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    <p>There are two separate concepts here:</p> <ul> <li><em>scope</em>, which determines where a name can be accessed, and</li> <li><em>storage duration</em>, which determines when a variable is created and destroyed.</li> </ul> <p><em>Local</em> variables (pedantically, variables with <em>block scope</em>) are only accessible within the block of code in which they are declared:</p> <pre><code>void f() { int i; i = 1; // OK: in scope } void g() { i = 2; // Error: not in scope } </code></pre> <p><em>Global</em> variables (pedantically, variables with <em>file scope</em> (in C) or <em>namespace scope</em> (in C++)) are accessible at any point after their declaration:</p> <pre><code>int i; void f() { i = 1; // OK: in scope } void g() { i = 2; // OK: still in scope } </code></pre> <p>(In C++, the situation is more complicated since namespaces can be closed and reopened, and scopes other than the current one can be accessed, and names can also have class scope. But that's getting very off-topic.)</p> <p><em>Automatic</em> variables (pedantically, variables with <em>automatic storage duration</em>) are local variables whose lifetime ends when execution leaves their scope, and are recreated when the scope is reentered.</p> <pre><code>for (int i = 0; i &lt; 5; ++i) { int n = 0; printf("%d ", ++n); // prints 1 1 1 1 1 - the previous value is lost } </code></pre> <p><em>Static</em> variables (pedantically, variables with <em>static storage duration</em>) have a lifetime that lasts until the end of the program. If they are local variables, then their value persists when execution leaves their scope.</p> <pre><code>for (int i = 0; i &lt; 5; ++i) { static int n = 0; printf("%d ", ++n); // prints 1 2 3 4 5 - the value persists } </code></pre> <p>Note that the <code>static</code> keyword has various meanings apart from static storage duration. On a global variable or function, it gives it <em>internal linkage</em> so that it's not accessible from other translation units; on a C++ class member, it means there's one instance per class rather than one per object. Also, in C++ the <code>auto</code> keyword no longer means automatic storage duration; it now means automatic type, deduced from the variable's initialiser.</p>
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    1. COthnks for answer, but i still cant get the diff between auto and local variable, isn't they both have storage duration of the running function, please clarify.
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    2. CO@user1779646: "automatic" means they have the *storage duration* of the current block, and are destroyed when leaving the block. "local" means they have the *scope* of the current block, and can't be accessed from outside the block. If a local variable is `static`, then it is not destroyed when leaving the block; it just becomes inaccessible until the block is reentered. See the `for`-loop examples in my answer for the difference between automatic and static storage duration for local variables.
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    3. COthanks, i finally understood it.
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