bit shifting in C, unexpected result

Question

when i pass n = 0x0, i get 0xffffffff on the screen which i expect should be 0x00000000 as i shift the word by 32 bits (Just Ignore the x! I didn't use it inside the function.)

void logicalShift(int x, int n) {
    int y = 32;
    int mask = 0xffffffff;
    printf("mask %x", mask << (y-n));
}

One of the interesting point is

void logicalShift(int x, int n) {
    int y = 32;
    int mask = 0xffffffff;
    printf("mask %x", mask << 32);
}

this will output what i expected. Do i miss out anything? Thank you!

Im running on ubuntu

see http://stackoverflow.com/questions/4009885/arithmetic-bit-shift-on-a-signed-integer — Martin Beckett, Aug 16 '12 at 12:59
`logicalShift(0xAAAAAAAA, 0);` prints `0` in release and `FFFFFFFF` in debug! :) — huysentruitw, Aug 16 '12 at 13:00
possible duplicate of [Why doesn't left bit-shift, "<<", for 32-bit integers work as expected when used more than 32 times?](http://stackoverflow.com/questions/7401888/why-doesnt-left-bit-shift-for-32-bit-integers-work-as-expected-when-used) — Bo Persson, Aug 16 '12 at 16:30

score 4 · Accepted Answer · edited May 23 '17 at 10:24

4

A shift left of 32 bits on a 32 bit value has undefined results. You can only shift 0 to 31 bits.

See also here: Why doesn't left bit-shift, "<<", for 32-bit integers work as expected when used more than 32 times?

edited May 23 '17 at 10:24

Community

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answered Aug 16 '12 at 13:02

Tom Tanner

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but the interesting point is when i do the code shown in the lower part, it seems work , it output 0 – Timothy Leung Aug 16 '12 at 13:05
@TimothyLeung That's what "undefined results" mean, you can't know what it will be, you're basically lucky it works in that case. – Some programmer dude Aug 16 '12 at 13:07

Joachim Isaksson · Answer 2 · 2012-08-16T13:08:24.103

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Here is the relevant quote from the C11 draft §6.5.7.3;

If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undeﬁned.

In other words, the result is undefined, and the compiler is free to generate any result.

edited Aug 16 '12 at 13:08

answered Aug 16 '12 at 13:02

Joachim Isaksson

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Your are citing the C++ standard ... there is no C++ in the question, no? – Jens Gustedt Aug 16 '12 at 13:04
@JensGustedt Updated with the relevant part of the C11 standard, quoted the C++ standard earlier just because I had it handy. – Joachim Isaksson Aug 16 '12 at 13:09

bit shifting in C, unexpected result

2 Answers2