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ccastingcharunsignedbit-masks

what is diffrence bettween masking a char with 0xFF and casting (unsigned char)

    unsigned char Mid;

if( (data[2]) == 0x9A){
    Mid = data[5];

    if( (Mid == 1) || (Mid == 2) || (Mid == 3) )
        return(Mid);

The code above gives:

warning: comparison is always false due to limited range of data type

error as I expected.

It's ok if I type

if( (unsigned char)data[2] == 0x9A){

or

if( (data[2]&0xFF) == 0x9A){

I understand the first one, but what happens in the second one?

What is the effect of masking with 0xFF?

Solution

    • When casting the value to unsigned char, you are doing an explicit type conversion.

    • When you mask the value with 0xFF, an implicit type conversion takes place.

    You have the signed char data[2] and 0xFF as operands to the bitwise AND operator. Because one of the operands (0xFF) cannot fit inside a signed char, both operands are implicitly converted into an unsigned type.