sendmail filter requirements
Because filters use threads, filters must be thread-safe. You can configure your filters to ensure their compatibility with threads.
Many operating systems provide support for POSIX threads in the standard C libraries. The compiler flag to link with threading support differs according to the compiler and linker used. If you are unsure of the local flag used, check the Makefile in your appropriate obj.*/libmilter build subdirectory.
Note: Because filters use threads, it might be necessary to alter the process
limits on your filter. For example, you might want to use setrlimit
to increase the number of open file descriptors if your filter will be busy;
otherwise, mail can be rejected.