Call Cgi Error
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of questions I get from my visitors at thesitewizard.com, I can see that one of the most dreaded errors that newcomers to CGI face is the "500 Internal Server Error". It is one of
Cgi Error Premature End Of Script Headers
the most uninformative error messages that can mean anything from an improper upload to cgi error ah01215 a bug in the script. This article attempts to give you some concrete, practical steps that you can take to narrow
Cgi Error Log
down the problem and hopefully eliminate it. For the uninitiated, a "500 Internal Server Error" is a message much like the common "404 File Not Found" message. You get the latter message in your browser https://success.trendmicro.com/solution/0119131 when you try to access a non-existent web page. You get the "500 Internal Server Error" message when you try to run a script with problems. For the purposes of this article, I am assuming that your CGI script is a Perl script, by far the most commonly available on the Internet. We will try to eliminate the most common errors first (and the easiest-to-eliminate ones): Location Did you http://www.thesitewizard.com/archive/servererror.shtml upload your scripts into the right place? This is not as obvious as it may seem. Some servers are configured to run your CGI scripts anywhere. Others will only run it when it is installed in a particular directory. It is not just a matter of creating a "cgi-bin" directory — for example, some hosts configure the server so that it will run scripts only from a particular directory outside the web directory structure (for security reasons). Your web page will still call the script "/cgi-bin/script.pl" but the server maps it to the actual directory. You will have to upload it in the right directory, regardless of what your web page calls it. Find out such information from your web host's documentation. File Upload Mode Did you upload your Perl script in ASCII mode? Your FTP client may have uploaded the script in binary mode if you did not take any special action. Perl scripts are ASCII files, and since different operating systems have different ways of representing the end of line character (eg Unix uses a line-feed, Windows uses a carriage-return and line-feed pair), it is important that you set the uploading method to ASCII, so that line-end translation is performed. Re-upload the script, this tim
expect things to always work as planned; experienced developers have learned otherwise. 5.5.1. Dying Gracefully The most common method that Perl developers use for handling errors is Perl's built-in die function. Here http://docstore.mik.ua/orelly/linux/cgi/ch05_05.htm is an example: open FILE, $filename or die "Cannot open $filename: $!"; If Perl is unable to open the file specified by $filename, die will print an error message to STDERR and terminate the script. http://kb.parallels.com/en/6034 The open function, like most Perl commands that interact with the system, sets $! to the reason for the error if it fails. Unfortunately, die is not always the best solution for handling errors in cgi error your CGI scripts. As you will recall from Chapter 3, "The Common Gateway Interface", output to STDERR is typically sent to the web server's error log, triggering the web server to return a 500 Internal Server Error . This is certainly not a very user-friendly response. You should determine a policy for handling errors on your site. You may decide that 500 Internal Server Error pages are acceptable for very uncommon call cgi error system errors like the inability to read or write to files. However, you may decide that you wish to display a formatted HTML page instead with information for users such as alternative actions they can take or who to notify about the problem. 5.5.1.1. Trapping die It is possible to trap die so that it does not generate a 500 Internal Server Error automatically. This is especially useful because many common third-party modules use die (and variants such as croak) as their manner for responding to errors. If you know that a particular subroutine may call die, you can catch this with an eval block in Perl: eval { dangerous_routine( ); 1; } or do { error( $q, $@ || "Unknown error" ); }; If dangerous_routine does call die, then eval will catch it, set the special variable $@ to the value of the die message, pass control to the end of the block, and return undef. This allows us to call another subroutine to display our error more gracefully. Note that an eval block will not trap exit. This works, but it certainly makes your code a lot more complex, and if your CGI script interacts with a lot of subroutines that might die, then you must either pla
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