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133 lines
5.7 KiB
HTML
133 lines
5.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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version published by the Free Software Foundation; with no Invariant Sections,
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<head>
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<title>Parameter Conventions (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="Parameter Conventions (GNU MP 6.1.0)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="GMP-Basics.html#GMP-Basics" rel="up" title="GMP Basics">
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<link href="Memory-Management.html#Memory-Management" rel="next" title="Memory Management">
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<link href="Variable-Conventions.html#Variable-Conventions" rel="prev" title="Variable Conventions">
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</head>
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<body lang="en">
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<a name="Parameter-Conventions"></a>
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<div class="header">
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<p>
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Next: <a href="Memory-Management.html#Memory-Management" accesskey="n" rel="next">Memory Management</a>, Previous: <a href="Variable-Conventions.html#Variable-Conventions" accesskey="p" rel="prev">Variable Conventions</a>, Up: <a href="GMP-Basics.html#GMP-Basics" accesskey="u" rel="up">GMP Basics</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Parameter-Conventions-1"></a>
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<h3 class="section">3.5 Parameter Conventions</h3>
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<a name="index-Parameter-conventions"></a>
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<a name="index-Conventions-for-parameters"></a>
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<p>When a GMP variable is used as a function parameter, it’s effectively a
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call-by-reference, meaning if the function stores a value there it will change
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the original in the caller. Parameters which are input-only can be designated
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<code>const</code> to provoke a compiler error or warning on attempting to modify
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them.
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</p>
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<p>When a function is going to return a GMP result, it should designate a
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parameter that it sets, like the library functions do. More than one value
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can be returned by having more than one output parameter, again like the
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library functions. A <code>return</code> of an <code>mpz_t</code> etc doesn’t return the
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object, only a pointer, and this is almost certainly not what’s wanted.
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</p>
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<p>Here’s an example accepting an <code>mpz_t</code> parameter, doing a calculation,
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and storing the result to the indicated parameter.
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</p>
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<div class="example">
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<pre class="example">void
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foo (mpz_t result, const mpz_t param, unsigned long n)
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{
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unsigned long i;
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mpz_mul_ui (result, param, n);
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for (i = 1; i < n; i++)
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mpz_add_ui (result, result, i*7);
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}
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int
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main (void)
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{
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mpz_t r, n;
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mpz_init (r);
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mpz_init_set_str (n, "123456", 0);
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foo (r, n, 20L);
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gmp_printf ("%Zd\n", r);
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return 0;
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}
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</pre></div>
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<p><code>foo</code> works even if the mainline passes the same variable for
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<code>param</code> and <code>result</code>, just like the library functions. But
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sometimes it’s tricky to make that work, and an application might not want to
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bother supporting that sort of thing.
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</p>
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<p>For interest, the GMP types <code>mpz_t</code> etc are implemented as one-element
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arrays of certain structures. This is why declaring a variable creates an
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object with the fields GMP needs, but then using it as a parameter passes a
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pointer to the object. Note that the actual fields in each <code>mpz_t</code> etc
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are for internal use only and should not be accessed directly by code that
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expects to be compatible with future GMP releases.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Memory-Management.html#Memory-Management" accesskey="n" rel="next">Memory Management</a>, Previous: <a href="Variable-Conventions.html#Variable-Conventions" accesskey="p" rel="prev">Variable Conventions</a>, Up: <a href="GMP-Basics.html#GMP-Basics" accesskey="u" rel="up">GMP Basics</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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