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HTML
197 lines
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Open processing for tape device files" />
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<meta name="abstract" content="Use this information to find out the considerations for opening tape device files." />
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<meta name="description" content="Use this information to find out the considerations for opening tape device files." />
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<meta name="DC.Relation" scheme="URI" content="../apis/qtardcap.htm" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Open processing for tape device files</title>
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</head>
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<body id="opcsd"><a name="opcsd"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Open processing for tape device files</h1>
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<div><p>Use this information to find out the considerations for opening
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tape device files.</p>
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<p>The following considerations apply to opening tape device files: </p>
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<ul><li>When the program opens a tape device file, the system merges any parameters
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that are specified in the file with the parameters that are specified in the
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program. The system then merges these parameters with the parameters that
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are specified on an OVRTAPF command.</li>
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<li>Specify the device names when you open the tape device file. If you specify
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DEV(*NONE) in the tape file, you must specify one or more device names on
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an OVRTAPF command. You can specify as many as four device names for a single
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tape device file (depending on how many magnetic tape devices you have). <p>The
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record length, block length, record-block-format, and buffer offset (for an
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ASCII file) always return to the program in the data management open feedback
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area. They return in the format in which they are written in the HDR2 file
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header label. This information is available regardless of the type of label
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processing that is used for the file.</p>
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</li>
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<li>The following data files support the read-backward operation for both
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single volume tape data files and multivolume-tape data files: <ul><li>fixed (*F)</li>
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<li>fixed block (*FB)</li>
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<li>undefined format (*U)</li>
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</ul>
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You request a read backward operation through a high-level language when
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you open the file. An escape message is signaled if a read-backward operation
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is attempted for variable-length (spanned or not spanned) or source records.
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<div class="note"><span class="notetitle">Note:</span> Use the Retrieve Device Capabilities (QTARDCAP) API to determine the
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device capabilities of your device. The following tape devices do not have
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read-backward capabilities: <ul><li>9348 tape unit</li>
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<li>8-mm cartridge device</li>
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<li>Some 1/4-inch cartridge devices</li>
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</ul>
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</div>
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<p>Reading
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a data file backwards, where you specify the device and volume list, you must position the volumes
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on the device in reverse order. For example, a device file with DEV(QTAPE1
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QTAPE2) VOL(VOL01 VOL02 VOL03) expects VOL03 on QTAPE1, VOL02 on QTAPE2, and
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then VOL01 on QTAPE1.</p>
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<div class="p">For a read-backward operation, the end-of-file
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condition occurs if the system recognizes the first volume of the data file
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from the header labels for the following label processing parameter values: <ul><li>Standard label processing (*SL)</li>
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<li>Bypass label processing (*BLP)</li>
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</ul>
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If the system does not recognize the header labels for the first volume
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of the data file, or if this is a *BLP file, the system signals the end-of-file
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condition when: <ul><li>The system processes the specified number of reels.</li>
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<li>The system processes the number of identifiers on the VOL parameter.</li>
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</ul>
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</div>
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</li>
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<li>Some high-level languages allow you to specify where to position the tape when the program opens an input tape device
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file. This indicates whether you process the tape in the forward or in the
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backward direction. These rules determine the first volume of a data file: <ul><li>HDR1 labels multivolume sequence field = 1 (ASCII or EBCDIC with no HDR2
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label) <p>or</p>
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</li>
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<li>HDR2 label volume switch indicator field = 0 (EBCDIC)</li>
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</ul>
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<p>The program specifies the record length according to the information that is shown in <a href="#opcsd__rft">Table 1</a>.</p>
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</li>
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<li>For source files, the record length used to determine the block length
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is the actual data length, not the data length plus 12 bytes (for sequence
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number and date).</li>
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<li>You must supply either a RCDLEN or BLKLEN parameter value
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for unspanned, deblock records (*F, *V, *D, *U).</li>
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<li>You must supply both RCDLEN and BLKLEN parameter values for spanned or
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blocked records (*FB, *VB, *DB, *VS, *VBS).</li>
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<li>When the file type specified in the tape device file is a source file: <ul><li>The system appends a date and sequence number to each record on input
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operations. The date field is always 0.</li>
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<li>The system removes the date and sequence number from each record on output
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operations</li>
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</ul>
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The
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program can check (if the high-level language you are using allows it) to
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determine if the input or output file is a source file. The record length
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must include 12 bytes for the date and sequence number. The block length and
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record length ratio remains the same for a source block and data record minus
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the 12 bytes allocated to source files. For example, if the actual data record
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length is 80, the record length becomes 92 for a source file. The block length
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remains unchanged.</li>
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<li>To process input files using standard labels the system will always use
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the block length in the file label. The device file block length is ignored. </li>
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<li>Variable-length (spanned or unspanned) records
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and undefined format records can be used for output files. If your high-level
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language does not support variable-length records, then all records for an
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output tape device file that uses variable format are maximum length.</li>
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<li>Specify the sequence number so you can find the data
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file. You cannot locate tape data files by label name.</li>
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<li>When you specify both the VOL and REELS parameters, the REELS parameter
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is ignored. If
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you want to use the REELS parameter (number of reels) to limit the number
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of input volumes that is processed, specify *NONE for the VOL parameter.</li>
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</ul>
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<div class="tablenoborder"><a name="opcsd__rft"><!-- --></a><table cellpadding="4" cellspacing="0" summary="" id="opcsd__rft" width="100%" frame="hsides" border="0" rules="none"><caption>Table 1. Specify record lengths
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by record and format type</caption><thead align="left"><tr><th align="left" valign="bottom" width="16.636528028933093%" id="d0e258">Record and format type</th>
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<th align="left" valign="bottom" width="16.636528028933093%" id="d0e260">Minimum record length for *DATA</th>
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<th align="left" valign="bottom" width="16.636528028933093%" id="d0e262">Minimum record length for *SRC</th>
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<th align="left" valign="bottom" width="16.636528028933093%" id="d0e264">Maximum record length for *DATA</th>
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<th align="left" valign="bottom" width="16.636528028933093%" id="d0e266">Maximum record length for *SRC</th>
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<th align="left" valign="bottom" width="16.817359855334537%" id="d0e268">Block length</th>
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</tr>
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</thead>
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<tbody><tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">Fixed blocked, *F, *FB, *U</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">18</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">30</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">32 767</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">32 767</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 ">Multiple of *DATA record length</td>
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</tr>
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<tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">Variable unblocked, *V</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">1</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">13</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">32 759 (See Note)</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">32 767</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 ">Equal to maximum *DATA record length plus
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8</td>
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</tr>
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<tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">D-type ASCII unblocked, *D</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">1</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">13</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">9 995 (See Note)</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">10 007 (See Note)</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 ">Equal to maximum *DATA record length plus
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4, plus buffer offset</td>
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</tr>
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<tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">Variable blocked, *VB</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">1</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">13</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">32 759</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">32 767</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 ">At least maximum *DATA record length plus
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8</td>
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</tr>
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<tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">D-type ASCII blocked, *DB</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">1</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">13</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">9 995 (See Note)</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">10 007 (See Note)</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 ">At least maximum *DATA record length plus
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4, plus buffer offset</td>
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</tr>
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<tr><td align="left" valign="top" width="16.636528028933093%" headers="d0e258 ">*VS, *VBS</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e260 ">1</td>
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<td align="center" valign="top" width="16.636528028933093%" headers="d0e262 ">13</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e264 ">32 759</td>
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<td align="left" valign="top" width="16.636528028933093%" headers="d0e266 ">32 759</td>
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<td align="left" valign="top" width="16.817359855334537%" headers="d0e268 "> </td>
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</tr>
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<tr><td colspan="6" align="left" valign="top" headers="d0e258 d0e260 d0e262 d0e264 d0e266 d0e268 "> <div class="note"><span class="notetitle">Note:</span> This
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is the maximum record length for a record being written to a tape. Input
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records can be padded to 32 767.</div>
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</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="usetfhll.htm" title="Understand the different processing types for tape device files used in high-level language.">Use tape device files in high-level language programs</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="../apis/qtardcap.htm">Retrieve Device Capabilities (QTARDCAP) API</a></div>
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</div>
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</div>
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</body>
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</html>
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