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Converting Existing CGI Scripts to ISAPI Extensions

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This section explains the basic requirements for converting existing CGI script executable files to ISAPI extension DLLs. As with other DLLs, Web server applications must be thread-safe. As more than one client will be running the same function at the same time, the code must follow safety procedures in modifying a global or static variable.

You can keep your ISAPI extension thread-safe by using appropriate synchronization techniques, such as creating critical sections and semaphores. For additional information about writing thread-safe DLLs, see the documentation in the Platform SDK and in the Microsoft Development Library.

There are several structural differences between an ISAPI extension and a CGI executable file. An ISAPI extension:

  • Receives most of its data through the lpbData member of the EXTENSION_CONTROL_BLOCK structure, as opposed to reading the data from the standard input (STDIN) handle for the process. For any additional data, the extension uses the ReadClient Function callback function.
  • Sends data back to the client with the WriteClient Function callback function instead of writing to the standard output (STDOUT) handle for the process.
  • Accesses common CGI variables through the EXTENSION_CONTROL_BLOCK structure. For other variables, GetServerVariable Function is called. In a CGI executable file, these are retrieved from the environment table by using getenv.
  • Specifies completion status by either sending the header directly using the WriteClient Function callback function, or calls the HSE_REQ_SEND_RESPONSE_HEADER_EXServerSupportFunction Function, instead of sending a "Status: NNN xxxxx..." to STDOUT.
  • Redirects requests with a Location: or URL: header. If the URL is local, instead of writing the header to STDOUT, use the HSE_REQ_SEND_URL. However, if the URL is remote or unknown, use HSE_REQ_SEND_URL_REDIRECT_RESP in the ServerSupportFunction Function callback function.