Bit shift and pointer oddities in C, looking for explanations
- by foo
Hi all, I discovered something odd that I can't explain. If someone here can see what or why this is happening I'd like to know. What I'm doing is taking an unsigned short containing 12 bits aligned high like this:
1111 1111 1111 0000
I then want to shif the bits so that each byte in the short hold 7bits with the MSB as a pad. The result on what's presented above should look like this:
0111 1111 0111 1100
What I have done is this:
unsigned short buf = 0xfff;
//align high
buf <<= 4;
buf >>= 1;
*((char*)&buf) >>= 1;
This gives me something like looks like it's correct but the result of the last shift leaves the bit set like this:
0111 1111 1111 1100
Very odd. If I use an unsigned char as a temporary storage and shift that then it works, like this:
unsigned short buf = 0xfff;
buf <<= 4;
buf >>= 1;
tmp = *((char*)&buf);
*((char*)&buf) = tmp >> 1;
The result of this is:
0111 1111 0111 1100
Any ideas what is going on here?
