257 lines
9.3 KiB
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257 lines
9.3 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>recvmsg</title>
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<body bgcolor="white">
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<basefont size="3"> <a name="recvmsg"></a> <a name="tag_03_602"></a><!-- recvmsg -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_602_01"></a>NAME</h4>
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<blockquote>recvmsg - receive a message from a socket</blockquote>
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<h4><a name="tag_03_602_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/sys/socket.h.html">sys/socket.h</a>><br>
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<br>
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ssize_t recvmsg(int</tt> <i>socket</i><tt>, struct msghdr *</tt><i>message</i><tt>, int</tt> <i>flags</i><tt>);<br>
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</tt></code></p>
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</blockquote>
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<h4><a name="tag_03_602_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>recvmsg</i>() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally used
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with connectionless-mode sockets because it permits the application to retrieve the source address of received data.</p>
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<p>The <i>recvmsg</i>() function takes the following arguments:</p>
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<dl compact>
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<dt><i>socket</i></dt>
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<dd>Specifies the socket file descriptor.</dd>
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<dt><i>message</i></dt>
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<dd>Points to a <b>msghdr</b> structure, containing both the buffer to store the source address and the buffers for the incoming
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message. The length and format of the address depend on the address family of the socket. The <i>msg_flags</i> member is ignored on
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input, but may contain meaningful values on output.</dd>
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<dt><i>flags</i></dt>
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<dd>Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the following
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values:
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<dl compact>
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<dt>MSG_OOB</dt>
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<dd>Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.</dd>
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<dt>MSG_PEEK</dt>
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<dd>Peeks at the incoming message.</dd>
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<dt>MSG_WAITALL</dt>
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<dd>On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. The function may
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return the smaller amount of data if the socket is a message-based socket, if a signal is caught, if the connection is terminated,
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if MSG_PEEK was specified, or if an error is pending for the socket.</dd>
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</dl>
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</dd>
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</dl>
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<p>The <i>recvmsg</i>() function shall receive messages from unconnected or connected sockets and shall return the length of the
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message.</p>
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<p>The <i>recvmsg</i>() function shall return the total length of the message. For message-based sockets, such as SOCK_DGRAM and
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SOCK_SEQPACKET, the entire message shall be read in a single operation. If a message is too long to fit in the supplied buffers,
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and MSG_PEEK is not set in the <i>flags</i> argument, the excess bytes shall be discarded, and MSG_TRUNC shall be set in the
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<i>msg_flags</i> member of the <b>msghdr</b> structure. For stream-based sockets, such as SOCK_STREAM, message boundaries shall be
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ignored. In this case, data shall be returned to the user as soon as it becomes available, and no data shall be discarded.</p>
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<p>If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first message.</p>
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<p>If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, <i>recvmsg</i>() shall
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block until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor,
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the <i>recvmsg</i>() function shall fail and set <i>errno</i> to [EAGAIN] or [EWOULDBLOCK].</p>
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<p>In the <b>msghdr</b> structure, the <i>msg_name</i> and <i>msg_namelen</i> members specify the source address if the socket is
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unconnected. If the socket is connected, the <i>msg_name</i> and <i>msg_namelen</i> members shall be ignored. The <i>msg_name</i>
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member may be a null pointer if no names are desired or required. The <i>msg_iov</i> and <i>msg_iovlen</i> fields are used to
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specify where the received data shall be stored. <i>msg_iov</i> points to an array of <b>iovec</b> structures; <i>msg_iovlen</i>
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shall be set to the dimension of this array. In each <b>iovec</b> structure, the <i>iov_base</i> field specifies a storage area and
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the <i>iov_len</i> field gives its size in bytes. Each storage area indicated by <i>msg_iov</i> is filled with received data in
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turn until all of the received data is stored or all of the areas have been filled.</p>
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<p>Upon successful completion, the <i>msg_flags</i> member of the message header shall be the bitwise-inclusive OR of all of the
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following flags that indicate conditions detected for the received message:</p>
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<dl compact>
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<dt>MSG_EOR</dt>
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<dd>End-of-record was received (if supported by the protocol).</dd>
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<dt>MSG_OOB</dt>
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<dd>Out-of-band data was received.</dd>
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<dt>MSG_TRUNC</dt>
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<dd>Normal data was truncated.</dd>
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<dt>MSG_CTRUNC</dt>
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<dd>Control data was truncated.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_03_602_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>recvmsg</i>() shall return the length of the message in bytes. If no messages are available to be
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received and the peer has performed an orderly shutdown, <i>recvmsg</i>() shall return 0. Otherwise, -1 shall be returned and
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<i>errno</i> set to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_602_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>recvmsg</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EAGAIN] or [EWOULDBLOCK]</dt>
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<dd><br>
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The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data
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is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await
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out-of-band data.</dd>
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<dt>[EBADF]</dt>
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<dd>The <i>socket</i> argument is not a valid open file descriptor.</dd>
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<dt>[ECONNRESET]</dt>
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<dd>A connection was forcibly closed by a peer.</dd>
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<dt>[EINTR]</dt>
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<dd>This function was interrupted by a signal before any data was available.</dd>
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<dt>[EINVAL]</dt>
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<dd>The sum of the <i>iov_len</i> values overflows a <b>ssize_t</b>, or the MSG_OOB flag is set and no out-of-band data is
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available.</dd>
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<dt>[EMSGSIZE]</dt>
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<dd>The <i>msg_iovlen</i> member of the <b>msghdr</b> structure pointed to by <i>message</i> is less than or equal to 0, or is
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greater than {IOV_MAX}.</dd>
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<dt>[ENOTCONN]</dt>
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<dd>A receive is attempted on a connection-mode socket that is not connected.</dd>
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<dt>[ENOTSOCK]</dt>
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<dd>The <i>socket</i> argument does not refer to a socket.</dd>
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<dt>[EOPNOTSUPP]</dt>
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<dd>The specified flags are not supported for this socket type.</dd>
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<dt>[ETIMEDOUT]</dt>
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<dd>The connection timed out during connection establishment, or due to a transmission timeout on active connection.</dd>
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</dl>
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<p>The <i>recvmsg</i>() function may fail if:</p>
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<dl compact>
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<dt>[EIO]</dt>
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<dd>An I/O error occurred while reading from or writing to the file system.</dd>
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<dt>[ENOBUFS]</dt>
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<dd>Insufficient resources were available in the system to perform the operation.</dd>
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<dt>[ENOMEM]</dt>
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<dd>Insufficient memory was available to fulfill the request.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_602_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_602_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <a href="../functions/select.html"><i>select</i>()</a> and <a href="../functions/poll.html"><i>poll</i>()</a> functions can
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be used to determine when data is available to be received.</p>
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</blockquote>
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<h4><a name="tag_03_602_08"></a>RATIONALE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_602_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_602_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="poll.html"><i>poll</i>()</a> , <a href="recv.html"><i>recv</i>()</a> , <a href="recvfrom.html"><i>recvfrom</i>()</a> ,
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<a href="select.html"><i>select</i>()</a> , <a href="send.html"><i>send</i>()</a> , <a href="sendmsg.html"><i>sendmsg</i>()</a> ,
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<a href="sendto.html"><i>sendto</i>()</a> , <a href="shutdown.html"><i>shutdown</i>()</a> , <a href=
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"socket.html"><i>socket</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/sys/socket.h.html"><i><sys/socket.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_602_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 6. Derived from the XNS, Issue 5.2 specification.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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