Local variable

In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. Local variables are contrasted with global variables.

In most languages, local variables are automatic variables stored on the call stack directly. This means that when a recursive function calls itself, local variables in each instance of the function are given separate memory address space. Hence variables of this scope can be declared, written to, and read, without any risk of side-effects to processes outside of the block in which they are declared.

Programming languages that employ call by value semantics provide a called subroutine with its own local copy of the arguments passed to it. In most languages, these local parameters are treated the same as other local variables within the subroutine. In contrast, call by reference and call by name semantics allow the parameters to act as aliases of the values passed as arguments, allowing the subroutine to modify variables outside its own scope.

Some advocate that all variables should be of local scope to avoid issues with side-effects.

Static local variables

A special type of local variable, called a static local, is available in many mainstream languages (including C/C++, Visual Basic, and VB.NET) which allows a value to be retained from one call of the function to another. In this case, recursive calls to the function also have access to the (single, statically allocated) variable. In all of the above languages, static variables are declared as such with a special storage class keyword (e.g., static).

Static locals in global functions can be thought of as global variables, because their value remains in memory for the life of the program.^[1] The only difference is that they are only accessible (i.e., scoped) to one function.

This is distinct from other usages of the static keyword, which has several different meanings in various languages.

local in Perl

Perl has a keyword, local, for “localizing” variables, but in this case, local isn't what most people think of as “local”.^[2] It gives a temporary, dynamically-scoped value to a global (package) variable, which lasts until the end of the enclosing block. However, the variable is visible to any function called from within the block.^[3]

To create lexical variables, which are more like the automatic variables discussed above, use the my operator instead.^[4]

References

1. ^ Current C standardPDF (3.61 MB) (as of 2009^[update]). In particular, see section 6.2.4 “Storage durations of objects”, page 32.
2. ^ perldoc.perl.org: local
3. ^ perldoc.perl.org: perlsub: Temporary Values via local()
4. ^ perldoc.perl.org: perlsub: Private Variables via my()