Instructions for writing a Desktop Picture Reader Extension for DESKPIC

By John Ridges

Good for you! You've decided to write an Desktop Picture Reader Extension. What do you need to know to write an extension? Well, a pretty good working knowledge of Presentation Manager wouldn't hurt. These instructions will attempt to tell you how to write an extension, but they won't tell you how to program in PM. To help, an example extension is provided and is a good place to start. When I write a new extension, I usually use 'PATTERN' as a framework and go from there.

A Desktop Picture Reader Extension is a Dynamic Link Library that contains four functions and has the file extension of '.DPR' instead of '.DLL'. The four functions must be exported with the following ordinals: deskpicextension @1 deskpicgetsize   @2 deskpicread      @3 deskpicinfo      @4

The four functions are specified by the following function prototypes: char far * far pascal _loadds deskpicextension(HMODULE); ULONG far pascal _loadds deskpicgetsize(HFILE); BOOL far pascal _loadds deskpicread(HFILE, SEL, BITMAPINFO far *); void far pascal _loadds deskpicinfo(char far *, HWND);

deskpicextension
This function is called by DESKPIC to determine the file type that the Desktop Picture Reader Extension can read. DESKPIC passes the module handle of the Desktop Picture Reader Extension in case any local resources need to be accessed. The function should return a pointer to a zero terminated string that contains a file extension that specifies the type of files that the Desktop Picture Reader Extension can read. The string must begin with a period and be in upper case (Example: ".XYZ"). This function should not allocate any resources because DESKPIC may call it and then choose not to call the other three functions. Note that while DESKPIC will call this function before calling the other three, it may call this function multiple times.

deskpicgetsize
This function is called by DESKPIC to determine the size of the area needed to contain the bitmap image data. DESKPIC passes an open file handle of the file it chose to be the desktop picture, with the file pointer at the beginning of the file. The function should read whatever information it needs to determine the size of the bitmap image data, and return that size (in bytes) to DESKPIC. In case of an error, the function should return zero.

deskpicread
This function is called by DESKPIC to read the bitmap image data. DESKPIC passes the open file handle of the desktop picture file (with the file pointer positioned wherever 'deskpicgetsize' left it), a selector to the bitmap image data area, and a pointer to the BITMAPINFO structure. The function should fill in both the bitmap image data area and the BITMAPINFO structure with information read from the picture file. When the function is finished, the data should conform to the formats of 'pbData' and 'pbmiData' for the OS/2 function GpiCreateBitmap. If the function is successful it should return FALSE, otherwise it should return TRUE in the case of an error. Since this function performs the bulk of the work of the Desktop Picture Reader Extension, it may use up to 16K of stack space. Note that the function should not close the file handle, as DESKPIC will do this itself.

deskpicinfo
DESKPIC calls this procedure when the user has clicked on the "info" button in the DESKPIC control dialog. Usually the procedure brings up a message box with credits and instructions, but a full fledged dialog box can be invoked if desired. DESKPIC passes a pointer to the pathname of the picture currently being displayed, and the handle of DESKPIC's control dialog window. Note that this procedure is optional: if the "deskpicinfo" ordinal is not exported, a default message box will be invoked.

That completes the explanation. Where the instructions are a little sketchy, use the example source code to see at least one way of doing it. Good luck, and may the bits be with you.