In C:

typedef int (*sieve2_callback_func) (
	sieve2_context_t * sieve2_context,
	void * user_data
);

typedef struct {
	sieve2_values_t value;
	sieve2_callback_func func;
} sieve2_callback_t;

sieve2_callback_t my_callbacks[] = {
{ SIEVE2_DEBUG_TRACE,           my_debug         },
{ SIEVE2_ERRCALL_PARSE,         my_errparse      },
{ SIEVE2_ERRCALL_RUNTIME,       my_errexec       },
{ SIEVE2_ERRCALL_PARSE,         my_errparse      },
{ SIEVE2_ACTION_FILEINTO,       my_fileinto      },
{ SIEVE2_ACTION_REDIRECT,       my_redirect      },
{ SIEVE2_ACTION_REJECT,         my_reject        },
{ SIEVE2_ACTION_NOTIFY,         my_notify        },
{ SIEVE2_ACTION_VACATION,       my_vacation      },
/* KEEP is essentially the default case of FILEINTO "INBOX". */
{ SIEVE2_ACTION_KEEP,           my_fileinto      },
{ SIEVE2_SCRIPT_GETSCRIPT,      my_getscript     },
/* We don't support one header at a time in this example. */
{ SIEVE2_MESSAGE_GETHEADER,     NULL             },
//{ SIEVE2_MESSAGE_GETHEADER,     my_getheader   },
/* libSieve can parse headers itself, so we'll use that. */
{ SIEVE2_MESSAGE_GETALLHEADERS, my_getheaders    },
{ SIEVE2_MESSAGE_GETSUBADDRESS, my_getsubaddress },
{ SIEVE2_MESSAGE_GETENVELOPE,   my_getenvelope   },
{ SIEVE2_MESSAGE_GETBODY,       my_getbody       },
{ SIEVE2_MESSAGE_GETSIZE,       my_getsize       },
{ 0 } };


What doesn't work in fpc:

   sieve2_callback_func = function(var sieve2_context; user_data : pointer) : integer; cdecl;
   sieve2_callback_t  = record
			  value	: sieve2_values_t;
			  func	: sieve2_callback_func;
		       end;

   callbackAr	: Array[1..5] of sieve2_callback_t = ( 
		(value:SIEVE2_ACTION_REDIRECT; func:@myRedirect),
		(value:SIEVE2_ACTION_KEEP; func:@myKeep),
		(value:SIEVE2_ACTION_DISCARD; func:@myDiscard),
		(value:SIEVE2_MESSAGE_GETSIZE; func:@myGetSize),
		NIL);