Note also that RFC-3875 The Common Gateway Interface (CGI) Version 1.1 says that the web server will execute the CGI script in a "system-defined manner". While it does say that the "system-defined method" for returning responses will be by writing to the "standard output" unless defined otherwise, it says nothing about standard error output (STDERR) as a channel for communication between the CGI script and the web server. If you are writing your scripts to be portable, you might consider it best to avoid any output to STDERR, returning any error messages as well formatted CGI responses or writing them to application specific log files.