variable

早期freebsd实现

Variable declarations

	Variables can be declared as either being global, local, or static.
	Global variables are visible to all functions and on the command
	line, and are permanent.  Local variables are visible only within
	a single function or command sequence.  When the function or command
	sequence returns, the local variables are deleted.  Static variables
	are permanent like global variables, but are only visible within the
	same input file or function where they are defined.

	To declare one or more variables, the 'local', 'global', or 'static'
	keywords are used, followed by the desired list of variable names,
	separated by commas.  The definition is terminated with a semicolon.
	Examples of declarations are:

		local	x, y, z;
		global	fred;
		local	foo, bar;
		static	var1, var2, var3;

	Variables may have initializations applied to them.  This is done
	by following the variable name by an equals sign and an expression.
	Global and local variables are initialized each time that control
	reaches them (e.g., at the entry to a function which contains them).
	Static variables are initialized once only, at the time that control
	first reaches them (but in future releases the time of initialization
	may change).  Unlike in C, expressions for static variables may
	contain function calls and refer to variables.  Examples of such
	initializations are:

		local	a1 = 7, a2 = 3;
		static	b = a1 + sin(a2);

	Within function declarations, all variables must be defined.
	But on the top level command line, assignments automatically define
	global variables as needed.  For example, on the top level command
	line, the following defines the global variable x if it had not
	already been defined:

		x = 7

	The static keyword may be used at the top level command level to
	define a variable which is only accessible interactively, or within
	functions defined interactively.

	Variables have no fixed type, thus there is no need or way to
	specify the types of variables as they are defined.  Instead, the
	types of variables change as they are assigned to or are specified
	in special statements such as 'mat' and 'obj'.  When a variable is
	first defined using 'local', 'global', or 'static', it has the
	value of zero.

	If a procedure defines a local or static variable name which matches
	a global variable name, or has a parameter name which matches a
	global variable name, then the local variable or parameter takes
	precedence within that procedure, and the global variable is not
	directly accessible.

	The MAT and OBJ keywords may be used within a declaration statement
	in order to initially define variables as that type.  Initialization
	of these variables are also allowed.  Examples of such declarations
	are:

		static mat table[3] = {5, 6, 7};
		local obj point p1, p2;

	There are no pointers in the calculator language, thus all
	arguments to user-defined functions are normally passed by value.
	This is true even for matrices, strings, and lists.  In order
	to circumvent this, the '&' operator is allowed before a variable
	when it is an argument to a function.  When this is done, the
	address of the variable is passed to the function instead of its
	value.  This is true no matter what the type of the variable is.
	This allows for fast calls of functions when the passed variable
	is huge (such as a large array).  However, the passed variable can
	then be changed by the function if the parameter is assigned into.
	The function being called does not need to know if the variable
	is being passed by value or by address.

	Built-in functions and object functions always accept their
	arguments as addresses, thus there is no need to use '&' when
	calling built-in functions.