There are three main types of handler functions for ProFTPD: configuration directive handlers, command handlers, and authentication handlers.
Configuration directive handlers are used to process the
configuration directives encountered in the main proftpd.conf configuration
file, any Included configuration files, and any .ftpaccess files
encountered. The context in which these functions operate is one without
connected clients; much of the "processing" entails syntax and availability
checks of directive parameters, and the recording of the configured settings.
The command handlers are responsible for the processing of the client-sent FTP commands; these are the heart of the server, providing the services requested by the connected user.
The last type, authentication handlers, are not really
handlers so much as they are replacement functions, to be used by ProFTPD for
the purposes of authentication (and authorization) in place of the system or
library-provided functions. These types of handlers are not as common as the
other two types of handlers. For the curious reader, however, the
sources for the mod_ldap, mod_auth_pam, and mod_auth_unix modules
illustrate how these handlers are implemented. Note that the tracing of the
calling of these functions during a live connection can be tricky.
While not quite true "handlers", per se, the response functions are also important piece of module handler development. These functions deal with how ProFTPD sends its responses back to the connected clients.
There are also several pre-defined macros to help in the checking of various handler argument conditions, such as argument number and syntax. These macros are described here.