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<p>Every C program coded to run in a hosted execution environment contains the definition (not the prototype) of a function called <code><b>main</b></code>, which is the designated start of the program.
</p>
<table class="t-sdsc-begin">
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</td></tr>
<tr class="t-sdsc">
<td> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> <code><b>main</b></code> <code><b>(void)</b></code> <code><b>{</b></code> <span class="t-spar">body</span> <code><b>} </b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> <code><b>main</b></code> <code><b>(</b></code><span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> <span class="t-spar">argc</span><code><b>,</b></code> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">char</span></span></span> <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span></span></span><span class="t-spar">argv</span><span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">]</span></span></span><code><b>)</b></code> <code><b>{</b></code> <span class="t-spar">body</span> <code><b>} </b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
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<td> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> <code><b>main</b></code> <code><b>(</b></code><span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> <span class="t-spar">argc</span><code><b>,</b></code> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">char</span></span></span> <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span></span></span><span class="t-spar">argv</span><span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">]</span></span></span> <code><b>,</b></code> <span class="t-spar">other_parameters</span> <code><b>)</b></code> <code><b>{</b></code> <span class="t-spar">body</span> <code><b>} </b></code>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <span class="t-c"><span class="mw-geshi c source-c"><span class="coMULTI">/* another implementation-defined signature */</span></span></span>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
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<p><br />
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="main_function.html#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="main_function.html#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="main_function.html#Explanation"><span class="tocnumber">3</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="main_function.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="main_function.html#References"><span class="tocnumber">5</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="main_function.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=1" title="Edit section: Parameters">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td> argc
</td>
<td> -
</td>
<td> Non-negative value representing the number of arguments passed to the program from the environment in which the program is run.
</td></tr>
<tr class="t-par">
<td> argv
</td>
<td> -
</td>
<td> Pointer to an array of pointers to null-terminated multibyte strings that represent the arguments passed to the program from the execution environment (<span class="t-c"><span class="mw-geshi c source-c">argv<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span></span></span> through <span class="t-c"><span class="mw-geshi c source-c">argv<span class="br0">[</span>argc<span class="sy2">-</span><span class="nu0">1</span><span class="br0">]</span></span></span>). The value of <span class="t-c"><span class="mw-geshi c source-c">argv<span class="br0">[</span>argc<span class="br0">]</span></span></span> is guaranteed to be <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">0</span></span></span>.
</td></tr>
<tr class="t-par">
<td> body
</td>
<td> -
</td>
<td> The body of the main function
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">other_parameters</span>
</td>
<td> -
</td>
<td> Implementations may allow additional forms of the main function. A very common extension is passing a third argument of type <code>char*[]</code> pointing at <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html">an array of pointers to the execution environment variables</a>.
</td></tr></table>
<p>The names <code>argc</code> and <code>argv</code> are arbitrary, as well as the representation of the types of the parameters: <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> ac, <span class="kw4">char</span><span class="sy2">**</span> av<span class="br0">)</span></span></span> is equally valid.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=2" title="Edit section: Return value">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span></h3>
<p>If the return statement is used, the return value is used as the argument to the implicit call to <span class="t-lc"><a href="../program/exit.html" title="c/program/exit">exit()</a></span> (see below for details). The values zero and <span class="t-lc"><a href="../program/EXIT_status.html" title="c/program/EXIT status">EXIT_SUCCESS</a></span> indicate successful termination, the value <span class="t-lc"><a href="../program/EXIT_status.html" title="c/program/EXIT status">EXIT_FAILURE</a></span> indicates unsuccessful termination.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=3" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The <code>main</code> function is called at program startup, after all objects with static storage duration are initialized. It is the designated entry point to a program that is executed in <i>hosted</i> environment (that is, with an operating system). The name and type of the entry point to any <i>freestanding</i> program (boot loaders, OS kernels, etc) are implementation-defined.
</p><p>The parameters of the two-parameter form of the main function allow arbitrary multibyte character strings to be passed from the execution environment (these are typically known as <i>command line arguments</i>). The pointers <code>argv[1] .. argv[argc-1]</code> point at the first characters in each of these strings. <code>argv[0]</code> is the pointer to the initial character of a null-terminated multibyte strings that represents the name used to invoke the program itself (or, if this is not supported by the execution environment, argv[0][0] is guaranteed to be zero).
</p><p>If the execution environment cannot distinguish between uppercase and lowercase letters, the command line arguments are converted to lower case.
</p><p>The strings are modifiable, and any modifications made persist until program termination, although these modifications do not propagate back to the execution environment: they can be used, for example, with <span class="t-lc"><a href="../string/byte/strtok.html" title="c/string/byte/strtok">strtok</a></span>.
</p><p>The size of the array pointed to by <code>argv</code> is at least <code>argc+1</code>, and the last element, <code>argv[argc]</code>, is guaranteed to be a null pointer.
</p><p>The <code>main</code> function has several special properties:
</p>
<div class="t-li1"><span class="t-li">1)</span> A prototype for this function cannot be supplied by the program</div>
<div class="t-li1"><span class="t-li">2)</span> If the return type of the main function is <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible</a> with <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span>, then the return from the initial call to main (but not the return from any subsequent, recursive, call) is equivalent to executing the <span class="t-lc"><a href="../program/exit.html" title="c/program/exit">exit</a></span> function, with the value that the main function is returning passed as the argument (which then calls the functions registered with <span class="t-lc"><a href="../program/atexit.html" title="c/program/atexit">atexit</a></span>, flushes and closes all streams, and deletes the files created with <span class="t-lc"><a href="../io/tmpfile.html" title="c/io/tmpfile">tmpfile</a></span>, and returns control to the execution environment).</div>
<div class="t-li1"><span class="t-li">3)</span> If the return type of the main function is not <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible</a> with <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> (e.g. <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span></span></span>), the value returned to the execution environment is unspecified</div>
<div class="t-li1"><span class="t-li">4)</span> The body of the main function does not need to contain the <a href="return.html" title="c/language/return">return statement</a>: if control reaches the end of <code>main</code> without encountering a return statement, the effect is that of executing <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span></span></span>.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=4" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><p> Demonstrates how to inform a program about where to find its input and where to write its results.
<p>Invocation: ./a.out indatafile outdatafile
</p>
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> argc, <span class="kw4">char</span> <span class="sy2">*</span>argv<span class="br0">[</span><span class="br0">]</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"argc = %d<span class="es1">\n</span>"</span>, argc<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> ndx <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">while</span> <span class="br0">(</span>ndx <span class="sy1"><</span> argc<span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"argv[%d] --> %s<span class="es1">\n</span>"</span>, ndx,argv<span class="br0">[</span>ndx<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="sy2">++</span>ndx<span class="sy4">;</span>
<span class="br0">}</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"argv[argc] = %s<span class="es1">\n</span>"</span>, argv<span class="br0">[</span>argc<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">argc = 3
argv[0] --> ./a.out
argv[1] --> indatafile
argv[2] --> outdatafile
argv[argc] = (null)</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=5" 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> 5.1.2.2.1 Program startup (p: 13)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 5.1.2.2.1 Program startup (p: 12)
</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> 2.1.2.2 Hosted environment
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/main_function&action=edit&section=6" 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/main_function.html" title="cpp/language/main function">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Main function</span></span></div>
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