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Concept
Global variable
Summary
In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the global environment or global state. In compiled languages, global variables are generally static variables, whose extent (lifetime) is the entire runtime of the program, though in interpreted languages (including command-line interpreters), global variables are generally dynamically allocated when declared, since they are not known ahead of time.
In some languages, all variables are global, or global by default, while in most modern languages variables have limited scope, generally lexical scope, though global variables are often available by declaring a variable at the top level of the program. In other languages, however, global variables do not exist; these are generally modular programming languages that enforce a module structure, or class-based object-oriented programming languages that enforce a class structure.
Interaction mechanisms with global variables are called global environment (see also global state) mechanisms. The global environment paradigm is contrasted with the local environment paradigm, where all variables are local with no shared memory (and therefore all interactions can be reconducted to message passing).
Global variables are used extensively to pass information between sections of code that do not share a caller/callee relation like concurrent threads and signal handlers. Languages (including C) where each file defines an implicit namespace eliminate most of the problems seen with languages with a global namespace though some problems may persist without proper encapsulation. Without proper locking (such as with a mutex), code using global variables will not be thread-safe except for read only values in protected memory.
Environment variables are a facility provided by some operating systems. Within the OS's shell (ksh in Unix, bash in Linux, COMMAND.COM in DOS and CMD.
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