86 lines
5.4 KiB
HTML
86 lines
5.4 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Use pointer data types in C and C++ applications that use SQL" />
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<meta name="abstract" content="You can also declare host variables that are pointers to the supported C and C++ data types, with the following restrictions." />
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<meta name="description" content="You can also declare host variables that are pointers to the supported C and C++ data types, with the following restrictions." />
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<meta name="DC.subject" content="pointer, C, C++, C program, host variable, using pointers, C++ program, using pointers" />
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<meta name="keywords" content="pointer, C, C++, C program, host variable, using pointers, C++ program, using pointers" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<title>Use pointer data types in C and C++ applications that use SQL</title>
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</head>
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<body id="rzajppointerdtc"><a name="rzajppointerdtc"><!-- --></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">Use pointer data types in C and C++ applications that use SQL</h1>
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<div><p>You can also declare host variables that are pointers to the supported
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C and C++ data types, with the following restrictions.</p>
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<div class="section"><ul><li>If a host variable is declared as a pointer, then that host variable must
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be declared with asterisks followed by a host variable. The following examples
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are all valid: <pre>short *mynum; /* Ptr to an integer */
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long **mynumptr; /* Ptr to a ptr to a long integer */
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char *mychar; /* Ptr to a single character */
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char(*mychara)[20]; /* Ptr to a char array of 20 bytes */
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struct { /* Ptr to a variable char array of 30 */
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short mylen; /* bytes. */
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char mydata[30];
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} *myvarchar;
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</pre>
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<div class="note"><span class="notetitle">Note:</span> Parentheses are only allowed when declaring a pointer to
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a NUL-terminated character array, in which case they are required. If the
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parentheses were not used, you would be declaring an array of pointers rather
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than the desired pointer to an array. For example: <pre>char (*a)[10]; /* pointer to a null-terminated char array */
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char *a[10]; /* pointer to an array of pointers */
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</pre>
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</div>
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</li>
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<li>If a host variable is declared as a pointer, then no other host variable
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can be declared with that same name within the same source file. For example,
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the second declaration below would be invalid: <pre>char *mychar; /* This declaration is valid */
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char mychar; /* But this one is invalid */
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</pre>
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</li>
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<li>When a host variable is referenced within an SQL statement, that host
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variable must be referenced exactly as declared, with the exception of pointers
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to NUL-terminated character arrays. For example, the following declaration
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required parentheses: <pre>char (*mychara)[20]; /* ptr to char array of 20 bytes */</pre>
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<p>However,
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the parentheses are not allowed when the host variable is referenced in an
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SQL statement, such as a SELECT: </p>
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<pre>EXEC SQL <strong>SELECT</strong> name <strong>INTO</strong> :*mychara <strong>FROM</strong> mytable;</pre>
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</li>
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<li>Only the asterisk can be used as an operator over a host variable name.</li>
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<li>The maximum length of a host variable name is affected by the number of
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asterisks specified, as these asterisks are considered part of the name.</li>
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<li>Pointers to structures are not usable as host variables except for variable
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character structures. Also, pointer fields in structures are not usable as
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host variables.</li>
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<li>SQL requires that all specified storage for based host variables be allocated.
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If the storage is not allocated, unpredictable results can occur.</li>
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</ul>
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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="rzajpc.htm" title="This topic describes the unique application and coding requirements for embedding SQL statements in a C or C++ program.">Code SQL statements in C and C++ applications</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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