C variable and constant value comparison not matching -

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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.


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