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Automatic variable
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<p>In <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, an automatic variable is a local <a href="/facts/Variable_(programming)/VoCKtwml">variable</a> which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The <a href="/facts/Scope_(computer_science)/bbj6AFMo">scope</a> is the lexical context, particularly the function or block in which a variable is defined. Local data is typically (in most languages) invisible outside the function or lexical context where it is defined. Local data is also invisible and inaccessible to a <i>called</i> function, but is not deallocated, coming back in scope as the <a href="/facts/Execution_thread/hh8WHyPd">execution thread</a> returns to the caller. </p><p>Automatic local variables primarily applies to <a href="/facts/Recursion_(computer_science)/2uXo18Gv"> recursive</a> <a href="/facts/Scope_(programming)/bbj6AFMo">lexically-scoped</a> languages. Automatic local variables are normally <a href="/facts/Stack-based_memory_allocation/Wem3dQa6">allocated in the stack frame</a> of the procedure in which they are declared. This was originally done to achieve <a href="/facts/Reentrant_(subroutine)/VMYdeEvw">re-entrancy</a> and allowing <a href="/facts/Recursion/CxSKxJoW">recursion</a>, a consideration that still applies today. The concept of automatic variables in recursive (and <a href="/facts/Nested_function/Rw2xzMLP">nested</a>) functions in a lexically scoped language was introduced to the wider audience with <a href="/facts/ALGOL/wlFJGNEx">ALGOL</a> in the late 1950s, and further popularized by its many descendants. </p><p>The term <i><a href="/facts/Local_variable/q8sIjyhx">local variable</a></i> is usually synonymous with automatic variable, since these are the same thing in many programming languages, but local is more general – most local variables are automatic local variables, but <a href="/facts/Static_local_variable/q8sIjyhx">static local variables</a> also exist, notably in C. For a static local variable, the allocation is static (the lifetime is the entire program execution), not automatic, but it is only in scope during the execution of the function. </p>