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<h1>15 Ports and Port Drivers</h1>
<p>Examples of how to use ports and port drivers are provided in
<span class="bold_code bc-13"><a href="javascript:erlhref('../../','doc/tutorial','introduction.html#interoperability%20tutorial');">Interoperability Tutorial</a></span>.
For information about the BIFs mentioned, see the
<span class="bold_code bc-18"><a href="javascript:erlhref('../../','erts','erlang.html');">erlang(3)</a></span> manual
page in ERTS.</p>
<h3><span onMouseOver="document.getElementById('ghlink-ports-idm281472323680088').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-ports-idm281472323680088').style.visibility = 'hidden';"><span id="ghlink-ports-idm281472323680088" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/system/doc/reference_manual/ports.xml#L39" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="ports" href="#ports">15.1
Ports</a></span></h3>
<p><strong>Ports</strong> provide the basic mechanism for communication
with the external world, from Erlang's point of view. They
provide a byte-oriented interface to an external program. When a
port has been created, Erlang can communicate with it by sending
and receiving lists of bytes, including binaries.</p>
<p>The Erlang process creating a port is said to be
the <strong>port owner</strong>, or the <strong>connected process</strong> of
the port. All communication to and from the port must go through
the port owner. If the port owner terminates, so does the port
(and the external program, if it is written correctly).</p>
<p>The external program resides in another OS process. By default,
it reads from standard input (file descriptor 0) and writes
to standard output (file descriptor 1). The external program
is to terminate when the port is closed.</p>
<h3><span onMouseOver="document.getElementById('ghlink-port-drivers-idm281472323675608').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-port-drivers-idm281472323675608').style.visibility = 'hidden';"><span id="ghlink-port-drivers-idm281472323675608" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/system/doc/reference_manual/ports.xml#L57" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="port-drivers" href="#port-drivers">15.2
Port Drivers</a></span></h3>
<p>It is possible to write a driver in C according to certain
principles and dynamically link it to the Erlang runtime system.
The linked-in driver looks like a port from the Erlang
programmer's point of view and is called a <strong>port driver</strong>.</p>
<div class="warning">
<div class="label">Warning</div>
<div class="content"><p>
<p>An erroneous port driver causes the entire Erlang runtime
system to leak memory, hang or crash.</p>
</p></div>
</div>
<p>For information about port drivers, see the
<span class="bold_code bc-18"><a href="javascript:erlhref('../../','erts','erl_driver.html');">erl_driver(4)</a></span>
manual page in ERTS,
<span class="bold_code bc-18"><a href="javascript:erlhref('../../','erts','driver_entry.html');">driver_entry(1)</a></span>
manual page in ERTS, and
<span class="bold_code bc-18"><a href="javascript:erlhref('../../','kernel','erl_ddll.html');">erl_ddll(3)</a></span>
manual page in Kernel.</p>
<h3><span onMouseOver="document.getElementById('ghlink-port-bifs-idm281472323669624').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-port-bifs-idm281472323669624').style.visibility = 'hidden';"><span id="ghlink-port-bifs-idm281472323669624" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/system/doc/reference_manual/ports.xml#L76" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="port-bifs" href="#port-bifs">15.3
Port BIFs</a></span></h3>
<p>To create a port:</p>
<div class="doc-table-wrapper">
<table class="doc-table"><tr>
<td align="left" valign="middle"><span class="code">open_port(PortName, PortSettings</span></td>
<td align="left" valign="middle">Returns a port identifier
<span class="code">Port</span> as the result of opening a new Erlang port.
Messages can be sent to, and received from, a port identifier,
just like a pid. Port identifiers can also be linked to
using <span class="code">link/1</span>, or registered under a name using
<span class="code">register/2</span>.</td>
</tr></table>
<p class="doc-table-caption">Table
15.1:
Port Creation BIF</p>
</div>
<p><span class="code">PortName</span> is usually a tuple <span class="code">{spawn,Command}</span>, where
the string <span class="code">Command</span> is the name of the external program.
The external program runs outside the Erlang workspace, unless a
port driver with the name <span class="code">Command</span> is found. If <span class="code">Command</span>
is found, that driver is started.</p>
<p><span class="code">PortSettings</span> is a list of settings (options) for the port.
The list typically contains at least a tuple <span class="code">{packet,N}</span>,
which specifies that data sent between the port and the external
program are preceded by an N-byte length indicator. Valid values
for N are 1, 2, or 4. If binaries are to be used instead of lists
of bytes, the option <span class="code">binary</span> must be included.</p>
<p>The port owner <span class="code">Pid</span> can communicate with the port
<span class="code">Port</span> by sending and receiving messages. (In fact, any
process can send the messages to the port, but the port owner must
be identified in the message).</p>
<p>As of Erlang/OTP R16, messages sent to ports are delivered truly
asynchronously. The underlying implementation previously
delivered messages to ports synchronously. Message passing has
however always been documented as an asynchronous operation. Hence,
this is not to be an issue for an Erlang program communicating
with ports, unless false assumptions about ports have been made.</p>
<p>In the following tables of examples, <span class="code">Data</span> must be an I/O list.
An I/O list is a binary or a (possibly deep) list of binaries
or integers in the range 0..255:</p>
<div class="doc-table-wrapper">
<table class="doc-table">
<tr>
<td align="left" valign="middle"><strong>Message</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Pid,{command,Data}}</span></td>
<td align="left" valign="middle">Sends <span class="code">Data</span> to the port.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Pid,close}</span></td>
<td align="left" valign="middle">Closes the port. Unless the
port is already closed, the port replies with
<span class="code">{Port,closed}</span> when all buffers have been flushed
and the port really closes.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Pid,{connect,NewPid}}</span></td>
<td align="left" valign="middle">Sets the port owner of
<span class="code">Port</span>to <span class="code">NewPid</span>. Unless the port is already closed,
the port replies with<span class="code">{Port,connected}</span> to the old
port owner. Note that the old port owner is still linked
to the port, but the new port owner is not.</td>
</tr>
</table>
<p class="doc-table-caption">Table
15.2:
Messages Sent To a Port</p>
</div>
<p></p>
<div class="doc-table-wrapper">
<table class="doc-table">
<tr>
<td align="left" valign="middle"><strong>Message</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Port,{data,Data}}</span></td>
<td align="left" valign="middle">
<span class="code">Data</span> is received from the external program.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Port,closed}</span></td>
<td align="left" valign="middle">Reply to <span class="code">Port ! {Pid,close}</span>.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{Port,connected}</span></td>
<td align="left" valign="middle">Reply to <span class="code">Port ! {Pid,{connect,NewPid}}</span>.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">{'EXIT',Port,Reason}</span></td>
<td align="left" valign="middle">If the port has terminated for some reason.</td>
</tr>
</table>
<p class="doc-table-caption">Table
15.3:
Messages Received From a Port</p>
</div>
<p>Instead of sending and receiving messages, there are also a
number of BIFs that can be used:</p>
<div class="doc-table-wrapper">
<table class="doc-table">
<tr>
<td align="left" valign="middle"><strong>Port BIF</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">port_command(Port,Data)</span></td>
<td align="left" valign="middle">Sends <span class="code">Data</span> to the port.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">port_close(Port)</span></td>
<td align="left" valign="middle">Closes the port.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">port_connect(Port,NewPid)</span></td>
<td align="left" valign="middle">Sets the port owner of
<span class="code">Port</span>to <span class="code">NewPid</span>. The old port owner <span class="code">Pid</span>
stays linked to the port and must call <span class="code">unlink(Port)</span>
if this is not desired.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">erlang:port_info(Port,Item)</span></td>
<td align="left" valign="middle">Returns information as specified by <span class="code">Item</span>.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">erlang:ports()</span></td>
<td align="left" valign="middle">Returns a list of all ports on the current node.</td>
</tr>
</table>
<p class="doc-table-caption">Table
15.4:
Port BIFs</p>
</div>
<p>Some additional BIFs that apply to port drivers:
<span class="code">port_control/3</span> and <span class="code">erlang:port_call/3</span>.</p>
</div>
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