Are the shift operators (<<, >>) arithmetic or logical in C?

When shifting left, there is no difference between arithmetic and logical shift. When shifting right, the type of shift depends on the type of the value being shifted.

(As background for those readers unfamiliar with the difference, a "logical" right shift by 1 bit shifts all the bits to the right and fills in the leftmost bit with a 0. An "arithmetic" shift leaves the original value in the leftmost bit. The difference becomes important when dealing with negative numbers.)

When shifting an unsigned value, the >> operator in C is a logical shift. When shifting a signed value, the >> operator is an arithmetic shift.

For example, assuming a 32 bit machine:

signed int x1 = 5;
assert((x1 >> 1) == 2);
signed int x2 = -5;
assert((x2 >> 1) == -3);
unsigned int x3 = (unsigned int)-5;
assert((x3 >> 1) == 0x7FFFFFFD);

According to K&R 2nd edition the results are implementation-dependent for right shifts of signed values.

Wikipedia says that C/C++ 'usually' implements an arithmetic shift on signed values.

Basically you need to either test your compiler or not rely on it. My VS2008 help for the current MS C++ compiler says that their compiler does an arithmetic shift.

In terms of the type of shift you get, the important thing is the type of the value that you're shifting. A classic source of bugs is when you shift a literal to, say, mask off bits. For example, if you wanted to drop the left-most bit of an unsigned integer, then you might try this as your mask:

~0 >> 1

Unfortunately, this will get you into trouble because the mask will have all of its bits set because the value being shifted (~0) is signed, thus an arithmetic shift is performed. Instead, you'd want to force a logical shift by explicitly declaring the value as unsigned, i.e. by doing something like this:

~0U >> 1;

Here are functions to guarantee logical right shift and arithmetic right shift of an int in C:

int logicalRightShift(int x, int n) {
    return (unsigned)x >> n;
}
int arithmeticRightShift(int x, int n) {
    if (x < 0 && n > 0)
        return x >> n | ~(~0U >> n);
    else
        return x >> n;
}

Well, I looked it up on wikipedia, and they have this to say:

C, however, has only one right shift operator, >>. Many C compilers choose which right shift to perform depending on what type of integer is being shifted; often signed integers are shifted using the arithmetic shift, and unsigned integers are shifted using the logical shift.

So it sounds like it depends on your compiler. Also in that article, note that left shift is the same for arithmetic and logical. I would recommend doing a simple test with some signed and unsigned numbers on the border case (high bit set of course) and see what the result is on your compiler. I would also recommend avoiding depending on it being one or the other since it seems C has no standard, at least if it is reasonable and possible to avoid such dependence.

gcc will typically use logical shifts on unsigned variables and for left-shifts on signed variables. The arithmetic right shift is the truly important one because it will sign extend the variable.

gcc will will use this when applicable, as other compilers are likely to do.

GCC does

1. for -ve - > Arithmetic Shift

2. For +ve -> Logical Shift

According to many c compilers:

1. << is an arithmetic left shift or bitwise left shift.
2. >> is an arithmetic right shiftor bitwise right shift.