989 lines
26 KiB
HTML
989 lines
26 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>read()--Read from Descriptor</title>
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<!-- Begin Header Records ========================================== -->
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- file cleaned -->
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<!-- Unix2 SCRIPT J converted by B2H R4.1 (346) (CMS) by V2KEA304 -->
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<!-- at RCHVMW2 on 17 Feb 1999 at 11:05:09 -->
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<!-- Change History: -->
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<!-- 010307 JTROUS Change to xref pread, pread64, pwrite, -->
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<!-- pwrite64 for V5R2, DCR 98686 -->
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<!-- 010703 JTROUS Add ERESTART, EINTR, V5R2, DCR 98686, add dev/zero-->
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<!-- 011022 JTROUS Changes from API Review 1, V5R2 -->
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<!-- 020618 EMIG: updated for NFS threadsafety, V5R3 -->
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<!-- 020624 RTHEIS: Add fclear() and fclear64(), V5R3 -->
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<!-- End Header Records -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<a name="Top_Of_Page"></a>
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<!-- Java sync-link -->
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<h2>read()--Read from Descriptor</h2>
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<div class="box" style="width: 60%;">
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<br>
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Syntax<br>
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<pre>
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#include <unistd.h>
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ssize_t read(int <em>file_descriptor</em>,
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void <em>*buf</em>, size_t <em>nbyte</em>);
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</pre>
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Service Program Name: QP0LLIB1<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Conditional; see <a href="#USAGE_NOTES">Usage
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Notes</a>.<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>From the file or socket indicated by <em>file_descriptor</em>, the <strong>
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read()</strong> function reads <em>nbyte</em> bytes of input into the memory
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area indicated by <em>buf</em>. If <em>nbyte</em> is zero, <strong>
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read()</strong> returns a value of zero without attempting any other
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action.</p>
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<p>If <em>file_descriptor</em> refers to a "regular file" (a stream file that
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can support positioning the file offset) or any other type of file on which the
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job can do an <strong>lseek()</strong> operation, <strong>read()</strong>
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begins reading at the file offset associated with <em>file_descriptor</em>. A
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successful <strong>read()</strong> changes the file offset by the number of
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bytes read.</p>
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<p>If <strong>read()</strong> is successful and <em>nbyte</em> is greater than
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zero, the access time for the file is updated.</p>
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<p><strong>read()</strong> is not supported for directories.</p>
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<p>If <em>file_descriptor</em> refers to a descriptor obtained using the
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<strong>open()</strong> function with O_TEXTDATA specified, the data is read
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from the file assuming it is in textual form. The maximum number of bytes on a
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single read that can be supported for text data is 2,147,483,408 (2GB - 240)
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bytes. The data is converted from the code page of the file to the code page of
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the application, job, or system as follows:</p>
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<ul>
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<li>When reading from a true stream file, any line-formatting characters (such
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as carriage return, tab, and end-of-file) are just converted from one code page
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to another.<br>
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</li>
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<li>When reading from record files that are being used as stream files,
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end-of-line characters are added to the end of the data in each record.</li>
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</ul>
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<p>There are some important considerations when the file is open for text
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conversion and the CCSIDs involved are not strictly single-byte:</p>
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<ul>
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<li>The <strong>read()</strong> will return the exact number of bytes
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requested. For some CCSIDs, this may mean that partial characters are returned
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at the end of the user buffer. In this case, the remainder of the character has
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been read from the file and internally buffered. The next consecutive read()
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will begin with the remainder of the partial character. However, if an <strong>
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lseek()</strong> is performed, the buffered data will be discarded. See <a
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href="lseek.htm">lseek()--Set File Read/Write Offset</a> for more
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information.<br>
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</li>
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<li>Because of the above consideration and because of the possible expansion or
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contraction of converted data, applications using the O_CCSID flag should avoid
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assumptions about data size and the current file offset. For example, a file
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might have a physical size of 100 bytes, but after an application has read 100
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bytes from the file, the current file offset may be 50. In order to read the
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whole file, the application might have to read 200 bytes or more, depending on
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the CCSIDs involved.</li>
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</ul>
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<p>If O_TEXTDATA was not specified on the <strong>open()</strong>, the data is
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read from the file without conversion. The application is responsible for
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handling the data.</p>
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<p>In the QSYS.LIB and independent ASP QSYS.LIB file systems, most end-of-file
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characters are symbolic; that is, they are stored outside the member. When
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reading:</p>
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<ul>
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<li>If O_TEXTDATA is specified, both symbolic and nonsymbolic end-of-file
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characters can be seen.<br>
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</li>
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<li>If O_TEXTDATA is not specified (binary mode), only nonsymbolic end-of-file
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characters can be seen.</li>
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</ul>
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<p>See the <em>Usage Notes</em> for <a href="write.htm">write()--Write to
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Descriptor</a>.</p>
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<p>When <em>file_descriptor</em> refers to a socket, the <strong>
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read()</strong> function reads from the socket identified by the socket
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descriptor.</p>
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<p>When attempting to read from an empty pipe or FIFO:</p>
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<ul>
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<li>If no job has the pipe or FIFO open for writing, <strong>read()</strong>
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return 0 to indicate end-of-file.<br>
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</li>
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<li>If some job has the pipe or FIFO open for writing and O_NONBLOCK was
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specified, <strong>read()</strong> will fail and <em>errno</em> will be set
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to [EAGAIN].<br>
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</li>
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<li>If some job has the pipe or FIFO open for writing and O_NONBLOCK was not
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specified, <strong>read()</strong> will block the calling thread until some
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data is written or until the pipe or FIFO is closed by all jobs that had the
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pipe or FIFO open for writing.</li>
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</ul>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>file_descriptor</strong></dt>
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<dd>(Input) The descriptor to be read.<br>
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</dd>
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<dt><strong>buf</strong></dt>
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<dd>(Output) A pointer to a buffer in which the bytes read are placed.<br>
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</dd>
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<dt><strong>nbyte</strong></dt>
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<dd>(Input) The number of bytes to be read.</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p>No authorization is required.</p>
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<br>
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<h3>Return Value</h3>
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<dl>
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<dt><em>value</em></dt>
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<dd><strong>read()</strong> was successful. The value returned is the number of
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bytes actually read and placed in <em>buf</em>. This number is less than or
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equal to <em>nbyte</em>. It is less than <em>nbyte</em> only if <strong>
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read()</strong> reached the end of the file before reading the requested number
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of bytes. If <strong>read()</strong> is reading a regular file and encounters a
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part of the file that has not been written (but before the end of the file),
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<strong>read()</strong> places bytes containing zeros into <em>buf</em> in
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place of the unwritten bytes.</dd>
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<dt><em>-1</em></dt>
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<dd><strong>read()</strong> was not successful. The <em>errno</em> global
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variable is set to indicate the error. If the value of <em>nbyte</em> is
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greater than SSIZE_MAX, <strong>read()</strong> sets <em>errno</em> to
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[EINVAL].</dd>
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</dl>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>read()</strong> is not successful, <em>errno</em> usually
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indicates one of the following errors. Under some conditions, <em>errno</em>
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could indicate an error other than those listed here.</p>
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<table cellpadding="5">
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<!-- cols="25 75" -->
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<tr>
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<th align="left" valign="bottom">Error condition</th>
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<th align="left" valign="bottom">Additional information</th>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EACCES">EACCES</a>]</em></td>
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<td align="left" valign="top">
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<p>If you are accessing a remote file through the Network File System, update
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operations to file permissions at the server are not reflected at the client
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until updates to data that is stored locally by the Network File System take
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place. (Several options on the Add Mounted File System (ADDMFS) command
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determine the time between refresh operations of local data.) Access to a
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remote file may also fail due to different mappings of user IDs (UID) or group
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IDs (GID) on the local and remote systems.</p>
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<p>This may occur if <em>file_descriptor</em> refers to a socket and the socket
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is using a connection-oriented transport service, and a <em>connect()</em> was
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previously completed. The thread, however, does not have the appropriate
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privileges to the objects that were needed to establish a connection. For
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example, the <em>connect()</em> required the use of an APPC device that the
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thread was not authorized to.</p>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EAGAIN">EAGAIN</a>]</em></td>
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<td align="left" valign="top">
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<p>If <em>file_descriptor</em> refers to a pipe or FIFO that has its O_NONBLOCK
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flag set, this error occurs if the <strong>read()</strong> would have blocked
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the calling thread.</p>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBADF">EBADF</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBADFID">EBADFID</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBUSY">EBUSY</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EDAMAGE">EDAMAGE</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EFAULT">EFAULT</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EINTR">EINTR</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EINVAL">EINVAL</a>]</em></td>
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<td align="left" valign="top">
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<p>This may occur if <em>file_descriptor</em> refers to a socket that is using
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a connectionless transport service, is not a socket of type <samp>
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SOCK_RAW</samp>, and is not bound to an address.</p>
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<p>The file resides in a file system that does not support large files, and the
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starting offset of the file exceeds 2GB minus 2 bytes.</p>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EIO">EIO</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOMEM">ENOMEM</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOTAVAIL">ENOTAVAIL</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOTSAFE">ENOTSAFE</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ENXIO">ENXIO</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EOVERFLOW">EOVERFLOW</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<p>The file is a regular file, <em>nbyte</em> is greater than 0, the starting
|
||
|
|
offset is before the end-of-file, and the starting offset is greater than or
|
||
|
|
equal to 2GB minus 2 bytes.</p>
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ERESTART">ERESTART</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ESTALE">ESTALE</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<p>If you are accessing a remote file through the Network File System, the file
|
||
|
|
may have been deleted at the server.</p>
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EUNKNOWN">EUNKNOWN</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<p>When the descriptor refers to a socket, <em>errno</em> could indicate one of
|
||
|
|
the following errors:</p>
|
||
|
|
|
||
|
|
<table cellpadding="5">
|
||
|
|
<!-- cols="25 75" -->
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<th align="left" valign="bottom">Error condition</th>
|
||
|
|
<th align="left" valign="bottom">Additional information</th>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNABORTED">ECONNABORTED</a>]</em></td>
|
||
|
|
|
||
|
|
<td>
|
||
|
|
<p>This error code indicates that the transport provider ended the connection
|
||
|
|
abnormally because of one of the following:</p>
|
||
|
|
<ul>
|
||
|
|
<li>The retransmission limit has been reached for data that was being sent on
|
||
|
|
the socket.</li>
|
||
|
|
<li>A protocol error was detected.<br></li>
|
||
|
|
</ul>
|
||
|
|
</td></tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNREFUSED">ECONNREFUSED</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNRESET">ECONNRESET</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EINTR">EINTR</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ENOTCONN">ENOTCONN</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<p>This error code is returned only on sockets that use a connection-oriented
|
||
|
|
transport service.</p>
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ETIMEDOUT">ETIMEDOUT</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<p>A non-blocking <strong>connect()</strong> was previously completed that
|
||
|
|
resulted in the connection timing out. No connection is established. This error
|
||
|
|
code is returned only on sockets that use a connection-oriented transport
|
||
|
|
service.</p>
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EUNATCH">EUNATCH</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EWOULDBLOCK">EWOULDBLOCK</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<p>If interaction with a file server is required to access the object, <em>
|
||
|
|
errno</em> could indicate one of the following errors:</p>
|
||
|
|
|
||
|
|
<table cellpadding="5">
|
||
|
|
<!-- cols="25 75" -->
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<th align="left" valign="bottom">Error condition</th>
|
||
|
|
<th align="left" valign="bottom">Additional information</th>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EADDRNOTAVAIL">EADDRNOTAVAIL</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNABORTED">ECONNABORTED</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNREFUSED">ECONNREFUSED</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ECONNRESET">ECONNRESET</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EHOSTDOWN">EHOSTDOWN</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EHOSTUNREACH">EHOSTUNREACH</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ENETDOWN">ENETDOWN</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ENETRESET">ENETRESET</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ENETUNREACH">ENETUNREACH</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ESTALE">ESTALE</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<p>If you are accessing a remote file through the Network File System, the file
|
||
|
|
may have been deleted at the server.</p>
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#ETIMEDOUT">ETIMEDOUT</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">
|
||
|
|
<em>[<a href="unix14.htm#EUNATCH">EUNATCH</a>]</em></td>
|
||
|
|
|
||
|
|
<td align="left" valign="top">
|
||
|
|
|
||
|
|
</td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<br>
|
||
|
|
<h3>Error Messages</h3>
|
||
|
|
|
||
|
|
<p>The following messages may be sent from this function:</p>
|
||
|
|
|
||
|
|
<table width="100%" cellpadding="5">
|
||
|
|
<tr>
|
||
|
|
<th align="left" valign="top">Message ID</th>
|
||
|
|
<th align="left" valign="top">Error Message Text</th>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">CPE3418 E</td>
|
||
|
|
<td align="left" valign="top">Possible APAR condition or hardware failure.</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">CPF3CF2 E</td>
|
||
|
|
<td align="left" valign="top">Error(s) occurred during running of &1
|
||
|
|
API.</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">CPF9872 E</td>
|
||
|
|
<td align="left" valign="top">Program or service program &1 in library
|
||
|
|
&2 ended. Reason code &3.</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">CPFA081 E</td>
|
||
|
|
<td align="left" valign="top">Unable to set return value or error code.</td>
|
||
|
|
</tr>
|
||
|
|
|
||
|
|
<tr>
|
||
|
|
<td align="left" valign="top">CPFA0D4 E</td>
|
||
|
|
<td align="left" valign="top">File system error occurred. Error number
|
||
|
|
&1.</td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<br>
|
||
|
|
<h3><a name="usage_notes">Usage Notes</a></h3>
|
||
|
|
|
||
|
|
<ol>
|
||
|
|
<li>This function will fail with error code [ENOTSAFE] when all the following
|
||
|
|
conditions are true:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Where multiple threads exist in the job.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The object on which this function is operating resides in a file system
|
||
|
|
that is not threadsafe. Only the following file systems are threadsafe for this
|
||
|
|
function:<br>
|
||
|
|
<br>
|
||
|
|
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>"Root" (/)</li>
|
||
|
|
|
||
|
|
<li>QOpenSys</li>
|
||
|
|
|
||
|
|
<li>User-defined</li>
|
||
|
|
|
||
|
|
<li>QNTC</li>
|
||
|
|
|
||
|
|
<li>QSYS.LIB</li>
|
||
|
|
|
||
|
|
<li>Independent ASP QSYS.LIB</li>
|
||
|
|
|
||
|
|
<li>QOPT</li>
|
||
|
|
|
||
|
|
<li>Network File System</li>
|
||
|
|
|
||
|
|
<li>QFileSvr.400</li>
|
||
|
|
</ul>
|
||
|
|
</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>QSYS.LIB and Independent ASP QSYS.LIB File System Differences
|
||
|
|
|
||
|
|
<p>This function will fail with error code [ENOTSAFE] if the object on which
|
||
|
|
this function is operation is a save file and multiple threads exist in the
|
||
|
|
job.</p>
|
||
|
|
|
||
|
|
<p>This function will fail with error code [EIO] if the file specified is a
|
||
|
|
save file and the file does not contain complete save file data.</p>
|
||
|
|
|
||
|
|
<p>The file access time for a database member is updated using the normal rules
|
||
|
|
that apply to database files. At most, the access time is updated once per
|
||
|
|
day.</p>
|
||
|
|
|
||
|
|
<p>If you previously used the integrated file system interface to manipulate a
|
||
|
|
member that contains an end-of-file character, you should avoid using other
|
||
|
|
interfaces (such as the Source Entry Utility or database reads and writes) to
|
||
|
|
manipulate the member. If you use other interfaces after using the integrated
|
||
|
|
file system interface, the end-of-file information will be lost.</p>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>QOPT File System Differences
|
||
|
|
|
||
|
|
<p>The file access time is not updated on a <strong>read()</strong>
|
||
|
|
operation.</p>
|
||
|
|
|
||
|
|
<p>When reading from files on volumes formatted in Universal Disk Format (UDF),
|
||
|
|
byte locks on the range being read are ignored.</p>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>Network File System Differences
|
||
|
|
|
||
|
|
<p>Local access to remote files through the Network File System may produce
|
||
|
|
unexpected results due to conditions at the server. Once a file is open,
|
||
|
|
subsequent requests to perform operations on the file can fail because file
|
||
|
|
attributes are checked at the server on each request. If permissions on the
|
||
|
|
file are made more restrictive at the server or the file is unlinked or made
|
||
|
|
unavailable by the server for another client, your operation on an open file
|
||
|
|
descriptor will fail when the local Network File System receives these updates.
|
||
|
|
The local Network File System also impacts operations that retrieve file
|
||
|
|
attributes. Recent changes at the server may not be available at your client
|
||
|
|
yet, and old values may be returned from operations. (Several options on the
|
||
|
|
Add Mounted File System (ADDMFS) command determine the time between refresh
|
||
|
|
operations of local data.)</p>
|
||
|
|
|
||
|
|
<p>Reading and writing to files with the Network File System relies on
|
||
|
|
byte-range locking to guarantee data integrity. To prevent data inconsistency,
|
||
|
|
use the <strong>fcntl()</strong> API to get and release these locks.</p>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>QFileSvr.400 File System Differences
|
||
|
|
|
||
|
|
<p>The largest buffer size allowed is 16 megabytes. If a larger buffer is
|
||
|
|
passed, the error EINVAL will be received.</p>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>For sockets that use a connection-oriented transport service (for example,
|
||
|
|
sockets with a type of <samp>SOCK_STREAM</samp>), a return value of zero
|
||
|
|
indicates one of the following:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>The partner program has issued a <strong>close()</strong> for the
|
||
|
|
socket.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The partner program has issued a <strong>shutdown()</strong> to disable
|
||
|
|
writing to the socket.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The connection is broken and the error was returned on a previously issued
|
||
|
|
socket function.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>A <strong>shutdown()</strong> to disable reading was previously done on the
|
||
|
|
socket.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The following applies to sockets that use a connectionless transport
|
||
|
|
service (for example, a socket with a type of <samp>SOCK_DGRAM</samp>).
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>If a <strong>connect()</strong> has been issued previously, then data can
|
||
|
|
be received only from the address specified in the previous <strong>
|
||
|
|
connect()</strong>.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The address from which data is received is discarded, since the <strong>
|
||
|
|
read()</strong> has no address parameter.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The entire message must be read in a single read operation. If the size of
|
||
|
|
the message is too large to fit in the user supplied buffer, the remaining
|
||
|
|
bytes of the message are discarded.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>A returned value of zero indicates one of the following:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>The partner program has sent a NULL message (a datagram with no user
|
||
|
|
data).<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>A <strong>shutdown()</strong> to disable reading was previously done on the
|
||
|
|
socket.<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The buffer length specified was zero.</li>
|
||
|
|
</ul>
|
||
|
|
</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>For file systems that do not support large files, <strong>read()</strong>
|
||
|
|
will return [EINVAL] if the starting offset exceeds 2GB minus 2 bytes,
|
||
|
|
regardless of how the file was opened. For the file systems that do support
|
||
|
|
large files, <strong>read()</strong> will return [EOVERFLOW] if the starting
|
||
|
|
offset exceeds 2GB minus 2 bytes and the file was not opened for large file
|
||
|
|
access.<br>
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>Using this function successfully on the /dev/null or /dev/zero character
|
||
|
|
special file results in a return value of zero. In addition, the access time
|
||
|
|
for the file is updated.</li>
|
||
|
|
</ol>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<h3>Related Information</h3>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>The <<strong>limits.h</strong>> file (see <a href="unix13.htm">Header
|
||
|
|
Files for UNIX-Type Functions</a>)
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The <<strong>unistd.h</strong>> file (see <a href="unix13.htm">Header
|
||
|
|
Files for UNIX-Type Functions</a>)
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="creat.htm">creat()</a>--Create or Rewrite File
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="dup.htm">dup()</a>--Duplicate Open File Descriptor
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="dup2.htm">dup2()</a>--Duplicate Open File Descriptor to Another
|
||
|
|
Descriptor
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="fclear.htm">fclear()</a>--Write (Binary Zeros) to Descriptor
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="fclear64.htm">fclear64()</a>--Write (Binary Zeros) to Descriptor
|
||
|
|
(Large File Enabled)</li>
|
||
|
|
|
||
|
|
<li><a href="fcntl.htm">fcntl()</a>--Perform File Control Command
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="ioctl.htm">ioctl()</a>--Perform I/O Control Request
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="lseek.htm">lseek()</a>--Set File Read/Write Offset
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="open.htm">open()</a>--Open File
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="pread.htm">pread()</a>--Read from Descriptor with Offset
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="pread64.htm">pread64()</a>--Read from Descriptor with Offset
|
||
|
|
(large file enabled)
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="pwrite.htm">pwrite()</a>--Write to Descriptor with Offset
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="pwrite64.htm">pwrite64()</a>--Write to Descriptor with Offset
|
||
|
|
(large file enabled)
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="readv.htm">readv()</a>--Read from Descriptor Using Multiple
|
||
|
|
Buffers
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="recv.htm">recv()</a>--Receive Data
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="recvfr.htm">recvfrom()</a>--Receive Data
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="recvms.htm">recvmsg()</a>--Receive Data or Descriptors or Both
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="write.htm">write()</a>--Write to Descriptor
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="writev.htm">writev()</a>--Write to Descriptor Using Multiple
|
||
|
|
Buffers</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<h3>Example</h3>
|
||
|
|
|
||
|
|
<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
|
||
|
|
for information pertaining to code examples.</p>
|
||
|
|
|
||
|
|
<p>The following example opens a file and reads input:</p>
|
||
|
|
|
||
|
|
<pre>
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <unistd.h>
|
||
|
|
#include <fcntl.h>
|
||
|
|
|
||
|
|
main() {
|
||
|
|
int ret, file_descriptor, rc;
|
||
|
|
char buf[]="Test text";
|
||
|
|
|
||
|
|
if ((file_descriptor = creat("test.output", S_IWUSR))!= 0)
|
||
|
|
perror("creat() error");
|
||
|
|
else {
|
||
|
|
if (-1==(rc=write(file_descriptor, buf, sizof(buf)-1)))
|
||
|
|
perror("write() error");
|
||
|
|
if (close(file_descriptor)!= 0)
|
||
|
|
perror("close() error");
|
||
|
|
}
|
||
|
|
|
||
|
|
if ((file_descriptor = open("test.output", O_RDONLY)) < 0)
|
||
|
|
perror("open() error");
|
||
|
|
else {
|
||
|
|
ret = read(file_descriptor, buf, sizeof(buf)-1));
|
||
|
|
buf[ret] = 0x00;
|
||
|
|
printf("block read: \n<%s>\", buf);
|
||
|
|
if (close(file_descriptor)!= 0)
|
||
|
|
perror("close() error");
|
||
|
|
}
|
||
|
|
if (unlink("test.output")!= 0)
|
||
|
|
perror("unlink() error");
|
||
|
|
}
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<p><strong>Output:</strong></p>
|
||
|
|
|
||
|
|
<pre>
|
||
|
|
block read:
|
||
|
|
<Test text>
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<hr>
|
||
|
|
API introduced: V3R1
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<center>
|
||
|
|
<table cellpadding="2" cellspacing="2">
|
||
|
|
<tr align="center">
|
||
|
|
<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
|
||
|
|
"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
</center>
|
||
|
|
</body>
|
||
|
|
</html>
|
||
|
|
|