volatile in struct

[Juergen Zeller]

Hello all,

I'm confused, I need help from the specialists!

I'm using rtems4.6.0 with an i386ex board. I was looking for an error 
for a while and now I figured out that it depends on a volatile 
definition in the following manner:

#if 0
#define VOLATILE /*THIS WORKS for millions of packets without the 
printk-msg 'lost bytes...' */
#else
#define VOLATILE volatile /*here I get the printk-msg 'lost bytes...' 
sometimes (about once per 2000 packets)*/
#endif


typedef struct {

...
unsigned char txbuf[256];
VOLATILE unsigned char txwr; // tx write pointer
VOLATILE unsigned char txrd; // tx read pointer
VOLATILE unsigned char txcount; // tx byte counter
VOLATILE unsigned char txmsgcount; // tx packet counter
...
} MS3_COMDATA;


static rtems_task tx_task (rtems_task_argument arg)
{
...
while (1) {

...
rtems_interrupt_disable (l);
com->txcount -= count;
com->txmsgcount--;
rtems_interrupt_enable (l);
...
}
}


int send (MS3_COMDATA *com, something else )
{
...
rtems_semaphore_obtain (com->sema, RTEMS_NO_TIMEOUT, 0);

/* if the last message was sent there shouldn't remain any bytes in the 
buffer*/
if (!com->txmsgcount && com->txcount) {
printk ("%d lost bytes in txbuffer[%d](%d)\n", com->txcount, port, 
counter[port]);
com->txcount = 0;
}
... /*Code for putting it into txbuf*/

com->txmsgcount++;
rtems_semaphore_release (com->sema);

}


Can anyone out there explain whats going on here? I never thought that 
volatile could be responsible for errors. Is it because it's inside a 
struct?

regards

juergen