How to access global variable when there is a local and global conflict

cglobal-variables

Code :

int a = 33;
int main()
{
  int a = 40; // local variables always win when there is a conflict between local and global.
  // Here how can i access global variable 'a' having value '33'.
}

If you ask : Why would someone want to do above thing? Why [a-zA-Z]* ?

My answer would be : Just to know that 'it is possible to do so'.

Thanks.

Best Answer

How about this old trick:

int main()
{
    int a = 40; // local variables always win when there is a conflict between local and global.

    {
        extern int a;
        printf("%d\n", a);
    }
}

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth 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 nth 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 nth 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 nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth 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);

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.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth 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.

When is it ok to use a global variable in C

Variables should always have a smaller scope possible. The argument behind that is that every time you increase the scope, you have more code that potentially modifies the variable, thus more complexity is induced in the solution.

It is thus clear that avoiding using global variables is preferred if the design and implementation naturally allow that. Due to this, I prefer not to use global variables unless they are really needed.

I can not agree with the 'never' statement either. Like any other concept, global variables are something that should be used only when needed. I would rather use global variables than using some artificial constructs (like passing pointers around), which would only mask the real intent.

Some good examples where global variables are used are singleton pattern implementations or register access in embedded systems.

On how to actually detect excessive usages of global variables: inspection, inspection, inspection. Whenever I see a global variable I have to ask myself: Is that REALLY needed at a global scope?