What the...? casting bug?

[gnormhurst]

'splain this, somebody:

#define STRAIGHT_DRIVE_DIRECTION ( 127 + 30 )

static int steeringValue = 0;
. . .
steeringValue = (int) ((STRAIGHT_DRIVE_DIRECTION));
printf( "nav: DTC %3d ", steeringValue );

That printed a value of -99! Why?

Changing the assignment to:

steeringValue = (int) ((unsigned char) (STRAIGHT_DRIVE_DIRECTION));

made it print "157" as expected the first time.

Is this weird, or is there something about type casting I don't understand?

[gnormhurst]

Ohhhhhhh!

Funny how you understand something as soon as you post it.

The compiler takes (127 + 30) and performs signed 8-bit arithmetic, yielding -99. Casting as a signed (int) duly kept the sign: -99. Casting as an unsigned char reinterpreted the value as 157, which was then cast as a signed int: 157.

My error was in assuming that constants were treated as unsigned chars, but they are treated as signed chars.

[Ryan M.]

Ohhhhhhh!

Funny how you understand something as soon as you post it.

The compiler takes (127 + 30) and performs signed 8-bit arithmetic, yielding -99. Casting as a signed (int) duly kept the sign: -99. Casting as an unsigned char reinterpreted the value as 157, which was then cast as a signed int: 157.

My error was in assuming that constants were treated as unsigned chars, but they are treated as signed chars.That is exactly what happened. :)

[Greg Ross]

Boy, you guys are fast. :)

Norm, I think if you had used

#define STRAIGHT_DRIVE_DIRECTION ( 127u + 30 )you would have been ok.

[Astronouth7303]

There's a compuler option for that. Project>Build Options...>Project. Go to MPLAB C18. check "Treat 'char' as unsigned" (under Category General). The command-line flag is '-k'