Interruption program

Can you put a picture of your design to well understand the context please.

Else, the variable context here is just to save values during ISR. For example in the count_binary example, you have the following :

 1 /* A variable to hold the value of the button pio edge capture register. */
 2 volatile int edge_capture;
 3 static void init_button_pio()
 4 {
 5     /* Recast the edge_capture pointer to match the alt_irq_register() function
 6      * prototype. */
 7     void* edge_capture_ptr = (void*) &edge_capture;
 8     /* Enable all 4 button interrupts. */
 9     IOWR_ALTERA_AVALON_PIO_IRQ_MASK(BUTTON_PIO_BASE, 0xf);
10     /* Reset the edge capture register. */
11     IOWR_ALTERA_AVALON_PIO_EDGE_CAP(BUTTON_PIO_BASE, 0x0);
12     /* Register the interrupt handler. */
13     alt_irq_register( BUTTON_PIO_IRQ, edge_capture_ptr,
14                       handle_button_interrupts );
15 }
16 static void handle_button_interrupts(void* context, alt_u32 id)
17 {
18     /* Cast context to edge_capture's type. It is important that this be 
19      * declared volatile to avoid unwanted compiler optimization.
20      */
21     volatile int* edge_capture_ptr = (volatile int*) context;
22     /* Store the value in the Button's edge capture register in *context. */
23     *edge_capture_ptr = IORD_ALTERA_AVALON_PIO_EDGE_CAP(BUTTON_PIO_BASE);
24     /* Reset the Button's edge capture register. */
25     IOWR_ALTERA_AVALON_PIO_EDGE_CAP(BUTTON_PIO_BASE, 0);
26     IORD_ALTERA_AVALON_PIO_EDGE_CAP(BUTTON_PIO_BASE); //An extra read call to clear of delay through the bridge
27 
28 }

In the ISR, you get the value of the edge capture register to know which button has been pushed, and store it at the address pointed by edge_capture_ptr. This variable point to the same address as context (line 21). And context has the same address as edge_capture variable (line 13 and 7).