C variable and constant value comparison not matching -

if have:

signed char * p; 

and comparison:

if ( *p == 0xff )    break; 

it never catch 0xff, if replace -1 will:

if ( *p == (signed char)0xff )    break; 

how can happen? sign flag? though 0xff == -1 == 255.

the value 0xff signed int value. c promote *p int when doing comparison, first if statement equivalent to:

if( -1 == 255 ) break; 

which of course false. using (signed char)0xff statement equivalent to:

if( -1 == -1 ) break; 

which works expect. key point here comparison done int types instead of signed char types.