sendfile() is a system call which streamlines the copying of data between the disk and the tcp socket. The call copied from the page cache directly rather than requiring a kernel -> user space -> kernel space copy for every read() and write() call. Generally the advantages are only felt on heavily loaded servers. The call is supported in ProFTPD for Linux and FreeBSD.
sendfile is not supported under 2.0.x, this is not an issue when compiling for 2.0.x on a 2.0.x system. However when compiling on a 2.2.x system for use on 2.0.x use the --disable-sendfile flag.
There are two patches available for runtime detection of sendfile() which gets round the 2.0.x problems.
Johnie Ingram (aka netgod)'s: http://www.proftpd.org/proftpd-devel-archive/99-10/msg00073.html
John Pierce <hawkfan@pyrotechnics.com> http://www.proftpd.org/proftpd-devel-archive/99-10/msg00112.html