-module(helloworld). 
-export([max/1,start/0]). 

max(N) -> 
   Max = erlang:system_info(process_limit), 
   io:format("Maximum allowed processes:~p~n" ,[Max]), 
   
   statistics(runtime), 
   statistics(wall_clock), 
   
   L = for(1, N, fun() -> spawn(fun() -> wait() end) end), 
   {_, Time1} = statistics(runtime), 
   {_, Time2} = statistics(wall_clock), lists:foreach(fun(Pid) -> Pid ! die end, L), 
   
   U1 = Time1 * 1000 / N, 
   U2 = Time2 * 1000 / N, 
   io:format("Process spawn time=~p (~p) microseconds~n" , [U1, U2]).
   wait() -> 
   
   receive 
      die -> void 
   end. 
 
for(N, N, F) -> [F()]; 
for(I, N, F) -> [F()|for(I+1, N, F)]. 

start()->
   max(1000), 
   max(100000).

On any machine which has a good processing power, both of the above max functions will pass. Following is a sample output from the above program.

Maximum allowed processes:262144

Process spawn time=47.0 (16.0) microseconds

Maximum allowed processes:262144

Process spawn time=12.81 (10.15) microseconds

Receive with a Timeout

Sometimes a receive statement might wait forever for a message that never comes. This could be for a number of reasons. For example, there might be a logical error in our program, or the process that was going to send us a message might have crashed before it sent the message. To avoid this problem, we can add a timeout to the receive statement. This sets a maximum time that the process will wait to receive a message.

Following is the syntax of the receive message with a timeout specified

Syntax

receive 
Pattern1 [when Guard1] -> 
Expressions1; 

Pattern2 [when Guard2] ->
Expressions2; 
... 
after Time -> 
Expressions 
end

The simplest example is to create a sleeper function as shown in the following program.

Example

-module(helloworld). 
-export([sleep/1,start/0]). 

sleep(T) ->
   receive 
   after T -> 
      true 
   end. 
   
start()->
   sleep(1000).

The above code will sleep for 1000 Ms before actually exiting.

Selective Receive

Each process in Erlang has an associated mailbox. When you send a message to the process, the message is put into the mailbox. The only time this mailbox is examined is when your program evaluates a receive statement.

Following is the general syntax of the Selective receive statement.

Syntax

receive 
Pattern1 [when Guard1] ->
Expressions1; 

Pattern2 [when Guard1] ->
Expressions1; 
... 
after 
Time ->
ExpressionTimeout 
end

This is how the above receive statement works −

• When we enter a receive statement, we start a timer (but only if an after section is present in the expression).

• Take the first message in the mailbox and try to match it against Pattern1, Pattern2, and so on. If the match succeeds, the message is removed from the mailbox, and the expressions following the pattern are evaluated.

• If none of the patterns in the receive statement matches the first message in the mailbox, then the first message is removed from the mailbox and put into a “save queue.” The second message in the mailbox is then tried. This procedure is repeated until a matching message is found or until all the messages in the mailbox have been examined.

• If none of the messages in the mailbox matches, then the process is suspended and will be rescheduled for execution the next time a new message is put in the mailbox. Note that when a new message arrives, the messages in the save queue are not rematched; only the new message is matched.

• As soon as a message has been matched, then all messages that have been put into the save queue are reentered into the mailbox in the order in which they arrived at the process. If a timer was set, it is cleared.

• If the timer elapses when we are waiting for a message, then evaluate the expressions ExpressionsTimeout and put any saved messages back into the mailbox in the order in which they arrived at the process.

Erlang – Performance

When talking about performance the following points need to be noted about Erlang.

• Funs are very fast − Funs was given its own data type in R6B and was further optimized in R7B.

• Using the ++ operator − This operator needs to be used in the proper way. The following example is the wrong way to do a ++ operation.

Example

-module(helloworld). 
-export([start/0]). 

start()->
   fun_reverse([H|T]) ->
   fun_reverse(T)++[H]; 
   fun_reverse([]) ->
   [].

As the ++ operator copies its left operand, the result is copied repeatedly, leading to quadratic complexity.

• Using Strings − String handling can be slow if done improperly. In Erlang, you need to think a little more about how the strings are used and choose an appropriate representation. If you use regular expressions, use the re-module in STDLIB instead of the obsolete regexp module.

• BEAM is a Stack-Based Byte-Code Virtual Machine − BEAM is a register-based virtual machine. It has 1024 virtual registers that are used for holding temporary values and for passing arguments when calling functions. Variables that need to survive a function call are saved to the stack. BEAM is a threaded-code interpreter. Each instruction is word pointing directly to executable C-code, making instruction dispatching very fast.

Erlang – Drivers

Sometimes we want to run a foreign-language program inside the Erlang Runtime System. In this case, the program is written as a shared library that is dynamically linked into the Erlang runtime system. The linked-in driver appears to the programmer as a port program and obeys exactly the same protocol as for a port program.

Creating a Driver

Creating a linked-in driver is the most efficient way of interfacing foreign-language code with Erlang, but it is also the most dangerous. Any fatal error in the linked-in driver will crash the Erlang System.

Following is an example of a driver implementation in Erlang −

Example

-module(helloworld). 
-export([start/0, stop/0]). 
-export([twice/1, sum/2]). 

start() ->
   start("example1_drv" ). 
start(SharedLib) ->
   case erl_ddll:load_driver("." , SharedLib) of 
   ok -> ok; 
      {error, already_loaded} -> ok; 
      _ -> exit({error, could_not_load_driver}) 
   end, 
   
   spawn(fun() -> init(SharedLib) end). 

init(SharedLib) -> 
   register(example1_lid, self()), 
   Port = open_port({spawn, SharedLib}, []), 
   loop(Port). 

stop() -> 
   example1_lid ! stop. 

twice(X) -> call_port({twice, X}). 
sum(X,Y) -> call_port({sum, X, Y}). call_port(Msg) -> 
   example1_lid ! {call, self(), Msg}, receive 
      {example1_lid, Result} -> 
      Result 
   end. 

LINKED-IN DRIVERS 223 
loop(Port) -> 
receive 
   {call, Caller, Msg} -> 
   Port ! {self(), {command, encode(Msg)}}, receive 
   {Port, {data, Data}} ->
   Caller ! {example1_lid, decode(Data)} 
   end, 

loop(Port); 
stop -> Port ! 
   {self(), close}, 
   receive 
      {Port, closed} -> 
      exit(normal) 
   end; 
   
      {''EXIT'', Port, Reason} -> 
      io:format("~p ~n" , [Reason]), 
      exit(port_terminated) 
   end. 

encode({twice, X}) -> [1, X]; 
encode({sum, X, Y}) -> [2, X, Y]. decode([Int]) -> Int.

Please note that working with drivers is extremely complex and care should be taken when working with drivers.

Erlang – Web Programming

In Erlang, the inets library is available to build web servers in Erlang. Let’s look at some of the functions available in Erlang for web programming. One can implement the HTTP server, also referred to as httpd to handle HTTP requests.

The server implements numerous features, such as −

• Secure Sockets Layer (SSL)
• Erlang Scripting Interface (ESI)
• Common Gateway Interface (CGI)
• User Authentication (using Mnesia, Dets or plain text database)
• Common Logfile Format (with or without disk_log(3) support)
• URL Aliasing
• Action Mappings
• Directory Listings

The first job is to start the web library via the command.

inets:start()

The next step is to implement the start function of the inets library so that the web server can be implemented.

Following is an example of creating a web server process in Erlang.

For example

-module(helloworld). 
-export([start/0]). 

start() ->
   inets:start(), 
   Pid = inets:start(httpd, [{port, 8081}, {server_name,"httpd_test"}, 
   {server_root,"D://tmp"},{document_root,"D://tmp/htdocs"},
   {bind_address, "localhost"}]), io:fwrite("~p",[Pid]).

The following points need to be noted about the above program.

• The port number needs to be unique and not used by any other program. The httpd service would be started on this port no.

• The server_root and document_root are mandatory parameters.

Output

Following is the output of the above program.

{ok,<0.42.0>}

To implement a Hello world web server in Erlang, perform the following steps −

Step 1 − Implement the following code −

-module(helloworld). 
-export([start/0,service/3]). 

start() ->
   inets:start(httpd, [ 
      {modules, [ 
         mod_alias, 
         mod_auth, 
         mod_esi, 
         mod_actions, 
         mod_cgi, 
         mod_dir,
         mod_get, 
         mod_head, 
         mod_log, 
         mod_disk_log 
      ]}, 
      
      {port,8081}, 
      {server_name,"helloworld"}, 
      {server_root,"D://tmp"}, 
      {document_root,"D://tmp/htdocs"}, 
      {erl_script_alias, {"/erl", [helloworld]}}, 
      {error_log, "error.log"}, 
      {security_log, "security.log"}, 
      {transfer_log, "transfer.log"}, 
      
      {mime_types,[ 
         {"html","text/html"}, {"css","text/css"}, {"js","application/x-javascript"} ]} 
   ]). 
         
service(SessionID, _Env, _Input) -> mod_esi:deliver(SessionID, [ 
   "Content-Type: text/htmlrnrn", "<html><body>Hello, World!</body></html>" ]).

Step 2 − Run the code as follows. Compile the above file and then run the following commands in erl.

c(helloworld).

You will get the following output.

{ok,helloworld}

The next command is −

inets:start().

You will get the following output.

ok

The next command is −

helloworld:start().

You will get the following output.

{ok,<0.50.0>}

Step 3 − You can now access the url – http://localhost:8081/erl/hello_world:service.