Sendmail 8.13.3 Programmer's Guide

Control Flow of Milter APIs
Resource Management
Chapter 3 53
Resource Management
You must deallocate per-connection resources because filter applications
exist for a long time and they handle many connections. The lifetime of a
connection depends on calls to the callbacks xxfi_connect() and
xxfi_close(). For more information on message-oriented and
connection-oriented APIs, see “Message Modification APIs” on page 25
and “Data Access APIs” on page 18, respectively. Only one
connection-specific data pointer is available for each connection.
Each message is marked by calls to the xxfi_envfrom() and
xxfi_eom() callbacks (or the xxfi_abort() callback), which implies
that message-specific resources are allocated and reclaimed from these
routines. Only one active message is available because the messages in a
connection are processed sequentially by each filter, and it is associated
with a given connection and filter (and connection-private data block).
The filter application must access these resources through the
smfi_getpriv() and smfi_setpriv() APIs and must reclaim the
resources using the xxfi_abort() API.