On using the mouse A brief introduction to LIBGPM the General Purpose Mouse programming interface Diversity of hardware As with SVGA devices there are computer mice from a diversity of vendors and no universal standard hardware design The customary solution a software layer is created which hides hardware differences For Linux platforms the libgpm package provides one such software layer though it was intended for text based applications Example the PS 2 Mouse The Mouse Report Format 3 bytes 7 Byte 0 6 5 4 Y data X data Y data X data over over sign sign flow flow 3 0 2 1 0 Middle Right Left Button Button Button down down down Byte 1 X coordinate data bits 7 0 Byte 2 Y coordinate data bits 7 0 Other types of computer mice use different report formats with possibly a different number of bytes e g 4 or 5 The Linux gpm package It s a mouse server for the Linux console It hides details about mouse hardware Intended for use with text based programs But we can use it with graphics programs Requires that the gpm daemon is running Type info gpm to see official information Also an online article by Pradeep Padala Programming steps Your client application must establish a connection with the gpm server daemon You need a header file include gpm h You declare an important data structure Gpm Connect conn You will need to initialize its four fields Then you call Gpm Open conn 0 Returns 1 if unsuccessful otherwise 0 Fields to be initialized conn eventMask 0 conn defaultMask 0 conn minMod 0 conn maxMod 0 events of interest to handle for you lowest modifier highest modifer Responding to mouse activity You create your own handler function for those mouse events that you wish to act upon Prototype of the handler function is int my handler Gpm Event evt void my data To install the handler use this assignment gpm handler my handler Whenever the mouse is moved or its buttons are pressed or released your function executes Useful fields in Gpm Event Gpm Event evt evt type 1 indicates a mouse move evt x evt y current mouse hot spot evt dx evt dy changes in position NOTE Remember that GPM was developed for text based applications so the hot spot coordinates are character cell locations not graphics pixel locations The GPM mouse event mask include gpm h L E A V E E N T E R H A R D M O V C L K T R I P L E D O U B L E S I N G L E U P D O W N D R A G Your event handler function will test one or more of these flag bits to determine what type of state change the mouse has undergone so that your application can respond to it in an appropriate manner M O V E A typical program loop This loop allows normal keyboard input to continue being processed e g echoed buffered while any mouse activities are processed by your handler or else by a default handler supplied by the daemon int c while c Gpm Getc stdin EOF Gpm Close A simple text mode demo Pradeep Padala has published a short C program that illustrates barebones usage of the gpm package from Linux Journal We have adapted his code for C Our demo is called trymouse cpp It s compiled like this g trymouse cpp lgpm o trymouse A simple graphics demo We have created a minimal graphics demo It shows how you could use the mouse to move a slider object e g in Pong game It s called gpmslide cpp You compile it like this g gpmslide cpp lgpm o gpmslide More elaborate graphics demo We also created a more elaborate mousedemo called seemouse cpp It defines its own custom mouse cursor It handles the four most common types of mouse events GPM MOVE GPM DOWN GPM DRAG GPM UP Our mouse cursor We built a 2 color 16 by 16 pixel image We used an array of character strings to define our image with only three character values 0 transparent color 1 background color 2 foreground color Our mouse cache Each time we want to draw our cursor we first copy the existing 16 by 16 pixel grid that occupies the planned cursor location Then we can draw our cursor threre When the mouse is moved we need to erase our cursor image by copying the saved background there and then saveand draw our cursor in the new location In class exercise 1 Can you create a different cursor image based on your own design ideas Will your design define a different hot spot Will you need to modify the boundaries to keep your cursor from going out of view Demo persists cpp We created a sound playing program that plays a continuous tone until a signal is received asynchronous notification that the user has pressed the ESCAPE key We used an ioctl command to RESET the playback device to stop the tone s sound almost immediately when our user presses the ESCAPE key In class exercise 2 Try commenting out the final ioctl call in our persists cpp demo then recompile it and execute that demo again Does the sound cease immediately Can you incorporate the sound effect into our seemouse cpp demo so that a user can start the tone by pressing the mousebutton and stop the tone by releasing it
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