tutorial5.xml

<article data-sblg-article="1" data-sblg-tags="tutorial" itemscope="itemscope" itemtype="http://schema.org/BlogPosting">
	<header>
		<h2 itemprop="name">
			CGI for C++ applications
		</h2>
		<address itemprop="author"><a href="https://kristaps.bsd.lv">Kristaps Dzonsons</a></address>
		<time itemprop="datePublished" datetime="2018-03-24">24 May, 2018</time>
	</header>
	<p>
		<aside itemprop="about">
			<span class="nm">kcgi</span> supports C++ callers just as easily as C.
			In this brief tutorial I'll take you through compiling a C++ application that uses the <span class="nm">kcgi</span>
			library.
		</aside>
	</p>
	<h3>
		Source Code
	</h3>
	<p>
		The hard part about this example is doing anything demonstrable with C++.
		So we'll do something very silly in just printing our HTTP output into the log file.
		For the <span class="nm">kcgi</span> inclusion, we'll need all the usual header files.
	</p>
	<figure class="sample">
		<pre class="prettyprint linenums">#include &lt;sys/types.h&gt; /* size_t, ssize_t */
#include &lt;stdarg.h&gt; /* va_list */
#include &lt;stddef.h&gt; /* NULL */
#include &lt;stdint.h&gt; /* int64_t */
#include &lt;kcgi.h&gt;</pre>
	</figure>
	<p>
		Next, we'll need our C++ bits.
		This is make-work, but serves to illustrate&#8230;
	</p>
	<figure class="sample">
		<pre class="prettyprint linenums">#include &lt;iostream&gt;</pre>
	</figure>
	<p>
		Now let's just jump directly into our main function.
		It will do nothing more than print <q>Hello, world!</q> to the browser.
		But it will also emit <q>Said hello!</q> into the error log.
		See your web server configuration for where this will appear.
		It's usually in the <span class="file">error.log</span> file.
		On OpenBSD's default server, it's often in 
		<span class="file">/var/www/logs/error.log</span>.
	</p>
	<figure class="sample">
		<pre class="prettyprint linenums">int 
main(void) {
    enum kcgi_err er;
    struct kreq r;
    const char *const pages[1] = { "index" };

    /* Set up our main HTTP context. */

    er = khttp_parse(&amp;r, NULL, 0, pages, 1, 0);
    if (er != KCGI_OK)
        return 0;
    khttp_head(&amp;r, kresps[KRESP_STATUS], 
        "%s", khttps[KHTTP_200]);
    khttp_head(&amp;r, kresps[KRESP_CONTENT_TYPE], 
        "%s", kmimetypes[r.mime]);
    khttp_body(&amp;r);
    khttp_puts(&amp;r, "Hello, world!\n");
    std::cerr &lt;&lt; "Said hello!";
    khttp_free(&amp;r);
    return 0;
}</pre>
	</figure>
	<p>
		Nothing to it&#8212;looks like any of our C examples.
		The difference is that we've used some C++ code to emit <q>Said hello!</q> to the error log.
		The next part is how we can compile this code.
		For that, we'll need to use a C++ compiler instead of the C compiler we've been using to date.
	</p>
	<figure class="sample">
		<pre class="prettyprint lang-sh linenums">% c++ `pkg-config --cflags kcgi` -c -o tutorial5.o tutorial5.cc
% c++ -static -o tutorial5 tutorial5.cc `pkg-config --libs kcgi`</pre>
	</figure>
	<p>
		Or just&#8230;
		(and noting again the <code>-static</code>, which we need by being in our file-system jail)
	</p>
	<figure class="sample">
		<pre class="prettyprint lang-sh linenums">% c++ -static `pkg-config --cflags --libs kcgi` -o tutorial5 tutorial5.cc</pre>
	</figure>
	<p>
		Now you can install your compiled CGI script just like any CGI script.
		See <a href="tutorial0.html">Getting Started with CGI in C</a> for these steps in detail.
		All of these steps work for FastCGI, of course.
		Enjoy!
		(On non-OpenBSD systems, you'll probably need to use <code>sudo</code> instead of <code>doas</code>.)
	</p>
	<figure class="sample">
		<pre class="prettyprint lang-sh linenums">% doas install -m 0555 tutorial5 /var/www/cgi-bin</pre>
	</figure>
</article>