Sun Microsystems Multitasking Safety Example, Code Example 2-1Native API for a Microwave Oven

You might find that you cannot organize the singleton’s maintenance in this way, because its state must be updated synchronously and atomically. Maintaining the foreground state is an example of this type of singleton. In this case, migrate a key piece of state into native memory and handle updates through calls to native methods.

Multitasking Safety Example

Consider a simple though somewhat contrived example of controlling an external device, a microwave oven. Assume that this device has a native API defined in the header file shown in CODE EXAMPLE 2-1.

CODE EXAMPLE 2-1Native API for a Microwave Oven

/* mw.h */

/*

*Status codes passed to cook callback. The cooking might have been

*interrupted, for instance, if the user pressed the STOP button or

*opened the oven door.

*/

typedef enum {

MW_DONE, /* cooking finished normally */ MW_INTERRUPTED, /* cooking interrupted */

} MWSTATUS;

/* Callback for the cook operation. */

typedef void (*MWCB)(MWSTATUS status, void *context);

/* Initializes the microwave oven. Must be called exactly once. */ extern void mw_init(void);

/* Sets the cook time for the next cook operation, in seconds. */ extern void mw_settime(int nsec);

/*

*Sets the cook power for the next cook operation, an integer

*in the range [1-100].

*/

extern void mw_setpower(int power);

/*

*Initiates the cooking operation. When the cooking finishes,

*the callback is called with the status code. The context pointer

*is passed to the callback for its own use, unmodified by the

*library.

Chapter 2 Multitasking Safety 9