sealink.imp

一个通讯程序源码

    SEAlink - Sliding window file transfer protocol

    XENIX System V version by Scott Reynolds
    additional XENIX modifications by Sanford Zelkovitz, without whose help
    this couldn't have been accomplished

    Based on:

    MS-DOS Version 1.16, created on 01/15/87 at 01:40:52
(C) COPYRIGHT 1986,87 by System Enhancement Associates; ALL RIGHTS RESERVED
    By:  Thom Henderson

    Description:

	This file contains a set of routines to illustrate the SEAlink
	sliding window file transfer protocol.  SEAlink is fully backward
	compatible to XMODEM, and can be easily adapted to most XMODEM
	variants.

	The intent of SEAlink is to provide a file transfer protocol that
	does not suffer from propagation delays, such as are introduced
	by satellite relays or packet switched networks.

    Instructions:

	Two routines are provided to implement SEAlink file transfers.

	int xmtfile(name)		/+ transmit a file +/
	char *name;			/+ name of file to transmit +/

	This routine is used to send a file.  One file is sent at a time.
	If the name is blank (name is null or *name points to a null),
	then only an end of transmission marker is sent.

	This routine returns a one if the file is successfully
	transmitted, or a zero if a fatal error occurs.

	char *rcvfile(name)		/+ receive a file +/
	char *name;			/+ name of file (optional) +/

	This routine is used to receive a file.  One file is received.
	The name, if given, takes precedence and will be the name of
	the resulting file.  If the name is blank (name is null or *name
	points to a null), then the name given by the transmitter is used.
	If the transmitter does not give a name, then the file transfer
	is aborted.

	This routine returns a pointer to the name of the file that
	was received.  If the file transfer is not successful, then
	a null pointer is returned.

	The pointer returned by rcvfile() points to a static data buffer.
	This does not have to be freed (and should not be), but it will
	be overwritten the next time rcvfile() is called.

	The rcvfile() function works on a temporary file whose name is
	the same as the final file, but with a period (".") added at the
	beginning.  If a file transfer is aborted, then this temporary
	file will be retained.  An aborted file transfer will not harm
	a pre-existing file of the same name.

    Programming notes:

	These routines can be used for either single or multiple file
	transfers.

	To send multiple files, send each one one at a time until either
	a transmit fails or all files are sent.  If all files are sent,
	then signal the end by calling xmtfile() with a null pointer.

	To receive multiple files, call rcvfile() repeatedly until it
	returns a null pointer.

	These routines pass a "block zero", which contains information
	about the original file name, size, and date/time of last
	modification.  If you cannot implement block zero, then you can
	leave it out.  If you cannot set any given field in block zero
	when transmitting, then you should leave it set to zeros.  If you
	cannot use any given field of block zero when receiving, then
	you should ignore it.

	These routines are fully compatible with XMODEM, including the
	original checksum method and later CRC adaptations.  It can be
	easily adapted to Modem7 protocol by adding a Modem7 filename
	transfer shell, though we do not recommend it.  The underlying
	logic, of course, can be adapted to almost any variant of XMODEM.

    License:

	You are granted a license to use this code in your programs, and
	to adapt it to your particular situation and needs, subject only
	to the following conditions:

	1)   You must refer to it as the SEAlink protocol, and you must
     	give credit to System Enhancement Associates.

	2)   If you modify it in such a way that your version cannot
     	converse with the original code as supplied by us, then
     	you should refer to it as "SEAlink derived", or as a
     	"variation of SEAlink", or words to that effect.

	In short, we're not asking for any money, but we'd like to
	get some credit for our work.

    Language:

	Computer Innovations C86
	Adapted for IBM PC XENIX 2.00.2 C using UNIX System V compatible calls

    Notes on XENIX modifications:

	The com_getc() routine has a minimum delay of .1 seconds, due
	to the nature of the read() system call.  Attempts to eliminate
	this delay have proven more costly than leaving it in.

	CRC maintenance functions were added to the original code, as
	they are not library calls under XENIX.

	All output is performed through file descriptor 0, and is done
	in blocks using the write() system call rather than individual
	character writes.  File descriptor 1 may be selected by invoking
	the program with a "-1" argument.

	Most low level routines utilize register class variables to
	decrease overhead and improve overall system response slightly.

	A rudimentary command line processor was added to the original
	routines to drive the transmitter and receiver.  The two
	options, "s" and "r", are for sending and receiving respectively.

	When invoked without proper arguments the program will display
	a short message including usage notes.

	-- Scott Reynolds, Sysop U.S.S. Enterprise BBS, [phone]