Apache uses the sendfile syscall on platforms where it is available in order to speed sending of responses. Unfortunately, on some systems, Apache will detect the presence of sendfile at compile-time, even when it does not work properly. This happens most frequently when using network or other non-standard file-system.

Symptoms of this problem include the above message in the error log and zero-length responses to non-zero-sized files. The problem generally occurs only for static files, since dynamic content usually does not make use of sendfile.

To fix this problem, simply use the EnableSendfile directive to disable sendfile for all or part of your server. Also see the EnableMMAP, which can help with similar problems.

If you get error messages related to the AcceptEx syscall on win32, see the Win32DisableAcceptEx directive.

Most problems with CGI scripts result in this message written in the error log together with an Internal Server Error delivered to the browser. A guide to helping debug this type of problem is available in the CGI tutorial.