Annotation of embedaddon/lrzsz/man/lsz.1, revision 1.1.1.1

1.1       misho       1: '\"
                      2: '\" Revision Level 
                      3: '\" Last Delta     04-21-88
                      4: .TH SZ 1 2.6.1996 lrzsz-0.12b
                      5: .SH NAME
                      6: sx, sb, sz \- XMODEM, YMODEM, ZMODEM file send
                      7: .SH SYNOPSIS
                      8: sz
                      9: .RB [\- +8abdefkLlNnopqTtuvyY ]
                     10: .I file ...
                     11: .br
                     12: sb
                     13: .RB [\- adfkqtuv ]
                     14: .I file ...
                     15: .br
                     16: sx
                     17: .RB [\- akqtuv ]
                     18: .I file
                     19: .br
                     20: sz
                     21: .RB [\- oqtv ]
                     22: .B "-c COMMAND"
                     23: .br
                     24: sz
                     25: .RB [\- oqtv ]
                     26: .B "-i COMMAND"
                     27: .br
                     28: sz -TT
                     29: .SH DESCRIPTION
                     30: .B Sz
                     31: uses the ZMODEM, YMODEM or XMODEM error correcting protocol to send
                     32: one or more files over a dial-in serial port to a variety of programs running under
                     33: PC-DOS, CP/M, Unix, VMS, and other operating systems.
                     34: 
                     35: While
                     36: .I rz
                     37: is smart enough to be called from
                     38: .I cu(1),
                     39: very few versions of
                     40: .I cu(1)
                     41: are smart enough to allow
                     42: .I sz
                     43: to work properly.
                     44: Unix flavors of Professional-YAM are available for such dial-out application.
                     45: 
                     46: 
                     47: .B Sz
                     48: sends one or more files with ZMODEM protocol.
                     49: 
                     50: ZMODEM
                     51: greatly simplifies file transfers compared to XMODEM.
                     52: In addition to a friendly user interface, ZMODEM
                     53: provides Personal Computer and other users
                     54: an efficient, accurate, and robust file transfer method.
                     55: 
                     56: ZMODEM provides complete
                     57: .B "END-TO-END"
                     58: data integrity between application programs.
                     59: ZMODEM's 32 bit CRC catches errors
                     60: that sneak into even the most advanced networks.
                     61: 
                     62: Advanced file management features include
                     63: AutoDownload (Automatic file Download initiated without user intervention),
                     64: Display of individual and total file lengths and transmission time estimates,
                     65: Crash Recovery,
                     66: selective file transfers,
                     67: and preservation of
                     68: exact file date and length.
                     69: 
                     70: Output from another program may be piped to
                     71: .B sz
                     72: for transmission by denoting standard input with "-":
                     73: .ce
                     74: ls -l | sz -
                     75: The program output is transmitted with the filename sPID.sz
                     76: where PID is the process ID of the
                     77: .B sz
                     78: program.
                     79: If the environment variable
                     80: .B ONAME
                     81: is set, that is used instead.
                     82: In this case, the Unix command:
                     83: .ce
                     84: ls -l | ONAME=con sz -ay -
                     85: will send a "file" to the PC-DOS console display.
                     86: The
                     87: .B -y
                     88: option instructs the receiver to open the file for writing unconditionally.
                     89: The
                     90: .B -a
                     91: option
                     92: causes the receiver to convert Unix newlines to PC-DOS carriage returns
                     93: and linefeeds.
                     94: 
                     95: 
                     96: .B Sb
                     97: batch sends one or more files with YMODEM or ZMODEM protocol.
                     98: The initial ZMODEM initialization is not sent.
                     99: When requested by the receiver,
                    100: .B sb
                    101: supports
                    102: .B YMODEM-g
                    103: with "cbreak" tty mode, XON/XOFF flow control,
                    104: and interrupt character set to CAN (^X).
                    105: .B YMODEM-g
                    106: (Professional-YAM
                    107: .B g
                    108: option)
                    109: increases throughput over error free channels
                    110: (direct connection, X.PC, etc.)
                    111: by not acknowledging each transmitted sector.
                    112: 
                    113: On
                    114: .SM Unix
                    115: systems, additional information about the file is transmitted.
                    116: If the receiving program uses this information,
                    117: the transmitted file length controls the exact number of bytes written to
                    118: the output dataset,
                    119: and the modify time and file mode
                    120: are set accordingly.
                    121: 
                    122: 
                    123: .B Sx
                    124: sends a single
                    125: .I file
                    126: with
                    127: .B XMODEM
                    128: or
                    129: .B XMODEM-1k
                    130: protocol
                    131: (sometimes incorrectly called "ymodem").
                    132: The user must supply the file name to both sending and receiving programs.
                    133: 
                    134: If
                    135: .B sz
                    136: is invoked with $SHELL set and iff that variable contains the
                    137: string
                    138: .I "rsh"
                    139: ,
                    140: .I "rbash"
                    141: or
                    142: .I "rksh"
                    143: (restricted shell),
                    144: .B sz
                    145: operates in restricted mode.
                    146: Restricted mode restricts pathnames to the current directory and
                    147: PUBDIR (usually /usr/spool/uucppublic) and/or subdirectories
                    148: thereof.
                    149: 
                    150: 
                    151: The fourth form sends a single COMMAND to a ZMODEM receiver for execution.
                    152: .B Sz
                    153: exits with the COMMAND return value.
                    154: If COMMAND includes spaces or characters special to the shell,
                    155: it must be quoted.
                    156: 
                    157: 
                    158: The fifth form sends a single COMMAND to a ZMODEM receiver for execution.
                    159: .B Sz
                    160: exits as soon as the receiver has correctly received the command,
                    161: before it is executed.
                    162: 
                    163: 
                    164: The sixth form (sz -TT)
                    165: attempts to output all 256 code combinations to the terminal.
                    166: In you are having difficulty sending files,
                    167: this command lets you see which character codes are being
                    168: eaten by the operating system.
                    169: 
                    170: 
                    171: If
                    172: .B sz
                    173: is invoked with stdout and stderr to different datasets,
                    174: Verbose is set to 2, causing frame by frame progress reports
                    175: to stderr.
                    176: This may be disabled with the
                    177: .B q
                    178: option.
                    179: .PP
                    180: The meanings of the available options are:
                    181: .PP
                    182: .PD 0
                    183: .TP
                    184: .B "-+, --append"
                    185: Instruct the receiver to append transmitted data to an existing file
                    186: (ZMODEM only).
                    187: .TP 
                    188: .B "-2, --twostop"
                    189: use two stop bits (if possible). Do not use this unless you know
                    190: what you are doing.
                    191: .TP
                    192: .B "-8, --try-8k"
                    193: Try to go up to 8KB blocksize. This is incompatible with standard zmodem,
                    194: but a common extension in the bbs world. (ZMODEM only).
                    195: .TP
                    196: .B "--start-8k"
                    197: Start with 8KB blocksize. Like --try-8k.
                    198: .TP
                    199: .B "-a, --ascii"
                    200: Convert NL characters in the transmitted file to CR/LF.
                    201: This is done by the sender for XMODEM and YMODEM, by the receiver
                    202: for ZMODEM.
                    203: .TP
                    204: .B "-b, --binary"
                    205: (ZMODEM) Binary override: transfer file without any translation.
                    206: .TP
                    207: .B "-B NUMBER, --bufsize NUMBER"
                    208: Use a readbuffer of 
                    209: .B NUMBER
                    210: bytes. Default ist 16384, which should be enough
                    211: for most situations. If you have a slow machine or a bad disk interface
                    212: or suffer from other hardware problems you might want to increase
                    213: the buffersize.
                    214: .B -1
                    215: or
                    216: .B auto
                    217: use a buffer large enough to buffer the whole file. Be careful with this
                    218: option - things normally get worse, not better, if the machine starts
                    219: to swap.
                    220: 
                    221: Using this option turns of memory mapping of the input file. This
                    222: increases memory and cpu usage.
                    223: .TP
                    224: .B "-c COMMAND, --command COMMAND"
                    225: Send COMMAND to the receiver for execution, return with COMMAND\'s exit status.
                    226: .TP
                    227: .B "-C N, --command-tries N"
                    228: Retry to send command N times (default: 11).
                    229: .TP
                    230: .B "-d, --dot-to-slash"
                    231: Change all instances of "." to "/" in the transmitted pathname.
                    232: Thus, C.omenB0000 (which is unacceptable to MSDOS or CP/M)
                    233: is transmitted as C/omenB0000.
                    234: If the resultant filename has more than 8 characters in the stem,
                    235: a "." is inserted to allow a total of eleven.
                    236: 
                    237: This option enables the 
                    238: .B "--full-path"
                    239: option.
                    240: .TP
                    241: .B "--delay-startup N"
                    242: Wait
                    243: .B N
                    244: seconds before doing anything.
                    245: .TP
                    246: .B "-e, --escape"
                    247: Escape all control characters;
                    248: normally XON, XOFF, DLE, CR-@-CR, and Ctrl-X are escaped.
                    249: .TP 
                    250: .B"-E, --rename"
                    251: Force the sender to rename the new file if a file with the same
                    252: name already exists.
                    253: .TP
                    254: .B "-f, --full-path"
                    255: Send Full pathname.
                    256: Normally directory prefixes are stripped from the transmitted
                    257: filename.
                    258: 
                    259: This is also turned on with to 
                    260: .B "--dot-to-slash"
                    261: option.
                    262: .TP
                    263: .B "-h, --help"
                    264: give help.
                    265: .TP
                    266: .B "-i COMMAND, --immediate-command COMMAND"
                    267: Send COMMAND to the receiver for execution, return immediately
                    268: upon the receiving program's successful recption of the command.
                    269: .TP
                    270: .B "-k, --1k"
                    271: (XMODEM/YMODEM) Send files using 1024 byte blocks
                    272: rather than the default 128 byte blocks.
                    273: 1024 byte packets speed file transfers at high bit rates.
                    274: (ZMODEM streams the data for the best possible throughput.)
                    275: .TP
                    276: .B "-L N, --packetlen N"
                    277: Use ZMODEM sub-packets of length N.
                    278: A larger N (32 <= N <= 1024) gives slightly higher throughput,
                    279: a smaller N speeds error recovery.
                    280: The default is 128 below 300 baud, 256 above 300 baud, or 1024 above 2400 baud.
                    281: .TP 
                    282: .B "-m N, --min-bps N"
                    283: Stop transmission if BPS-Rate (Bytes Per Second) falls below N for a 
                    284: certain time (see --min-bps-time option).
                    285: .TP
                    286: .B "-M N, --min-bps-time"
                    287: Used together with --min-bps. Default is 120 (seconds).
                    288: .TP
                    289: .B "-l N, --framelen N"
                    290: Wait for the receiver to acknowledge correct data every
                    291: .B N
                    292: (32 <= N <= 1024)
                    293: characters.
                    294: This may be used to avoid network overrun when XOFF flow control is lacking.
                    295: .TP
                    296: .B "-n, --newer"
                    297: (ZMODEM) Send each file if
                    298: destination file does not exist.
                    299: Overwrite destination file if
                    300: source file is newer than the destination file.
                    301: .TP
                    302: .B "-N, --newer-or-longer"
                    303: (ZMODEM) Send each file if
                    304: destination file does not exist.
                    305: Overwrite destination file if
                    306: source file is newer or longer than the destination file.
                    307: .TP
                    308: .B "-o, --16-bit-crc"
                    309: (ZMODEM) Disable automatic selection of 32 bit CRC.
                    310: .TP
                    311: .B "-O, --disable-timeouts"
                    312: Disable read timeout handling. This makes lsz hang if the other side
                    313: doesn't send anything, but increases performance (not much) and
                    314: decreases system load (reduces number of system calls by about 50
                    315: percent).
                    316: 
                    317: Use this option with care.
                    318: .TP
                    319: .B "-p, --protect"
                    320: (ZMODEM) Protect existing destination files by skipping transfer if the
                    321: destination file exists.
                    322: .TP
                    323: .B "-q, --quiet"
                    324: Quiet suppresses verbosity.
                    325: .TP
                    326: .B "-R, --restricted"
                    327: Restricted mode: restricts pathnames to the current directory and
                    328: PUBDIR (usually /usr/spool/uucppublic) and/or subdirectories
                    329: thereof.
                    330: .TP
                    331: .B "-r, --resume"
                    332: (ZMODEM) Resume interrupted file transfer.
                    333: If the source file is longer than the destination file,
                    334: the transfer commences at the offset in the source file that equals
                    335: the length of the destination file.
                    336: .TP
                    337: .B "-s HH:MM, --stop-at HH:MM"
                    338: Stop transmission at
                    339: .B HH 
                    340: hours, 
                    341: .B MM
                    342: minutes. Another variant, using 
                    343: .B +N 
                    344: instead of 
                    345: .B HH:MM,
                    346: stops transmission in
                    347: .B N
                    348: seconds.
                    349: .TP
                    350: .B "-S, --timesync"
                    351: enable timesync protocol support. See timesync.doc for further 
                    352: information.
                    353: 
                    354: This option is incompatible with standard zmodem. Use it with care.
                    355: .TP
                    356: .B "--syslog[=off]"
                    357: turn syslogging on or off. the default is set at configure time.
                    358: This option is ignored if no syslog support is compiled in.
                    359: .TP
                    360: .B "-t TIM, --timeout TIM"
                    361: Change timeout to
                    362: .I TIM
                    363: tenths of seconds.
                    364: .TP
                    365: .B "-T, --turbo"
                    366: Do not escape certain characters (^P, ^P|0x80, telenet escape sequence
                    367: [CR + @]). This improves performance by about 1 percent and shouldn't
                    368: hurt in the normal case (but be careful - ^P might be useful if connected
                    369: through a terminal server).
                    370: .TP
                    371: .B "--tcp"
                    372: Try to initiate a TCP/IP connection. lsz will ask the receiving zmodem
                    373: to open a TCP/IP connection. All handshaking (which address / port to
                    374: use) will be done by the zmodem programs.
                    375: 
                    376: You will normally not want to use this option as lrzsz is the only
                    377: zmodem which understands what to do (private extension). You might
                    378: want to use this option if the two programs are connected
                    379: (stdin/out) over a slow or bad (not 8bit clean) network connection.
                    380: 
                    381: Use of this option imposes a security risk, somebody else could connect
                    382: to the port in between. See
                    383: .B SECURITY
                    384: for details.
                    385: .TP
                    386: .B "--tcp-client ADDRESS:PORT"
                    387: Act as a tcp/ip client: Connect to the given port.
                    388: 
                    389: See
                    390: .B "--tcp-server"
                    391: for more information.
                    392: 
                    393: .TP
                    394: .B "--tcp-server"
                    395: Act as a server: Open a socket, print out what to do, wait for connection.
                    396: 
                    397: You will normally not want to use this option as lrzsz is the only
                    398: zmodem which understands what to do (private extension). You might
                    399: want to use this if you have to use zmodem (for which reason whatever),
                    400: and cannot use the
                    401: .B --tcp
                    402: option of
                    403: .I lsz
                    404: (perhaps because your telnet doesn't allow to spawn a local program
                    405: with stdin/stdout connected to the remote side).
                    406: 
                    407: If you use this option you have to start
                    408: .I lsz
                    409: with the
                    410: .B --tcp-client ADDRESS:PORT
                    411: option.
                    412: .I lrz will print the address and port on startup.
                    413: 
                    414: Use of this option imposes a security risk, somebody else could connect
                    415: to the port in between. See
                    416: .B SECURITY
                    417: for details.
                    418: 
                    419: .TP
                    420: .B "-u"
                    421: Unlink the file after successful transmission.
                    422: .TP
                    423: .B "-U, --unrestrict"
                    424: Turn off restricted mode (this is not possible if running under a 
                    425: restricted shell).
                    426: .TP
                    427: .B "-w N, --windowsize N"
                    428: Limit the transmit window size to N bytes (ZMODEM).
                    429: .TP
                    430: .B "-v, --verbose"
                    431: Verbose output to stderr. More v's generate more output.
                    432: .TP
                    433: .B "-X, --xmodem"
                    434: use XMODEM protocol.
                    435: .TP
                    436: .B "-y, --overwrite"
                    437: Instruct a ZMODEM receiving program to overwrite any existing file
                    438: with the same name.
                    439: .TP
                    440: .B "-Y, --overwrite-or-skip"
                    441: Instruct a ZMODEM receiving program to overwrite any existing file
                    442: with the same name,
                    443: and to skip any source files that do have a file with the same
                    444: pathname on the destination system.
                    445: .TP
                    446: .B "--ymodem"
                    447: use ZMODEM protocol.
                    448: .TP
                    449: .B "-Z, --zmodem"
                    450: use ZMODEM protocol.
                    451: .PD
                    452: .SH SECURITY
                    453: Restricted mode restricts pathnames to the current directory
                    454: and PUBDIR (usually /var/spool/uucppublic)
                    455: and/or subdirectories thereof, and disables remote command
                    456: execution.
                    457: 
                    458: Restricted mode is entered if the 
                    459: .B R
                    460: option is given or if lsz detects that it runs under a restricted
                    461: shell or if the environment variable ZMODEM_RESTRICTED is found.
                    462: 
                    463: Restricted mode can be turned of with the 
                    464: .B U
                    465: option if not running under a restricted shell.
                    466: 
                    467: .TP
                    468: Use of the
                    469: .B --tcp-client
                    470: or
                    471: .B --tcp-server
                    472: options imposes a security risk, as somebody else could connect to
                    473: the port before you do it, and grab your data. If there's strong
                    474: demand for a more secure mode i might introduce some sort of
                    475: password challenge.
                    476: 
                    477: 
                    478: .SH ENVIRONMENT
                    479: .TP
                    480: .B ZNULLS
                    481: may be used to specify the number of nulls to send before a ZDATA frame.
                    482: .TP
                    483: .B SHELL
                    484: lsz recognizes a restricted shell if this variable includes
                    485: .I "rsh"
                    486: or
                    487: .I "rksh"
                    488: .TP
                    489: .B ZMODEM_RESTRICTED
                    490: lrz enters restricted mode if the variable is set.
                    491: .TP
                    492: .B TMPDIR
                    493: If this environment variable is set its content is used as the 
                    494: directory to place in the answer file to a 
                    495: .B timesync 
                    496: request.
                    497: .B TMP
                    498: Used instead of TMPDIR if TMPDIR is not set. If neither TMPDIR nor
                    499: TMP is set /tmp will be used.
                    500: .SH EXAMPLES
                    501: .ne 7
                    502: .B "ZMODEM File Transfer"
                    503: (Unix to DSZ/ZCOMM/Professional-YAM)
                    504: .br
                    505: .B "% sz \-a *.c"
                    506: .br
                    507: This single command transfers all .c files in the current Unix directory
                    508: with conversion
                    509: .RB ( \-a )
                    510: to end of line conventions appropriate to the receiving environment.
                    511: With ZMODEM AutoDownload enabled, Professional-YAM  and ZCOMM
                    512: will automatically recieve
                    513: the files after performing a security check.
                    514: 
                    515: .br
                    516: .B "% sz \-Yan *.c *.h"
                    517: .br
                    518: Send only the .c and .h files that exist on both systems,
                    519: and are newer on the sending system than the
                    520: corresponding version on the receiving system, converting Unix to
                    521: DOS text format.
                    522: .br
                    523: .B
                    524: $ sz -\\Yan file1.c file2.c file3.c foo.h baz.h
                    525: .R
                    526: (for VMS)
                    527: .br
                    528: 
                    529: .B "ZMODEM Command Download"
                    530: (Unix to Professional-YAM)
                    531: .br
                    532:  cpszall:all
                    533:     sz \-c "c:;cd /yam/dist"
                    534:     sz \-ya $(YD)/*.me
                    535:     sz \-yqb y*.exe
                    536:     sz \-c "cd /yam"
                    537:     sz \-i "!insms"
                    538: .br
                    539: This Makefile fragment uses
                    540: .B sz
                    541: to issue commands to Professional-YAM to change current disk and directory.
                    542: Next,
                    543: .B sz
                    544: transfers the
                    545: .I .me
                    546: files from the $YD directory, commanding the receiver to overwrite the old files
                    547: and to convert from Unix end of line conventions to PC-DOS conventions.
                    548: The third line transfers some
                    549: .I .exe
                    550: files.
                    551: The fourth and fifth lines command Pro-YAM to
                    552: change directory and execute a PC-DOS batch file
                    553: .I insms .
                    554: Since the batch file takes considerable time, the
                    555: .B "\-i"
                    556: form is used to allow
                    557: .B sz
                    558: to exit immediately.
                    559: 
                    560: .B "XMODEM File Transfer"
                    561: (Unix to Crosstalk)
                    562: .br
                    563: %
                    564: .B "sx \-a foo.c"
                    565: .br
                    566: .B "ESC"
                    567: .br
                    568: .B "rx foo.c"
                    569: .br
                    570: The above three commands transfer a single file
                    571: from Unix to a PC and Crosstalk with
                    572: .I sz
                    573: translating Unix newlines to DOS CR/LF.
                    574: This combination is much slower and far less reliable than ZMODEM.
                    575: .SH ERROR MESSAGES
                    576: "Caught signal 99"
                    577: indicates the program was not properly compiled,
                    578: refer to "bibi(99)" in rbsb.c for details.
                    579: .SH SEE ALSO
                    580: rz(omen),
                    581: ZMODEM.DOC,
                    582: YMODEM.DOC,
                    583: Professional-YAM,
                    584: crc(omen),
                    585: sq(omen),
                    586: todos(omen),
                    587: tocpm(omen),
                    588: tomac(omen),
                    589: yam(omen)
                    590: 
                    591: Compile time options required for various operating systems are described in
                    592: the source file.
                    593: .SH "VMS VERSION"
                    594: The VMS version does not support wild cards.
                    595: Because of VMS DCL, upper case option letters muse be represented
                    596: by \\ proceding the letter.
                    597: 
                    598: The current VMS version does not support XMODEM, XMODEM-1k, or YMODEM.
                    599: 
                    600: VMS C Standard I/O and RMS may interact to modify the file contents.
                    601: .SH FILES
                    602: 32 bit CRC code courtesy Gary S. Brown.
                    603: 
                    604: sz.c, crctab.c, rbsb.c, zm.c, zmodem.h Unix source files
                    605: 
                    606: sz.c, crctab.c, vrzsz.c, zm.c, zmodem.h, vmodem.h, vvmodem.c,
                    607: VMS source files.
                    608: 
                    609: /tmp/szlog stores debugging output (sz -vv)
                    610: (szlog on VMS).
                    611: .SH "TESTING FEATURE"
                    612: The command "sz -T file"
                    613: exercises the
                    614: .B Attn
                    615: sequence error recovery by commanding
                    616: errors with unterminated packets.
                    617: The receiving program should complain five times about
                    618: binary data packets being too long.
                    619: Each time
                    620: .B sz
                    621: is interrupted,
                    622: it should send a ZDATA header followed by another defective packet.
                    623: If the receiver does not detect five long data packets,
                    624: the
                    625: .B Attn
                    626: sequence is not interrupting the sender, and the
                    627: .B Myattn
                    628: string in
                    629: .B sz.c
                    630: must be modified.
                    631: 
                    632: After 5 packets,
                    633: .B sz
                    634: stops the "transfer" and
                    635: prints the total number of characters "sent" (Tcount).
                    636: The difference between Tcount and 5120 represents the number of characters
                    637: stored in various buffers when the Attn sequence is generated.
                    638: .SH BUGS
                    639: Calling
                    640: .I sz
                    641: from most versions of cu(1) doesn't work because cu's receive process
                    642: fights
                    643: .I sz
                    644: for characters from the modem.
                    645: 
                    646: On at least one BSD system, sz would hang or exit when it got within
                    647: a few kilobytes of the end of file.
                    648: Using the "-w 8192" flag fixed the problem.
                    649: The real cause is unknown, perhaps a bug in the kernel TTY output routines.
                    650: 
                    651: Programs that do not properly implement the specified file transfer protocol
                    652: may cause
                    653: .I sz
                    654: to "hang" the port for a minute or two.
                    655: This problem is corrected by using
                    656: ZCOMM, Pro-YAM, or other program with a correct implementation
                    657: of the specified protocol.
                    658: 
                    659: Many programs claiming to support YMODEM only support XMODEM with 1k blocks,
                    660: and they often don't get that quite right.
                    661: 
                    662: XMODEM transfers add up to 127 garbage bytes per file.
                    663: XMODEM-1k and YMODEM-1k transfers use 128 byte blocks
                    664: to avoid extra padding.
                    665: 
                    666: YMODEM programs use the file length transmitted at the beginning of the
                    667: transfer to prune the file to the correct length; this may cause problems with
                    668: source files that grow during the course of the transfer.
                    669: This problem does not pertain to ZMODEM transfers, which preserve the exact
                    670: file length unconditionally.
                    671: 
                    672: Most ZMODEM options are merely passed to the receiving program;
                    673: some do not implement all these options.
                    674: 
                    675: Circular buffering and a ZMODEM sliding window should be used
                    676: when input is from pipes instead of acknowledging frames each 1024 bytes.
                    677: If no files can be opened,
                    678: .B sz
                    679: sends a ZMODEM command to echo a suitable complaint;
                    680: perhaps it should check for the presence of at least one accessible file before
                    681: getting hot and bothered.
                    682: The test mode leaves a zero length file on the receiving system.
                    683: 
                    684: A few high speed modems have a firmware bug that drops characters when the
                    685: direction of high speed transmissson is reversed.
                    686: The environment variable ZNULLS may be used to specify the number of nulls to
                    687: send before a ZDATA frame.
                    688: Values of 101 for a 4.77 mHz PC and 124 for an AT are typical.

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