%02X doesn't work in printf (Debian 5)

[Shimmey]

Debian Release 5.0.4 (lenny)
Kernel Linux 2.6.26-2-686
gcc (Debian 4.3.2-1.1) 4.3.2

The format character "%02X" doesn't work when and only when the value of char is "0xFF"in the printf function.
I got 8 Fs instead of 2 Fs.

Here is an example:
PROGRAM:

int main (int argc, char *argv[])
{
char xc[3];
char *p = xc;

memset ( xc, 0, 3);
xc[1] = 0xff;

for ( p = xc; p < xc + 3; p++)
printf ("%02X\n", *p);

return 0;
}

RESULT:
00
FFFFFFFF
00

I don't know why. Who could tell me the reason of that?
Thanks very much!

[Rubberman]

Try %.02x instead - the dot is essential here. Read the printf man pages for more information about what the formatting strings REALLY do. In your case, it specifies a MINIMUM field width of 2 characters, zero padded. The char input is interpreted as an integer. The dot means you want 2 digits of PRECISION, so it will only print 2 characters of precision, giving you the results you expect.

[Shimmey]

Oh, I see!
Thanks very much!

The parameter was converted to int type first in the printf function.
In the converting process, the C extend the char type into int by extending the sign bit.
So, char is signed int type in fact by default.
I changed the type into unsighed char, and it works very well now.

Thanks for your explanation very much!

C is really a wonderful programming language.