Hi,

Thanks for the responses. I don't get the url math, though. Can you direct me to a tutorial?

The only way I can understand it is if I say to myself, "The current directory is cgi-bin, and the ../ directory refers to cgi-bin's parent directory. Then according to the virtual directory structure, cgi-bin's parent directory is the root directory, which is htdocs, and then you descend from htdocs to the my_imgs directory--where the file is found.

Ok, I think I get it now. Relative paths are resolved based on the directory structure specified in the request url. For instance, suppose you have a web page that is requested using the following url:

www.acme.com/dir1/dir2/page.htm

You can see a directory structure in that url, and that directory structure will be used to turn relative paths into absolute paths. If page.htm contains a link to another page and the link specifies a relative path:

<a href = "../results.htm">click me</a>
[download]

then you look at the url in the original request to sort out the path:

www.acme.com/dir1/dir2/page.htm

In this case, the original page.htm is in the directory dir2, which is the current directory. Looking at the request url, the parent directory of dir2 is dir1. Therefore, the browser resolves the relative path:

../results.htm

into the absolute path: www.acme.com/dir1/results.htm

If the href had used the path: ../../dir3/results.htm, then the browser would resolve the relative path into the absolute path:

href = "../../dir3/results.htm" <-----relative path
www.acme.com/dir1/dir2/page.htm <---request url
www.acme.com/dir3/results.htm <----absolute path

In my case, the url that was used to request my cgi script was:

http://localhost/cgi-bin/prog1.pl

The current directory in that url is cgi-bin. So when the perl script produced an <img> tag with the relative path:

<img src="../my_imgs/blue_square.jpg">

the parent directory was localhost. Therefore, the relative url corresponds to the absolute url: localhost/my_imgs/blue_sqare.jpg.

Now it's Apaches turn. Apache maps the host, in this case localhost (in the first example it was www.acme.com), to the htdocs directory. Therefore, the url tells Apache to look for a subdirectory in htdocs called my_imgs and then looks in my_imgs for a file called blue_square.jpg.

Almost. You convert from virtual to real too early.

The current virtual directory is /cgi-bin, and the ../ directory refers to /cgi-bin's parent directory, the root directory. Then you descend to the my_imgs directory and the pics.jpg file. The final url is therefore /my_imgs/pic.jpg. The server maps that url to the file htdocs/my_imgs/pic.jpg.

Web path	 Disk path
/	         $DOCUMENT_ROOT/htdocs
/cgi-bin	 $DOCUMENT_ROOT/cgi-bin
/my_imgs/pic.jpg $DOCUMENT_ROOT/htdocs/my_imgs/pic.jpg

Hope this helps.

I don't think that is correct. My DocumentRoot is set to /Library/Apache2/htdocs, so it wouldn't make sense to say that the web path for / is the disk path $DOCUMENT_ROOT/htdocs, which would be /Library/Apache2/htdocs/htdocs. I found this on the apache website:

DocumentRoot directive

Syntax: DocumentRoot directory-path
Default: DocumentRoot /usr/local/apache/htdocs
Context: server config, virtual host
Status: core

This directive sets the directory from which httpd will serve files. 
Unless matched by a directive like Alias, the server appends the
path from the requested URL to the document root to make the
path to the document. Example:

    DocumentRoot /usr/web

then an access to http://www.my.host.com/index.html refers to /usr/web/index.html.

There appears to be a bug in mod_dir which causes problems
when the DocumentRoot has a trailing slash (i.e., "DocumentRoot
/usr/web/") so please avoid that.

One thing I discovered: when a browser converts a relative path to an absolute path prior to requesting a resource, if a relative path tries to move up the hierarchy of a url too far with ../../../, the extra ones are ignored. For instance, if the page's url is:

http://localhost/cgi-bin/prog1.pl

the current directory is cgi-bin. However, in that url cgi-bin does not have a parent directory. Therefore, if the cgi script produces a page with an image that uses this relative path::

<img src="../../../../my_imgs/blue_square.jpg"

the ../../../../ part of the relative path just gets you:

http://localhost

then the rest of the path, /my_imgs/blue_square.jpg, gets appended to that, giving you:

http://localhost/my_imgs/blue_square.jpg

Subsequently, when apache receives the request for that url, as the passage from the apache website above says, everything after the host gets appended to the document root, which in my case yields this:

/Library/Apache2/htdocs/my_imgs/blue_square.jpg

That is a real path on the filesystem. To summarize there is a two step process:

1) The browser converts a relative path (used by an html element on a page) to an absolute path by looking at the page's url, then sends a request for that url to the Apache server.

2) Apache takes the part of the url after the host name and appends it to the DocumentRoot (as specified in httpd.conf). For instance, if apache receives a request for this url

http://www.mysite.com/dir1/dir2/page.htm

the host name is www.mysite.com, and with my DocumentRoot (= /Library/Apache2/htdocs) Apache would create the following path to the requested resource:

/Library/Apace2/htdos/dir1/dir2/page.htm

That's my current mental model of what's going on. I'll adjust it as required.

See above for correction :-)

As far as your understanding of the translation, I think you have it. As with a "real" file system, cd .. from the root directory will still leave you at the root directory, and by extension (ignoring symlinks, the shell's idea of $cwd, and so on), cd ../../../foo from the directory /bar/blatz will traverse to /bar, /, /, and then /foo. The same thing holds with the virtual directory structure on the web server.

If you are familiar with the concept of mount points, there are many directives in the Apache configuration language that can create the moral equivalence of a mount point. The stock cgi-bin directory is one, Alias is another. The client sees a view of this virtual directory structure as defined by the directives in the Apache control files.

--MidLifeXis