Question

I’ve been trying for a while pack & unpack some chars into an integer. Although there are some topics related to this question, my problem is related with the signed shift. I don’t get the ‘trick’ to unpack a signed value, i.e.:

char c1 = -119;
char c2 = 26;

// pack
int packed = (unsigned char)c1 | (c2 << 8);
// unpack
c1 = packed >> 0;
c2 = packed >> 8;

// printf(c1, c2) -> Unpacked data: -119 | 26

That works as expected but when i try to pack more data, i.e:

char c0 = -42;
char c1 = -119;
char c2 = 26;

// pack
int packed = (unsigned char)c0 | (unsigned char)(c1 << 8) | (c2 << 16);
// unpack
c0 = packed >> 0;
c1 = packed >> 8;
c2 = packed >> 16;

// printf -> Unpacked data: -42 | 0 | 26

c1 value is missed. I guess It’s related to something with the sign bit is shifted into the high-order position.

How could i get back c1 value?

Thanks in advance.

Answer 1

You are casting c1 to unsigned char after shifting it out of the range of that type, so the result of the cast is zero. You should do the cast before shifting:

int packed = (unsigned char)c0 | ((unsigned char)c1 << 8) | (c2 << 16);