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1. A hardware timer is programmed to periodically inerrupt the CPU (called timer interrupt).Time elapsed between two successive interrupts is called a jiffie.Which is 10ms in case of Linux system....

one more good example is to calculate factoraial.

factorial(int n)
{
if(n==1)
return 1;
...

You use extern to DECLARE a variable.It tells the compiler to look for the DEFINITON of the variable in one of the included header files. No memory is allocated when you declare it using extern.

will this also work?
int i;
i='A';
printf("%d",i)//prints ASCII value of 'A'