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 <h3 class="section">25.1 Program Arguments</h3>

 <p><a name="index-program-arguments-3006"></a><a name="index-command-line-arguments-3007"></a><a name="index-arguments_002c-to-program-3008"></a>
 <a name="index-program-startup-3009"></a><a name="index-startup-of-program-3010"></a><a name="index-invocation-of-program-3011"></a><a name="index-g_t_0040code_007bmain_007d-function-3012"></a><a name="index-main-3013"></a>The system starts a C program by calling the function <code>main</code>.  It
 is up to you to write a function named <code>main</code>&mdash;otherwise, you
 won't even be able to link your program without errors.

    <p>In ISO&nbsp;C<!-- /@w --> you can define <code>main</code> either to take no arguments, or to
 take two arguments that represent the command line arguments to the
 program, like this:

 <pre class="smallexample">     int main (int <var>argc</var>, char *<var>argv</var>[])
 </pre>
    <p><a name="index-argc-_0028program-argument-count_0029-3014"></a><a name="index-argv-_0028program-argument-vector_0029-3015"></a>The command line arguments are the whitespace-separated tokens given in
 the shell command used to invoke the program; thus, in &lsquo;<samp><span class="samp">cat foo
 bar</span></samp>&rsquo;, the arguments are &lsquo;<samp><span class="samp">foo</span></samp>&rsquo; and &lsquo;<samp><span class="samp">bar</span></samp>&rsquo;.  The only way a
 program can look at its command line arguments is via the arguments of
 <code>main</code>.  If <code>main</code> doesn't take arguments, then you cannot get
 at the command line.

    <p>The value of the <var>argc</var> argument is the number of command line
 arguments.  The <var>argv</var> argument is a vector of C strings; its
 elements are the individual command line argument strings.  The file
 name of the program being run is also included in the vector as the
 first element; the value of <var>argc</var> counts this element.  A null
 pointer always follows the last element: <var>argv</var><code>[</code><var>argc</var><code>]</code>
 is this null pointer.

    <p>For the command &lsquo;<samp><span class="samp">cat foo bar</span></samp>&rsquo;, <var>argc</var> is 3 and <var>argv</var> has
 three elements, <code>"cat"</code>, <code>"foo"</code> and <code>"bar"</code>.

    <p>In Unix systems you can define <code>main</code> a third way, using three arguments:

 <pre class="smallexample">     int main (int <var>argc</var>, char *<var>argv</var>[], char *<var>envp</var>[])
 </pre>
    <p>The first two arguments are just the same.  The third argument
 <var>envp</var> gives the program's environment; it is the same as the value
 of <code>environ</code>.  See <a href="Environment-Variables.html#Environment-Variables">Environment Variables</a>.  POSIX.1 does not
 allow this three-argument form, so to be portable it is best to write
 <code>main</code> to take two arguments, and use the value of <code>environ</code>.

 <ul class="menu">
 <li><a accesskey="1" href="Argument-Syntax.html#Argument-Syntax">Argument Syntax</a>:              By convention, options start with a hyphen.
 <li><a accesskey="2" href="Parsing-Program-Arguments.html#Parsing-Program-Arguments">Parsing Program Arguments</a>:    Ways to parse program options and arguments.
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	<h3 class="section">25.1 Program Arguments</h3>

	<p><a name="index-program-arguments-3006"></a><a name="index-command-line-arguments-3007"></a><a name="index-arguments_002c-to-program-3008"></a>
	<a name="index-program-startup-3009"></a><a name="index-startup-of-program-3010"></a><a name="index-invocation-of-program-3011"></a><a name="index-g_t_0040code_007bmain_007d-function-3012"></a><a name="index-main-3013"></a>The system starts a C program by calling the function <code>main</code>. It
	is up to you to write a function named <code>main</code>—otherwise, you
	won't even be able to link your program without errors.

	<p>In ISO C<!-- /@w --> you can define <code>main</code> either to take no arguments, or to
	take two arguments that represent the command line arguments to the
	program, like this:

	<pre class="smallexample"> int main (int <var>argc</var>, char *<var>argv</var>[])
	</pre>
	<p><a name="index-argc-_0028program-argument-count_0029-3014"></a><a name="index-argv-_0028program-argument-vector_0029-3015"></a>The command line arguments are the whitespace-separated tokens given in
	the shell command used to invoke the program; thus, in ‘<samp><span class="samp">cat foo
	bar</span></samp>’, the arguments are ‘<samp><span class="samp">foo</span></samp>’ and ‘<samp><span class="samp">bar</span></samp>’. The only way a
	program can look at its command line arguments is via the arguments of
	<code>main</code>. If <code>main</code> doesn't take arguments, then you cannot get
	at the command line.

	<p>The value of the <var>argc</var> argument is the number of command line
	arguments. The <var>argv</var> argument is a vector of C strings; its
	elements are the individual command line argument strings. The file
	name of the program being run is also included in the vector as the
	first element; the value of <var>argc</var> counts this element. A null
	pointer always follows the last element: <var>argv</var><code>[</code><var>argc</var><code>]</code>
	is this null pointer.

	<p>For the command ‘<samp><span class="samp">cat foo bar</span></samp>’, <var>argc</var> is 3 and <var>argv</var> has
	three elements, <code>"cat"</code>, <code>"foo"</code> and <code>"bar"</code>.

	<p>In Unix systems you can define <code>main</code> a third way, using three arguments:

	<pre class="smallexample"> int main (int <var>argc</var>, char <var>argv</var>[], char <var>envp</var>[])
	</pre>
	<p>The first two arguments are just the same. The third argument
	<var>envp</var> gives the program's environment; it is the same as the value
	of <code>environ</code>. See <a href="Environment-Variables.html#Environment-Variables">Environment Variables</a>. POSIX.1 does not
	allow this three-argument form, so to be portable it is best to write
	<code>main</code> to take two arguments, and use the value of <code>environ</code>.

	<ul class="menu">
	<li><a accesskey="1" href="Argument-Syntax.html#Argument-Syntax">Argument Syntax</a>: By convention, options start with a hyphen.
	<li><a accesskey="2" href="Parsing-Program-Arguments.html#Parsing-Program-Arguments">Parsing Program Arguments</a>: Ways to parse program options and arguments.
	</ul>

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