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Diffstat (limited to 'system/doc/tutorial/c_portdriver.xmlsrc')
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1 files changed, 57 insertions, 55 deletions
diff --git a/system/doc/tutorial/c_portdriver.xmlsrc b/system/doc/tutorial/c_portdriver.xmlsrc index 2fd6fb0aac..61187703a4 100644 --- a/system/doc/tutorial/c_portdriver.xmlsrc +++ b/system/doc/tutorial/c_portdriver.xmlsrc @@ -4,7 +4,7 @@ <chapter> <header> <copyright> - <year>2000</year><year>2013</year> + <year>2000</year><year>2015</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> @@ -21,46 +21,45 @@ </legalnotice> - <title>Port drivers</title> + <title>Port Drivers</title> <prepared></prepared> <docno></docno> <date></date> <rev></rev> <file>c_portdriver.xml</file> </header> - <p>This is an example of how to solve the <seealso marker="example">example problem</seealso> by using a linked in port driver.</p> + <p>This section outlines an example of how to solve the example problem + in <seealso marker="example">Problem Example</seealso> + by using a linked-in port driver.</p> + <p>A port driver is a linked-in driver that is accessible as a port + from an Erlang program. It is a shared library (SO in UNIX, DLL in + Windows), with special entry points. The Erlang runtime system + calls these entry points when the driver is started and when data + is sent to the port. The port driver can also send data to + Erlang.</p> + <p>As a port driver is dynamically linked into the emulator process, + this is the fastest way of calling C-code from Erlang. Calling + functions in the port driver requires no context switches. But it + is also the least safe way, because a crash in the port driver + brings the emulator down too.</p> + <p>The scenario is illustrated in the following figure:</p> <image file="../tutorial/port_driver.gif"> - <icaption>Port Driver Communication.</icaption> + <icaption>Port Driver Communication</icaption> </image> <section> - <title>Port Drivers</title> - <p>A port driver is a linked in driver that is accessible as a - port from an Erlang program. It is a shared library (SO in Unix, - DLL in Windows), with special entry points. The Erlang runtime - calls these entry points, when the driver is started and when - data is sent to the port. The port driver can also send data to - Erlang.</p> - <p>Since a port driver is dynamically linked into the emulator - process, this is the fastest way of calling C-code from Erlang. - Calling functions in the port driver requires no context - switches. But it is also the least safe, because a crash in the - port driver brings the emulator down too.</p> - </section> - - <section> <title>Erlang Program</title> - <p>Just as with a port program, the port communicates with a Erlang + <p>Like a port program, the port communicates with an Erlang process. All communication goes through one Erlang process that is the <em>connected process</em> of the port driver. Terminating this process closes the port driver.</p> <p>Before the port is created, the driver must be loaded. This is done with the function <c>erl_dll:load_driver/1</c>, with the name of the shared library as argument.</p> - <p>The port is then created using the BIF <c>open_port/2</c> with + <p>The port is then created using the BIF <c>open_port/2</c>, with the tuple <c>{spawn, DriverName}</c> as the first argument. The string <c>SharedLib</c> is the name of the port driver. The second - argument is a list of options, none in this case.</p> + argument is a list of options, none in this case:</p> <pre> -module(complex5). -export([start/1, init/1]). @@ -77,9 +76,9 @@ init(SharedLib) -> register(complex, self()), Port = open_port({spawn, SharedLib}, []), loop(Port).</pre> - <p>Now it is possible to implement <c>complex5:foo/1</c> and - <c>complex5:bar/1</c>. They both send a message to the - <c>complex</c> process and receive the reply.</p> + <p>Now <c>complex5:foo/1</c> and <c>complex5:bar/1</c> + can be implemented. Both send a message to the + <c>complex</c> process and receive the following reply:</p> <pre> foo(X) -> call_port({foo, X}). @@ -92,10 +91,14 @@ call_port(Msg) -> {complex, Result} -> Result end.</pre> - <p>The <c>complex</c> process encodes the message into a sequence - of bytes, sends it to the port, waits for a reply, decodes the - reply and sends it back to the caller. - </p> + <p>The <c>complex</c> process performs the following:</p> + <list type="bulleted"> + <item>Encodes the message into a sequence of bytes.</item> + <item>Sends it to the port.</item> + <item>Waits for a reply.</item> + <item>Decodes the reply.</item> + <item>Sends it back to the caller:</item> + </list> <pre> loop(Port) -> receive @@ -108,59 +111,58 @@ loop(Port) -> loop(Port) end.</pre> <p>Assuming that both the arguments and the results from the C - functions will be less than 256, a very simple encoding/decoding - scheme is employed where <c>foo</c> is represented by the byte + functions are less than 256, a simple encoding/decoding scheme + is employed. In this scheme, <c>foo</c> is represented by byte 1, <c>bar</c> is represented by 2, and the argument/result is - represented by a single byte as well. - </p> + represented by a single byte as well:</p> <pre> encode({foo, X}) -> [1, X]; encode({bar, Y}) -> [2, Y]. - + decode([Int]) -> Int.</pre> - <p>The resulting Erlang program, including functionality for - stopping the port and detecting port failures is shown below.</p> + <p>The resulting Erlang program, including functions for stopping + the port and detecting port failures, is as follows:</p> <codeinclude file="complex5.erl" type="erl"/> </section> <section> <title>C Driver</title> <p>The C driver is a module that is compiled and linked into a - shared library. It uses a driver structure, and includes the + shared library. It uses a driver structure and includes the header file <c>erl_driver.h</c>.</p> <p>The driver structure is filled with the driver name and function pointers. It is returned from the special entry point, declared with the macro <c><![CDATA[DRIVER_INIT(<driver_name>)]]></c>.</p> - <p>The functions for receiving and sending data, are combined into + <p>The functions for receiving and sending data are combined into a function, pointed out by the driver structure. The data sent - into the port is given as arguments, and the data the port - sends back is sent with the C-function <c>driver_output</c>.</p> - <p>Since the driver is a shared module, not a program, no main - function should be present. All function pointers are not used - in our example, and the corresponding fields in the + into the port is given as arguments, and the replied data is sent + with the C-function <c>driver_output</c>.</p> + <p>As the driver is a shared module, not a program, no main + function is present. All function pointers are not used + in this example, and the corresponding fields in the <c>driver_entry</c> structure are set to NULL.</p> - <p>All functions in the driver, takes a handle (returned from - <c>start</c>), that is just passed along by the erlang + <p>All functions in the driver takes a handle (returned from + <c>start</c>) that is just passed along by the Erlang process. This must in some way refer to the port driver instance.</p> - <p>The example_drv_start, is the only function that is called with - a handle to the port instance, so we must save this. It is - customary to use a allocated driver-defined structure for this - one, and pass a pointer back as a reference.</p> - <p>It is not a good idea to use a global variable; since the port - driver can be spawned by multiple Erlang processes, this - driver-structure should be instantiated multiple times. + <p>The <c>example_drv_start</c>, is the only function that is called with + a handle to the port instance, so this must be saved. It is + customary to use an allocated driver-defined structure for this + one, and to pass a pointer back as a reference.</p> + <p>It is not a good idea to use a global variable as the port + driver can be spawned by multiple Erlang processes. This + driver-structure is to be instantiated multiple times: </p> <codeinclude file="port_driver.c" tag="" type="none"></codeinclude> </section> <section> <title>Running the Example</title> - <p>1. Compile the C code.</p> + <p><em>Step 1.</em> Compile the C code:</p> <pre> unix> <input>gcc -o exampledrv -fpic -shared complex.c port_driver.c</input> windows> <input>cl -LD -MD -Fe exampledrv.dll complex.c port_driver.c</input></pre> - <p>2. Start Erlang and compile the Erlang code.</p> + <p><em>Step 2.</em> Start Erlang and compile the Erlang code:</p> <pre> > <input>erl</input> Erlang (BEAM) emulator version 5.1 @@ -168,7 +170,7 @@ Erlang (BEAM) emulator version 5.1 Eshell V5.1 (abort with ^G) 1> <input>c(complex5).</input> {ok,complex5}</pre> - <p>3. Run the example.</p> + <p><em>Step 3.</em> Run the example:</p> <pre> 2> <input>complex5:start("example_drv").</input> <0.34.0> |