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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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