Ned Konz wrote:
Ron wrote:
Hi Folks. I would have thought that with -Os the code below should work.
static volatile char* Ptr;
.
.
static Send(char* p)
{
if (p)
{
while (Ptr != NULL) {;}
Ptr = p;
UDR1 = *Ptr++;
}
}
.
.
SIGNAL(SIG_UART1_TRANS)
{
if (*Ptr == '\0')
Ptr = NULL;
else
UDR1 = *Ptr++;
}
However, the while loop becomes:
LDS R24,0x018D
LDS R25,0x018E
SBIW R24,0x00
BRNE PC-0x01
The code is as expected with -O0. Version is 3.4.6. Something I'm
missing?
Probably.
You didn't declare Ptr to be volatile.
Instead you declared Ptr to *point to* a volatile char.
So in the while loop (while you're comparing Ptr to NULL) there's no
reason to re-read Ptr because it's not volatile.
That's correct. In particular,
"volatile char* ptr" parses as "(volatile char) *ptr".
What you want is "char * volatile ptr", meaning that the pointer itself
is volatile, not the thing it points to.
A good rule is that a variable declaration should not consist of more
than two parts (excluding "static" or "extern", and the variable
itself). Any time you need more complex types, use typedefs. Thus:
typedef char *pchar;
volatile pchar Ptr; // volatile pointer to a char
typedef volatile char vchar;
vchar *Ptr; // pointer to volatile char
typedef vchar *pvchar;
volatile pvchar Ptr; // volatile pointer to volatile char
Then there is (almost!) no chance of getting things wrong.
mvh.,
David
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