Ever wondered why C does not have a built-in operator to get the number of elements in an array? That's because such functionality can be easily implemented by a programmer in a one-line macro:
/* Returns the number of elements in the array */
#define NumElm(array) (sizeof (array) / sizeof ((array)[0]))
This macro computes the number of elements in an array by dividing the total size of an array by the size of a single element. The most important point to note is that this calculation is done at compile time; this not only makes the use of this macro as efficient at run-time as replacing the macro call with a constant, but also allows the use of the macro call in a context where constant expressions are required, so you can write something like this:
int a[10];
int b[NumElm(a)];/* equivalent to int b[10]; */
This is a standard way to calculate array length in C; it is described in K&R.
Note that this works only with "real" arrays; the result of this macro when given a pointer as an argument is meaningless (there is no general way to get a number of objects pointed to by a pointer -- pointers to objects and pointers to arrays of objects are the same thing in C. That's a feature.)
