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structures,
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defined in
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<I><<A HREF="file:///usr/include/sys/uio.h">sys/uio.h</A>></I>
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as:
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<P>
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struct iovec {
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<BR> void *iov_base; /* Starting address */
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<BR> size_t iov_len; /* Number of bytes to transfer */
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};
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<P>
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The
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<B>readv</B>()
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system call works just like
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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except that multiple buffers are filled.
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<P>
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The
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<B>writev</B>()
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system call works just like
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<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2)
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except that multiple buffers are written out.
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<P>
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Buffers are processed in array order.
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This means that
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<B>readv</B>()
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completely fills
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<I>iov</I>[0]
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before proceeding to
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<I>iov</I>[1],
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and so on.
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(If there is insufficient data, then not all buffers pointed to by
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<I>iov</I>
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may be filled.)
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Similarly,
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<B>writev</B>()
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writes out the entire contents of
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<I>iov</I>[0]
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before proceeding to
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<I>iov</I>[1],
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and so on.
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<P>
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The data transfers performed by
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<B>readv</B>()
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and
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<B>writev</B>()
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are atomic: the data written by
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<B>writev</B>()
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is written as a single block that is not intermingled with output
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from writes in other processes (but see
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<B><A HREF="/cgi-bin/man/man2html?7+pipe">pipe</A></B>(7)
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for an exception);
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analogously,
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<B>readv</B>()
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is guaranteed to read a contiguous block of data from the file,
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regardless of read operations performed in other threads or processes
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that have file descriptors referring to the same open file description
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(see
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)).
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<A NAME="lbAE"> </A>
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<H3>preadv() and pwritev()</H3>
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The
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<B>preadv</B>()
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system call combines the functionality of
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<B>readv</B>()
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and
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<B><A HREF="/cgi-bin/man/man2html?2+pread">pread</A></B>(2).
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It performs the same task as
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<B>readv</B>(),
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but adds a fourth argument,
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<I>offset</I>,
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which specifies the file offset at which the input operation
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is to be performed.
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<P>
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The
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<B>pwritev</B>()
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system call combines the functionality of
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<B>writev</B>()
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and
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<B><A HREF="/cgi-bin/man/man2html?2+pwrite">pwrite</A></B>(2).
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It performs the same task as
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<B>writev</B>(),
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but adds a fourth argument,
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<I>offset</I>,
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which specifies the file offset at which the output operation
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is to be performed.
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<P>
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The file offset is not changed by these system calls.
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The file referred to by
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<I>fd</I>
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must be capable of seeking.
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<A NAME="lbAF"> </A>
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<H3>preadv2() and pwritev2()</H3>
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<P>
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These system calls are similar to
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<B>preadv</B>()
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and
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<B>pwritev</B>()
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calls, but add a fifth argument,
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<I>flags</I>,
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which modifies the behavior on a per-call basis.
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<P>
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Unlike
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<B>preadv</B>()
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and
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<B>pwritev</B>(),
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if the
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<I>offset</I>
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argument is -1, then the current file offset is used and updated.
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<P>
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The
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<I>flags</I>
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argument contains a bitwise OR of zero or more of the following flags:
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<DL COMPACT>
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<DT id="1"><B>RWF_DSYNC</B> (since Linux 4.7)
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<DD>
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Provide a per-write equivalent of the
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<B>O_DSYNC</B>
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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flag.
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This flag is meaningful only for
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<B>pwritev2</B>(),
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and its effect applies only to the data range written by the system call.
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<DT id="2"><B>RWF_HIPRI</B> (since Linux 4.6)
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<DD>
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High priority read/write.
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Allows block-based filesystems to use polling of the device,
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which provides lower latency, but may use additional resources.
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(Currently, this feature is usable only on a file descriptor opened using the
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<B>O_DIRECT</B>
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flag.)
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<DT id="3"><B>RWF_SYNC</B> (since Linux 4.7)
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<DD>
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Provide a per-write equivalent of the
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<B>O_SYNC</B>
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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flag.
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This flag is meaningful only for
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<B>pwritev2</B>(),
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and its effect applies only to the data range written by the system call.
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<DT id="4"><B>RWF_NOWAIT</B> (since Linux 4.14)
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<DD>
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Do not wait for data which is not immediately available.
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If this flag is specified, the
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<B>preadv2</B>()
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system call will return instantly if it would have to read data from
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the backing storage or wait for a lock.
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If some data was successfully read, it will return the number of bytes read.
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If no bytes were read, it will return -1 and set
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<I>errno</I>
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to
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<B>EAGAIN</B>.
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Currently, this flag is meaningful only for
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<B>preadv2</B>().
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<DT id="5"><B>RWF_APPEND</B> (since Linux 4.16)
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<DD>
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Provide a per-write equivalent of the
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<B>O_APPEND</B>
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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flag.
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This flag is meaningful only for
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<B>pwritev2</B>(),
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and its effect applies only to the data range written by the system call.
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The
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<I>offset</I>
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argument does not affect the write operation;
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the data is always appended to the end of the file.
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However, if the
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<I>offset</I>
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argument is -1, the current file offset is updated.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>RETURN VALUE</H2>
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On success,
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<B>readv</B>(),
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<B>preadv</B>()
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and
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<B>preadv2</B>()
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return the number of bytes read;
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<B>writev</B>(),
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<B>pwritev</B>()
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and
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<B>pwritev2</B>()
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return the number of bytes written.
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<P>
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Note that it is not an error for a successful call to transfer fewer bytes
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than requested (see
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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and
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<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2)).
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<P>
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On error, -1 is returned, and <I>errno</I> is set appropriately.
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<A NAME="lbAH"> </A>
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<H2>ERRORS</H2>
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The errors are as given for
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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and
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<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2).
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Furthermore,
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<B>preadv</B>(),
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<B>preadv2</B>(),
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<B>pwritev</B>(),
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and
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<B>pwritev2</B>()
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can also fail for the same reasons as
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<B><A HREF="/cgi-bin/man/man2html?2+lseek">lseek</A></B>(2).
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Additionally, the following errors are defined:
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<DL COMPACT>
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<DT id="6"><B>EINVAL</B>
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<DD>
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The sum of the
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<I>iov_len</I>
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values overflows an
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<I>ssize_t</I>
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value.
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<DT id="7"><B>EINVAL</B>
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<DD>
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The vector count,
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<I>iovcnt</I>,
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is less than zero or greater than the permitted maximum.
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<DT id="8"><B>EOPNOTSUPP</B>
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<DD>
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An unknown flag is specified in <I>flags</I>.
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</DL>
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<A NAME="lbAI"> </A>
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<H2>VERSIONS</H2>
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<B>preadv</B>()
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and
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<B>pwritev</B>()
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first appeared in Linux 2.6.30; library support was added in glibc 2.10.
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<P>
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<B>preadv2</B>()
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and
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<B>pwritev2</B>()
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first appeared in Linux 4.6.
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Library support was added in glibc 2.26.
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<A NAME="lbAJ"> </A>
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<H2>CONFORMING TO</H2>
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<B>readv</B>(),
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<B>writev</B>():
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POSIX.1-2001, POSIX.1-2008,
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4.4BSD (these system calls first appeared in 4.2BSD).
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<P>
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<B>preadv</B>(),
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<B>pwritev</B>():
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nonstandard, but present also on the modern BSDs.
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<P>
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<B>preadv2</B>(),
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<B>pwritev2</B>():
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nonstandard Linux extension.
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<A NAME="lbAK"> </A>
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<H2>NOTES</H2>
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POSIX.1 allows an implementation to place a limit on
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the number of items that can be passed in
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<I>iov</I>.
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An implementation can advertise its limit by defining
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<B>IOV_MAX</B>
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in
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<I><<A HREF="file:///usr/include/limits.h">limits.h</A>></I>
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or at run time via the return value from
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<I>sysconf(_SC_IOV_MAX)</I>.
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On modern Linux systems, the limit is 1024.
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Back in Linux 2.0 days, this limit was 16.
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<A NAME="lbAL"> </A>
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<H3>C library/kernel differences</H3>
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The raw
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<B>preadv</B>()
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and
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<B>pwritev</B>()
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system calls have call signatures that differ slightly from that of the
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corresponding GNU C library wrapper functions shown in the SYNOPSIS.
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The final argument,
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<I>offset</I>,
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is unpacked by the wrapper functions into two arguments in the system calls:
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<P>
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<B> unsigned long </B><I>pos_l</I><B>, unsigned long </B><I>pos</I>
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<P>
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These arguments contain, respectively, the low order and high order 32 bits of
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<I>offset</I>.
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<A NAME="lbAM"> </A>
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<H3>Historical C library/kernel differences</H3>
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To deal with the fact that
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<B>IOV_MAX</B>
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was so low on early versions of Linux,
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the glibc wrapper functions for
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<B>readv</B>()
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and
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<B>writev</B>()
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did some extra work if they detected that the underlying kernel
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system call failed because this limit was exceeded.
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In the case of
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<B>readv</B>(),
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the wrapper function allocated a temporary buffer large enough
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for all of the items specified by
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<I>iov</I>,
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passed that buffer in a call to
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2),
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copied data from the buffer to the locations specified by the
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<I>iov_base</I>
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fields of the elements of
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<I>iov</I>,
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and then freed the buffer.
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The wrapper function for
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<B>writev</B>()
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performed the analogous task using a temporary buffer and a call to
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<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2).
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<P>
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The need for this extra effort in the glibc wrapper functions
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went away with Linux 2.2 and later.
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However, glibc continued to provide this behavior until version 2.10.
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Starting with glibc version 2.9,
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the wrapper functions provide this behavior only if the library detects
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that the system is running a Linux kernel older than version 2.6.18
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(an arbitrarily selected kernel version).
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And since glibc 2.20
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(which requires a minimum Linux kernel version of 2.6.32),
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the glibc wrapper functions always just directly invoke the system calls.
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<A NAME="lbAN"> </A>
|
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<H2>EXAMPLE</H2>
|
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|
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The following code sample demonstrates the use of
|
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<B>writev</B>():
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<P>
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char *str0 = "hello ";
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char *str1 = "world\n";
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struct iovec iov[2];
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ssize_t nwritten;
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<P>
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iov[0].iov_base = str0;
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iov[0].iov_len = strlen(str0);
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|
iov[1].iov_base = str1;
|
|
iov[1].iov_len = strlen(str1);
|
|
<P>
|
|
nwritten = writev(STDOUT_FILENO, iov, 2);
|
|
|
|
|
|
<A NAME="lbAO"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+pread">pread</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2)
|
|
|
|
<A NAME="lbAP"> </A>
|
|
<H2>COLOPHON</H2>
|
|
|
|
This page is part of release 5.05 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
|
|
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="9"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="10"><A HREF="#lbAC">SYNOPSIS</A><DD>
|
|
<DT id="11"><A HREF="#lbAD">DESCRIPTION</A><DD>
|
|
<DL>
|
|
<DT id="12"><A HREF="#lbAE">preadv() and pwritev()</A><DD>
|
|
<DT id="13"><A HREF="#lbAF">preadv2() and pwritev2()</A><DD>
|
|
</DL>
|
|
<DT id="14"><A HREF="#lbAG">RETURN VALUE</A><DD>
|
|
<DT id="15"><A HREF="#lbAH">ERRORS</A><DD>
|
|
<DT id="16"><A HREF="#lbAI">VERSIONS</A><DD>
|
|
<DT id="17"><A HREF="#lbAJ">CONFORMING TO</A><DD>
|
|
<DT id="18"><A HREF="#lbAK">NOTES</A><DD>
|
|
<DL>
|
|
<DT id="19"><A HREF="#lbAL">C library/kernel differences</A><DD>
|
|
<DT id="20"><A HREF="#lbAM">Historical C library/kernel differences</A><DD>
|
|
</DL>
|
|
<DT id="21"><A HREF="#lbAN">EXAMPLE</A><DD>
|
|
<DT id="22"><A HREF="#lbAO">SEE ALSO</A><DD>
|
|
<DT id="23"><A HREF="#lbAP">COLOPHON</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:05:34 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|