In C89 and C++, array dimensions must be declared using integer constant expressions. This allows the compiler to compute the array's size at compile time. In C99, this rule was relaxed: you can now use any integer expression to declare array dimensions. The size of such an array is known only at runtime. For example:
int func(int dim)
int arr[dim]; // possible only in C99
The number of elements in arr may change every time func() is called. For example:
printf("enter array's size : ");
Because the size of a variable length array can't be determined at compile-time, C99 also changed the sizeof operator. In general, sizeof calculates the size of an object at compile time. However, when applied to variable length arrays, sizeof calculates the array's size at runtime. Remember that variable length arrays aren't dynamic arrays. That is, they can't change their size during their lifetime. They differ from ordinary arrays in that they may have a different size every time their declaration is encountered. Variable arrays can only be local. Note that C++ doesn't support variable length arrays. However, because many C++ compilers are also C compilers, certain C++ compilers may support this feature as a non-standard extension.