Using EXTERN

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Using EXTERN

• Dave Doulton

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Contents

• What are EXTERN functions?
• How do you use them?
• How do you create them?
• Examples of usage.

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What are EXTERN Functions?

• EXTERN and EXTERNS are two functions held in the
  extern.dll (Dynamically Linked Library).
• These functions can do anything you wish provided
  you can create the dll.
• Your dll is placed in the SIR directory in place
  of the supplied version.
• Note they can both take string or numeric
  parameters which can do different things but only
  return a numeric from EXTERN and a string from
  EXTERNS

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How do you use them?

• EXTERN
• num EXTERN ( X )
• Invokes a user-supplied external function from
  the EXTERN dll.
• The function can take a numeric or string
  parameter and calls a different user function for
  each case.
• The extern.dll library supplied by SIR contains
  dummy functions which return zero.
• EXTERNS
• str EXTERNS ( X )
• Invokes a user-supplied external function from
  the EXTERN dll.
• The function can take a numeric or string
  parameter and calls a different user function for
  each case.
• The extern.dll library supplied by SIR contains
  dummy functions which return blank (a zero length
  string).

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How do you create them?

• To create the functions you can used the supplied
  template which can be found in the api/examples
  subdirectory of the directory where SIR is
  installed.

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How do you create them?

• //
• // Skeleton of user defined EXTERN function.
• //
• // Use this to build your own extern.dll to use
  the PQL EXTERN and EXTERNS functions.
• // See the examples in the API directory.
• //
• ifndef __StdCall
• ifdef _WIN32
• define __StdCall __stdcall
• else
• define __StdCall
• endif
• endif
• //
• // The sirdbms executable links to these
  functions as defined
• //
• const char __StdCall SIRExtern1 (char
  buffer)
• double __StdCall SIRExtern2 (char buffer)

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How do you create them?

• //
• // PQL Function strEXTERNS(string)
• //
• const char __StdCall SIRExtern1 (char buffer)
  
• return ""
• //
• // PQL Function numEXTERN(string)
• //
• double __StdCall SIRExtern2 (char buffer)
• return 0
• //
• // PQL Function strEXTERNS(num)
• //
• const char __StdCall SIRExtern3 (double num)

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Examples of usage

• David Baxter has made a dll that reads web pages.
• Note not all web pages can be read. It is a
  matter of trial and error to check if the pages
  you want are readable.
• The web addresses are entered without the http//
  in the version I am using and that will be
  available on the SUG website.

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Examples of usage

• I have 5 example programs
• Writing the contents of a web page to a file.
• Reading Share prices from the msn money website.
• Reading Weather information from the BBC weather
  website.
• Reading Train Live Running from the National rail
  website.
• Reading Formula 1 results from F1 web pages

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Examples of usage

• The descriptions of the extern functionality in
  this dll are as follows
• numEXTERN(string)
• It takes a url filename and returns a socket id.
  Its quick and dirty and just goes to port 80
• strEXTERNS(num)
• Num is the socket id from before and it returns a
  single character at a time from the http server.
• numEXTERN(num)
• Num is the socket id from before and cleans up.

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Examples of usage

• The example programs use a subprocedure to read
  from the web server up to the next .
• This usually results in fairly small chunks of
  text to analyse.
• The lines are searched for predetermined text
  that allows the extraction of the required data.

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Examples of usage

• See the example programs.

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Any Questions?