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Print capturing example

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 </p>
 [[Category:Janus Sockets]]

JAL2: Created page with "==Retrieving web pages: HTTP, HTML== The examples in this section show techniques for composing HTTP requests, sending them to a web server, and retrieving the response. The cu..."

2012-02-14T22:52:31Z

Created page with "==Retrieving web pages: HTTP, HTML== The examples in this section show techniques for composing HTTP requests, sending them to a web server, and retrieving the response. The cu..."

New page

==Retrieving web pages: HTTP, HTML==
The examples in this section show techniques for composing HTTP requests,
sending them to a web server, and retrieving the response.

The current specification for HTTP ([http:///www.w3.org/Protocols/rfc2616/rfc2616.html HTTP/1.1])
gives something like the following simple structure for using HTTP:
<ol>
<li>Client ("browser") sends a request, which contains:
<ol>
<li>The request line, ended by CRLF (x'0D0A'), and containing:
<ol>
<li>The method, for example, GET to get a web page
<li>The ID, or "URL"
<li>The protocol version, for example, HTTP/1.1
</ol>
<li>Additional header lines, each ended by CRLF
<li>An empty line, that is, an immediate CRLF
<li>The body
</ol>
<li>Server interprets the request and sends a response, which contains:
<ol>
<li>The response status line, ended by CRLF, and containing:
<ol>
<li>The HTTP version ID (for example, HTTP/1.1), followed by spaces
<li>The status code number, followed by spaces
<li>The status code message
</ol>
<li>Additional header lines, each ended by CRLF
<li>An empty line, that is, an immediate CRLF
<li>The body
</ol>
</ol>

Header lines contain a case-insensitive field name, followed by a colon (<tt>:</tt>),
and spaces followed by the field value.

The ASCII string of carriage-return followed by linefeed (0D0A) is
used in the HTTP protocol to separate header lines and to separate the header from the body.
The response body is the web "page,"
which may contain a variety of data.
Frequently, the page will contain HTML, which is frequently separated into
"lines;" however, the separators of the lines are commonly either 0D0A, 0D, or 0A.
Although this presents ambiguous parse tokens, as discussed in [[#Ambiguous PRSTOK strings|"Ambiguous PRSTOK strings"]],
the pitfalls of the general request-response protocols do not apply with a single
request-response when the only ambiguity occurs in the response stream.

===Simple echo of HTTP/HTML===
This example shows a complete program that communicates with a remote
web server and that parses and prints the returned HTML stream.

Note that the third argument to <var>[[$Sock_RecvPrs]]</var> is -1, which means that there
is no limit to the length of each line parsed, hence no limit to the number of bytes discarded
at the end of the line (only the first 78 bytes (<code>%s</code> size) of each line of HTML is examined
by the <var class="product">User Language</var> program).
[[#Echo of HTTP/HTML with continuation lines|"Echo of HTTP/HTML with continuation lines"]]
has an example that examines complete lines, printing continuation lines as needed.
JANUS DEFINE MYBROWSE * CLSOCK 60 REMOTE * 80 LINEND 0D0A
JANUS START MYBROWSE
Begin
%socket Is Float
%rc Is Float
%host Is String Len 255
%page Is String Len 255
%s Is String Len 78
%host = 'www.sirius-software.com'
%page = 'main.html'

%socket = $sock_conn('TEST', %host)
If %socket Lt 0 Then
Stop
End If
%rc = $sock_send(%socket, 'GET /')
%rc = $sock_send(%socket, %page)
%rc = $sock_sendln(%socket, ' HTTP/1.0')
* Null line indicates end of request:
%rc = $sock_sendln(%socket, '')

%s = $sock_set(%socket, 'PRSTOK', 'AMBIG|0D0A|0A|0D')
Repeat
%rc = $sock_recvprs(%socket, %s, -1)
Print %s
If %rc Le 0 Then
Loop End
End If
End Repeat
%rc = $sock_close(%socket)
End


===Echo of HTTP/HTML with continuation lines===
This example shows a complete program that communicates with a remote
web server and which parses and prints the returned HTML stream.
This program differs from that in
[[#Simple echo of HTTP/HTML|"imple echo of HTTP/HTML"] because it examines complete lines,
printing continuation lines as needed.

The fourth argument to <var>[[$Sock_RecvPrs]]</var> (<code>%x</code> in the example below) is
the index of the separator string found in the received data.
When this value is 0, it means that, within the data that has
been received upon return from <var>$Sock_RecvPrs</var>, no separator has been found.
This indicates one of the following:
<ul>
<li>All data has been received (or all
data within <var>RECVLIM</var>, which is not used in this example)
<li>The limit set by its third argument (here defaulting to 0, which means
the size of the second argument) caused <var>$Sock_RecvPrs</var> to return before it
might have found a separator.
</ul>

This information (<code>%x Eq 0</code>) works adequately for the example.
JANUS DEFINE MYBROWSE * CLSOCK 60 REMOTE * 80 LINEND 0D0A
JANUS START MYBROWSE
Begin
%socket Is Float
%rc Is Float
%x Is Float
%host Is String Len 255
%page Is String Len 255
%s Is String Len 75
%pre Is String Len 2
%host = 'www.sirius-software.com'
%page = 'main.html'

%socket = $sock_conn('TEST', %host)
If %socket Lt 0 Then
Stop
End If
%rc = $sock_send(%socket, 'GET /')
%rc = $sock_send(%socket, %page)
%rc = $sock_sendln(%socket, ' HTTP/1.0')
* Null line indicates end of request:
%rc = $sock_sendln(%socket, '')

%s = $sock_set(%socket, 'PRSTOK', 'AMBIG|0D0A|0A|0D')
%pre = ' '
Repeat
%rc = $sock_recvprs(%socket, %s, , %x)
Print %pre And %s
If %rc Le 0 Then
Loop End
End If
%pre = ' '
If %x Eq 0 Then
%pre = '->'
End If
End Repeat
%rc = $sock_close(%socket)
End


==Request-response protocol==
The first parts of this section exhibit a very simple
protocol which can be used between any two sockets platforms.
This example is shown using <var class="product">Janus Sockets</var> for both the client and
server, but either the client or the server portion can be easily
implemented on any platform with a sockets interface.

This protocol makes use of the <var>[[$Sock_RecvPrs]]</var> function to receive the
client request and server response.
The request consists of multiple strings, each
separated by an "end of line" sequence.
This is a natural sequence for various platforms;
for example, you might want to copy the contents of a text file as some of the data in the request.

In some environments, end of line is <code>X'0D'</code> and in others <code>X'0D0A'</code>.
Since one line-end alternative is a prefix of another, it is necessary to
use the <var>AMBIG</var> specification on the <var>PRSTOK</var> parameter.
The last part of this section ([[#Ambiguous PRSTOK strings|"Ambiguous PRSTOK strings"]])
discusses some considerations for ambiguous <var>PRSTOK</var> parameters.

===Single request client code===
This client sends a single request and receives a response.
A request contains an action, followed by the data for the request,
followed by a distinct line for the end of the request.
After sending the request, it receives the response from the server.

This example could easily be written for another sockets platform,
and the data could be conveniently obtained from a text file.

As discussed in [[#Ambiguous PRSTOK strings|"Ambiguous PRSTOK strings"]], it is important to use an
unambiguous separator string at the end of the request, one that
is not a prefix of any other separator.
In our protocol, we are using a single byte of hex 07 (the ASCII "Bell" character) as this
separator to "turn around" the connection from both the client and server perspective.
JANUS DEFINE SOCKA * CLSOCK 10 REMOTE SOCKA_SRV 7733
JANUS START SOCKA
Begin
%x Float
%r Float
%status String Len 10
%x = $sock_conn('SOCKA')
If %x Lt 0 Then
Stop
End If
%r = $sock_sendln(%X, 'ACTION1')
Fr Where RECTYPE = 'MYDATA'
%r = $sock_sendln(%x, DATA)
End For
%r = $sock_sendln(%x, 'GO', 'LINEnd 07')
%r = $sock_recvprs(%x, %status, , 'PRSTOK 07')
If %status Ne 'GOOD' Then
Stop
End If
%r = $sock_close(%x)


===Single request server code===
The server receives a single request and sends a response.
See details about the protocol in [[#Single request client code|"Single request client code"]].

Note:
As always, the connection initiating <var>SRVSOCK</var> processing is accessible as socket number 1.
JANUS DEFINE SOCKS 7733 SRVSOCK 10 CMD 'INCLUDE SINGLE_REQ'
JANUS START SOCKS
PROCEDURE SINGLE_REQ
Begin
%r Float
%sepx Float
%data String Len 255
%r = $sock_set(1, 'PRSTOK', 'AMBIG|0D0A|0D|0A|07')
%r = $sock_set(1, 'LINEND', '07')
%r = $sock_recvprs(1, %data)
If %r Le 0 Then
Stop
End If
If %data Eq 'ACTION1' Then
Repeat
%r = $sock_Recvprs(1, %data, %sepx)
If %r Le 0 Then
Stop
Elseif %sepx Le 4 Then
If %data Ne 'GO' Then
%r = $sock_sendln(1, 'BAD')
Else
%r = $sock_sendln(1, 'GOOD')
End If
Stop
End If
... processing a line of data
End Repeat
Else If %data Eq ... other operation
Repeat
%r = $sock_recvprs(1, %data, %sepx)
... (same structure as above)
End Repeat
Else If %data Eq ... other operations
...
End If
%r = $sock_close(1)
End
END PROCEDURE


===Multiple request client code===
This client sends a series of requests and receives a response from each one.
The protocol is the same as shown in [[#Single request client code|"Single request client code"]], except that
at the end of the series of requests it sends a special action to
indicate the end of the conversation.

In this example, the use of the unambiguous separator at the end of
the response is more important than in the previous example, because
the server is going to receive after sending the response.
With a single request-response protocol, as long as you are sure that
the server will close the connection after sending the response, the
client will receive the complete response even if it is terminated by an ambiguous separator.

It is good practice, however, to use unambiguous separators to end
each request and response, since, for example, the server may be poorly
designed and the connection could stay open for an extended time.
JANUS DEFINE SOCKA * CLSOCK 10 REMOTE SOCKA_SRV 7733
JANUS START SOCKA
Begin
%x Float
%r Float
%status String Len 10
%x = $sock_conn('SOCKA')
If %x Lt 0 Then
Stop
End If
%r = $sock_set(%x, 'PRSTOK', '07')
Fr Where RECTYPE = 'REQUEST'
%r = $sock_sendln(%x, REQ_TYPE)
D: FEO DATA_LINE
%r = $sock_sendln(%x, VALUE IN D)
End For
%r = $sock_sendln(%x, 'GO', 'LINEND 07')
%r = $sock_recvprs(%x, %status)
If %status Ne 'GOOD' Then
Stop
End If
End For
%r = $sock_sendln(%x, 'DONE')
%r = $sock_close(%x)


===Multiple request server code===
The server receives a series of requests and sends a response for each one.
The protocol is the same as shown in [[#Single request client code|"Single request client code"]], except that
at the end of the series of requests the client sends a special action to
indicate the end of the conversation.

Note:
As always, the connection initiating <var>SRVSOCK</var> processing is
accessible as socket number 1.
JANUS DEFINE SOCKS 7733 SRVSOCK 10 CMD 'INCLUDE MULTIPLE_REQ'
JANUS START SOCKS
PROCEDURE MULTIPLE_REQ
Begin
%r Float
%sepx Float
%data String Len 255
%r = %sock_set(1, 'PRSTOK', 'AMBIG|0D0A|0D|0A|07')
%r = %sock_set(1, 'LINEND', '07')
Repeat
%r = %sock_recvprs(1, %data)
If %r Le 0 Then
Loop End
End If
If %data Eq 'DONE' Then
Loop End
Elseif %data Eq 'ACTION1' Then
Repeat
%r = %sock_recvprs(1, %data, %sepx)
If %r Le 0 Then
Stop
Elseif %sepx Eq 4 Then
If %data Eq 'DONE' Then
%r = %sock_sendln(1, 'BAD')
Loop End
End If
%r = %sock_sendln(1, 'GOOD')
Loop End
End
... processing a line of data
End Repeat
Else If %data Eq ... another operation
Repeat
%r = %sock_recvprs(1, %data, %sepx)
... (same structure as above)
End Repeat
Else If %data Eq ... other operations
...
End If
End Repeat
%r = %sock_close(1)
End
END PROCEDURE


===Ambiguous PRSTOK strings===
In the example code shown in the preceding sections, the lines of data are sent with
either CRLF, CR, or LF (0D0A|0D|0A) separating them, but the line that signals the end of
a request is terminated with hex 07.
The separators <code>0D0A</code> and <code>0D</code> are '''ambiguous''' because <code>0D</code> is a
prefix of <code>0D0A</code>.
When a <code>0D</code> is received, <var class="product">Janus Sockets</var> waits until it receives an additional byte
to distinguish which of the two separators was sent.
The unambiguous 07 is used in the example in [[#Multiple request client code|"Multiple request client code"]]
to end the <code>GO</code> line, because using an ambiguous separator when
going from send to receive mode can cause both sides to wait for data that is not forthcoming.

In a request-response protocol, when one side finishes sending (the
client finishes a request, or the server finishes a response), it then
does a receive (the client to get the response, the server to get the next request).
When either side does the receive, it must be ensured that the other side does not
continue to wait for data on its own receive.
Otherwise, both sides are receiving and nothing is being sent for them to receive.
In this case, they could wait forever, until the <var>TIMEOUT</var> value expires at one side or the other,
or until (using <var class="product">Model 204</var>) one side is bumped.

If the server accepts 0D0A|0D|0A (or even just 0D0A|0D) as the terminator
of the <code>GO</code> line, and the client sends <code>0D</code> as its <var>LINEND</var> and
then does a receive, the server will wait to receive a byte after the
<code>0D</code> to determine whether the separator was <code>0D0A</code> or just <code>0D</code>.
This situation where both sides wait forever is avoided by using an unambiguous separator
(<code>07</code> in this case) as the terminator of the <code>GO</code> line.
Similarly, the multiple request server uses the unambiguous <code>07</code> to terminate the response, so that
the client can send the next request.

If possible, use unambiguous separators when designing your protocols.
If that is very inconvenient, such as the case of copying text files with the
ambiguous linend 0D0A|0D|0A strings, you must use the <code>AMBIG|</code> specification
at the start of your <var>PRSTOK</var> string, and you should take special care when composing
the terminator of a line that is the last one before a switch to do a receive.
The <code>AMBIG|</code> indicator highlights the necessity to ensure that your
protocol, that is, the sockets application processing logic, avoids the pitfalls
introduced by ambiguous separators.

Since <code>0A</code> and <code>0D0A</code> are not prefixes of another separator, either
could be used as <code>GO</code> terminator, if the line <code>GO</code> is guaranteed
not to occur as a data line.
The server code (for example,
in [[#Single request server code|"Single request server code"]]) uses the extra protection of a unique
line and a unique separator to double-check that the client is not violating the protocol.

Again, the "wait forever" problem that ambiguous separators may produce only
applies to a separator that occurs at a send/receive boundary.
Thus, as mentioned in [[#Retrieving web pages: HTTP, HTML|"Retrieving web pages: HTTP, HTML"]], a
single request-response protocol presents no problems if the only ambiguity
is in the response stream.

Other than "wait forever" problems, the other question raised by ambiguous separators concerns null "lines."
The simple rule is that the longest separator possible is chosen.
So, consider the following somewhat silly client code that "hand codes" its separators:
%r = $sock_send(%sok, 'Line 1')
%r = $sock_send(%sok, $x2c('0D'), 'BINARY')
%line2 = ''
%r = $sock_send(%sok, %line2)
%r = $sock_send(%sok, $X2C('0A'), 'BINARY')
%r = $sock_send(%sok, 'Line 3')
%r = $sock_send(%sok, $x2c('0D07'), 'BINARY')

If the corresponding server code is:
%r = $sock_set(1, 'PRSTOK', 'AMBIG|0D0A|0D|0A|07')
Repeat
%r = $sock_recv(1, %targ)
If %r Le 0 Then
Loop End
End
Print 'Length ' $Len(%targ) ': ' %targ

The results will be:
Length 6: Line 1
Length 6: Line 3
Length 0:

What might appear to be the sending of three lines by the client, with
the second line null, is actually seen by the server as two lines, because
the hexadecimal string <code>0D0A</code> is seen as one separator, not two.

===Print capturing example===

This example shows the use of print capturing.
This application uses <var>WRITE IMAGE ON TERMINAL</var> for output, which might
be appropriate, for example, in a <var>BATCH2</var> input stream.

Note that the final <var>Print</var> statement goes to the terminal, since
there are no sockets with print captured at that point.

For more information about print capturing, see [[$Sock_Capture#Print capturing hierarchy and other considerations|"Print capturing hierarchy and other considerations"]].
Begin
IMAGE LIN
S IS STRING LEN 255
END IMAGE
OPEN TERMINAL FOR OUTPUT
%sock = $sock_conn('SOCKEM')
If %sock Lt 0 Then
Stop
End If
%s = $sock_capture(%sock, 'ON')
Prepare Image LIN
Print 'Hello, world!'
%lin:S = 'Look at me - I sent a line!'
Write Image LIN On Terminal
Print 'Goodbye, world!'
%x = $sock_close
Print 'Mission accomplished'
End


[[Category:Janus Sockets]]

← Older revision		Revision as of 15:14, 26 April 2013
Line 461:		Line 461:
	==See also==		==See also==
	<ul>		<ul>
			<li>[[Janus Sockets User Language coding considerations]]
	<li>Socket-level interfaces:		<li>Socket-level interfaces:
	<ul>		<ul>

← Older revision		Revision as of 19:13, 25 April 2013
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	==See also==		==See also==
	<ul>		<ul>
	~~<li>[[Sample Janus Sockets programs]]~~
	<li>Socket-level interfaces:		<li>Socket-level interfaces:
	<ul>		<ul>

Sample Janus Sockets programs - Revision history

JAL: /* See also */

JAL: /* See also */

JAL: /* Print capturing example */

JAL2: Created page with "==Retrieving web pages: HTTP, HTML== The examples in this section show techniques for composing HTTP requests, sending them to a web server, and retrieving the response. The cu..."