bit shifting in c returning wrong result

Question

I'm writing code to translate machine code. readFiveBytes is inside of a loop and supposed to take one byte at a time and place each byte into its correct place and return an address in the end.

void readFiveBytes(int c) {
    if (counter != 0) {
        currentIns = currentIns | (c << (counter * 4));
        printf("reading byte: %X; address: %08X; counter: %d; type: 5\n",c, currentIns, counter);
        counter = counter - 2;
    } else {
        currentIns = currentIns | c;
        printType3(currentIns);
        printf("reading byte: %X; address: %08X; counter: %d; type: 5\n",c, currentIns, counter);
        currentIns = 0x00000000000000;
        isFinishReading = true;
    }
}

the output is

reading byte: 71; address: 00000071; counter: 8; type: 5
reading byte: 26; address: 26000071; counter: 6; type: 5
reading byte: 0; address: 26000071; counter: 4; type: 5
reading byte: 0; address: 26000071; counter: 2; type: 5
reading byte: 0; address: 26000071; counter: 0; type: 5

I'm wondering why the first byte has not been shifted to the most left? (It seems like second one is working well)

"bit shifting in c not working properly" – gotta love these " not working" questions… surely it **is** working? At most *you* can't use it properly… — The Paramagnetic Croissant, Feb 07 '15 at 18:07
This type of function is the reason why people say that global variables are bad. This function doesn't even do anything by itself, it's the body of a loop that has been forcefully removed from its rightful place, and now communicates through some globals in a way that is not obvious and, from the looks of it, broken. Don't even fix this function, replace it by something better. — harold, Feb 07 '15 at 18:16

score 1 · Accepted Answer · answered Feb 07 '15 at 18:14

1

Shifting the bit width or more is undefined behavior.

The first pass, counter = 8 attempts currentIns = currentIns | (c << 32);. Assuming currentIns is a 32-bit integer, this is undefined behavior.

The typical undefined behavior in this case is only a shift of (counter * 4)%32 occurs. With counter = 8, this is a shift of 0.

answered Feb 07 '15 at 18:14

chux - Reinstate Monica

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currentIns's type is unsigned and unsigned int is 32 bit. That's why it is not working. Thank you! – user3397656 Feb 07 '15 at 18:20

bit shifting in c returning wrong result

1 Answers1