Setting a bit
Use the bitwise OR operator (|
) to set a bit.
number |= 1UL << n;
That will set the n
th bit of number
. n
should be zero, if you want to set the 1
st bit and so on upto n-1
, if you want to set the n
th bit.
Use 1ULL
if number
is wider than unsigned long
; promotion of 1UL << n
doesn't happen until after evaluating 1UL << n
where it's undefined behaviour to shift by more than the width of a long
. The same applies to all the rest of the examples.
Clearing a bit
Use the bitwise AND operator (&
) to clear a bit.
number &= ~(1UL << n);
That will clear the n
th bit of number
. You must invert the bit string with the bitwise NOT operator (~
), then AND it.
Toggling a bit
The XOR operator (^
) can be used to toggle a bit.
number ^= 1UL << n;
That will toggle the n
th bit of number
.
Checking a bit
You didn't ask for this, but I might as well add it.
To check a bit, shift the number n to the right, then bitwise AND it:
bit = (number >> n) & 1U;
That will put the value of the n
th bit of number
into the variable bit
.
Changing the nth bit to x
Setting the n
th bit to either 1
or 0
can be achieved with the following on a 2's complement C++ implementation:
number ^= (-x ^ number) & (1UL << n);
Bit n
will be set if x
is 1
, and cleared if x
is 0
. If x
has some other value, you get garbage. x = !!x
will booleanize it to 0 or 1.
To make this independent of 2's complement negation behaviour (where -1
has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.
number ^= (-(unsigned long)x ^ number) & (1UL << n);
or
unsigned long newbit = !!x; // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);
It's generally a good idea to use unsigned types for portable bit manipulation.
or
number = (number & ~(1UL << n)) | (x << n);
(number & ~(1UL << n))
will clear the n
th bit and (x << n)
will set the n
th bit to x
.
It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.
Remove existing files from the repository:
find . -name .DS_Store -print0 | xargs -0 git rm -f --ignore-unmatch
Add this line:
.DS_Store
to the file .gitignore
, which can be found at the top level of your repository (or create the file if it isn't there already). You can do this easily with this command in the top directory:
echo .DS_Store >> .gitignore
Then commit the file to the repo:
git add .gitignore
git commit -m '.DS_Store banished!'
Best Answer
The CPU can be switched from 64 bit execution mode to 32 bit when it traps into kernel context, and a 32 bit kernel can still be constructed to understand the structures passed in from 64 bit user-space apps.
The MacOS X kernel does not directly dereference pointers from the user app anyway, as it resides its own separate address space. A user-space pointer in an ioctl call, for example, must first be resolved to its physical address and then a new virtual address created in the kernel address space. It doesn't really matter whether that pointer in the ioctl was 64 bits or 32 bits, the kernel does not dereference it directly in either case.
So mixing a 32 bit kernel and 64 bit binaries can work, and vice-versa. The thing you cannot do is mix 32 bit libraries with a 64 bit application, as pointers passed between them would be truncated. MacOS X supplies more of its frameworks in both 32 and 64 bit versions in each release.