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<p>A function is a C language construct that associates a <a href="statements.html#Compound_statements" title="c/language/statements">compound statement</a> (the function body) with an <a href="identifier.html" title="c/language/identifier">identifier</a> (the function name). Every C program begins execution from the <a href="main_function.html" title="c/language/main function">main function</a>, which either terminates, or invokes other, user-defined or library functions.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co1">// function definition.</span>
<span class="co1">// defines a function with the name "sum" and with the body "{ return x+y; }"</span>
<span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span> x, <span class="kw4">int</span> y<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> x <span class="sy2">+</span> y<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Functions may accept zero or more <i>parameters</i>, which are initialized from the <i>arguments</i> of a <a href="operator_other.html#Function_call" title="c/language/operator other">function call operator</a>, and may return a value to its caller by means of the <a href="return.html" title="c/language/return">return statement</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> n <span class="sy1">=</span> sum<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// parameters x and y are initialized with the arguments 1 and 2</span></pre></div></div>
<p>The body of a function is provided in a <a href="function_definition.html" title="c/language/function definition">function definition</a>. Each function must be defined only once in a program, unless the function is <a href="inline.html" title="c/language/inline">inline</a>.
</p><p>There are no nested functions (except where allowed through non-standard compiler extensions): each function definition must appear at file scope, and functions have no access to the local variables from the caller:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="co1">// the main function definition</span>
<span class="br0">{</span>
<span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// function declaration (may appear at any scope)</span>
<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// local variable in main</span>
sum<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// function call</span>
 
<span class="co1">// int sum(int a, int b) // error: no nested functions</span>
<span class="co1">// {</span>
<span class="co1">// return a + b; </span>
<span class="co1">// }</span>
<span class="br0">}</span>
<span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="co1">// function definition</span>
<span class="br0">{</span>
<span class="co1">// return x + a + b; // error: main's x is not accessible within sum</span>
<span class="kw1">return</span> a <span class="sy2">+</span> b<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/functions&action=edit&section=1" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 6.7.6.3 Function declarators (including prototypes) (p: 133-136)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.9.1 Function definitions (p: 156-158)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 6.7.5.3 Function declarators (including prototypes) (p: 118-121)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.9.1 Function definitions (p: 141-143)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990):
</li></ul>
<dl><dd><ul><li> 3.5.4.3 Function declarators (including prototypes)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.7.1 Function definitions
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/functions&action=edit&section=2" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../cpp/language/function.html" title="cpp/language/function">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Declaring functions</span></span></div>
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