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| 8 | This file is part of the GNU MPFR Library.
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| 10 | The GNU MPFR Library is free software; you can redistribute it and/or modify
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| 29 | <title>Frequently Asked Questions about GNU MPFR</title>
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| 112 | </head>
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| 113 |
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| 114 | <body>
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| 115 |
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| 116 | <h1>Frequently Asked Questions about <cite><acronym>GNU</acronym> <acronym>MPFR</acronym></cite></h1>
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| 117 |
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| 118 | <p><strong>Important notice: Problems with a particular version of
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| 119 | <cite><acronym>MPFR</acronym></cite> are discussed in the corresponding
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| 120 | bugs page.</strong></p>
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| 121 |
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| 122 | <p>The latest version of this <acronym>FAQ</acronym> is available at
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| 123 | <a href="https://www.mpfr.org/faq.html">https://www.mpfr.org/faq.html</a>.
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| 124 | Please look at this version if possible.</p>
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| 125 |
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| 126 | <ol>
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| 127 | <li><a href="#mpfr_vs_mpf">What are the differences between
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| 128 | <cite><acronym>MPF</acronym></cite> from <cite><acronym>GMP</acronym></cite>
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| 129 | and <cite><acronym>MPFR</acronym></cite>?</a></li>
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| 130 | <li><a href="#mpf2mpfr">How to convert my program written using
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| 131 | <cite><acronym>MPF</acronym></cite> to
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| 132 | <cite><acronym>MPFR</acronym></cite>?</a></li>
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| 133 | <li><a href="#no_libgmp">At configure time, I get the error: <q>libgmp not found or uses a different ABI.</q></a></li>
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| 134 | <li><a href="#undef_ref1">I get undefined reference to <code>__gmp_get_memory_functions</code>.</a></li>
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| 135 | <li><a href="#undef_ref2">When I link my program with
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| 136 | <cite><acronym>MPFR</acronym></cite>, I get undefined reference
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| 137 | to <code>__gmpXXXX</code>.</a></li>
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| 138 | <li><a href="#crash_high_prec">My program crashes with high precisions.</a></li>
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| 139 | <li><a href="#accuracy">Though I have increased the precision, the results
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| 140 | are not more accurate.</a></li>
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| 141 | <li><a href="#detect_mpfr">How can I detect <cite><acronym>MPFR</acronym></cite>
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| 142 | installation using <cite>autoconf</cite> or <cite>pkg-config</cite>?</a></li>
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| 143 | <li><a href="#cite">How to cite <cite><acronym>MPFR</acronym></cite> in a
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| 144 | scientific publication?</a></li>
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| 145 | <li><a href="#fpic">When I build <cite><acronym>MPFR</acronym></cite>, I get
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| 146 | an error asking me to recompile with <samp>-fPIC</samp>.</a></li>
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| 147 | </ol>
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| 148 |
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| 149 | <dl class="faq">
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| 150 |
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| 151 | <dt id="mpfr_vs_mpf">1. What are the differences between
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| 152 | <cite><acronym>MPF</acronym></cite> from <cite><acronym>GMP</acronym></cite>
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| 153 | and <cite><acronym>MPFR</acronym></cite>?</dt>
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| 154 |
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| 155 | <dd><p>The main differences are:</p>
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| 156 | <ul>
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| 157 | <li><p>The precision of a <cite><acronym>MPFR</acronym></cite> variable
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| 158 | is the <em>exact</em> number of bits used for its mantissa, whereas in
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| 159 | <cite><acronym>MPF</acronym></cite>, the precision requested by the user
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| 160 | is a minimum value (<cite><acronym>MPF</acronym></cite> generally uses a
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| 161 | higher precision). With the additional difference below, this implies that
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| 162 | the <cite><acronym>MPFR</acronym></cite> results do not depend on the
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| 163 | number of bits (16, 32, 64 or more) of the underlying architecture.</p></li>
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| 164 | <li><p>As a consequence, <cite><acronym>MPFR</acronym></cite> uses a
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| 165 | base-2 exponent, whereas in <cite><acronym>MPF</acronym></cite>, this
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| 166 | is a base-2<sup>32</sup> or base-2<sup>64</sup> exponent, depending on
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| 167 | the limb size. For this reason (and other internal ones), the maximum
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| 168 | exponent range in <cite><acronym>MPFR</acronym></cite> is different
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| 169 | (and smaller, if the exponent is represented by the same type as in
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| 170 | <cite><acronym>MPF</acronym></cite>).</p></li>
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| 171 | <li><p><cite><acronym>MPFR</acronym></cite> provides an additional rounding
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| 172 | mode argument to its functions; furthermore, it is guaranteed that the
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| 173 | result of any operation is the nearest possible floating-point value from
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| 174 | the exact result (considering the input variables as exact values), taking
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| 175 | into account the precision of the destination variable and the rounding
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| 176 | mode. <cite><acronym>MPFR</acronym></cite> also says whether the rounded
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| 177 | result is above or below the exact result.</p></li>
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| 178 | <li><p><cite><acronym>MPFR</acronym></cite> supports much more functions
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| 179 | (in particular transcendental functions such as exponentials, logarithms,
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| 180 | trigonometric functions and so on) and special values: signed zeros,
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| 181 | infinities, not-a-number (NaN).</p></li>
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| 182 | </ul></dd>
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| 183 |
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| 184 | <dt id="mpf2mpfr">2. How to convert my program written using
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| 185 | <cite><acronym>MPF</acronym></cite> to
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| 186 | <cite><acronym>MPFR</acronym></cite>?</dt>
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| 187 |
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| 188 | <dd><p>You need to add <q><code>r</code></q> to the function names, and to
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| 189 | specify the rounding mode (<code>MPFR_RNDN</code> for rounding to nearest,
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| 190 | <code>MPFR_RNDZ</code> for rounding toward zero, <code>MPFR_RNDU</code>
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| 191 | for rounding toward plus infinity, <code>MPFR_RNDD</code> for rounding
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| 192 | toward minus infinity). You can also define macros as follows:
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| 193 | <code class="block-code">#define mpf_add(a, b, c) mpfr_add(a, b, c, MPFR_RNDN)</code></p>
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| 194 | <p>The header file <samp>mpf2mpfr.h</samp> from the
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| 195 | <cite><acronym>MPFR</acronym></cite> distribution automatically
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| 196 | redefines all <cite><acronym>MPF</acronym></cite> functions in this
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| 197 | way, using the default <cite><acronym>MPFR</acronym></cite> rounding
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| 198 | mode. Thus you simply need to add the following line in all your files
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| 199 | using <cite><acronym>MPF</acronym></cite> functions:
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| 200 | <code class="block-code">#include <mpf2mpfr.h></code>
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| 201 | just after the <samp>gmp.h</samp> and <samp>mpfr.h</samp>
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| 202 | header files. If the program uses <cite><acronym>MPF</acronym></cite>
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| 203 | internals (such as direct access to <code>__mpf_struct</code> members),
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| 204 | additional changes will be needed.</p></dd>
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| 205 |
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| 206 | <dt id="no_libgmp">3. At configure time, I get the error: <q>libgmp not found or uses a different ABI.</q></dt>
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| 207 |
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| 208 | <dd><p>This test (<samp>checking for __gmpz_init in -lgmp</samp>) comes
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| 209 | after the <samp>gmp.h</samp> detection. The failure occurs either because
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| 210 | the <cite><acronym>GMP</acronym></cite> library could not be found
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| 211 | (as it is not in the provided library search paths) or because the
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| 212 | <cite><acronym>GMP</acronym></cite> library that was found does not have
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| 213 | the expected <acronym title="Application Binary Interface">ABI</acronym>
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| 214 | (<abbr>e.g.</abbr> 32-bit <abbr>vs</abbr> 64-bit). The former problem can be
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| 215 | due to the fact that a static build of <cite><acronym>MPFR</acronym></cite>
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| 216 | was requested while only a shared <cite><acronym>GMP</acronym></cite> library
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| 217 | is installed (or the opposite, but another error can also show up in this
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| 218 | case, see the <a href="#fpic">question about <samp>-fPIC</samp></a>). The
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| 219 | latter problem can have several causes:</p>
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| 220 | <ul>
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| 221 | <li>A wrong libgmp library has been picked up. This can occur if you have
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| 222 | several <cite><acronym>GMP</acronym></cite> versions installed on the
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| 223 | machine and something is wrong with the provided library search paths.</li>
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| 224 | <li>Wrong compiler options (<samp>CFLAGS</samp>) were given. In general, the
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| 225 | presence or absence of the <samp>-m64</samp> compiler option must match the
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| 226 | library <acronym title="Application Binary Interface">ABI</acronym>.</li>
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| 227 | <li>A wrong <samp>gmp.h</samp> file has been picked up (if you have several
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| 228 | <cite><acronym>GMP</acronym></cite> versions installed). Indeed, by default,
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| 229 | <cite><acronym>MPFR</acronym></cite> gets the compiler options from the
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| 230 | <samp>gmp.h</samp> file (with <cite><acronym>GMP</acronym></cite> 4.2.3
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| 231 | or later); this is needed because <cite><acronym>GMP</acronym></cite> does
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| 232 | not necessarily use the default <acronym>ABI</acronym>. The consequence is
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| 233 | that if the <samp>gmp.h</samp> file is associated with a library using a
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| 234 | different <acronym>ABI</acronym>, the <acronym>ABI</acronym>-related options
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| 235 | will be incorrect. Hence the failure.</li>
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| 236 | </ul>
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| 237 | <p>Note: The <samp>config.log</samp> output gives more information
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| 238 | than the error message. In particular, see the output of the test:
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| 239 | <samp>checking for CC and CFLAGS in gmp.h</samp>; it should give you
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| 240 | the default compiler options (from <samp>gmp.h</samp>).</p>
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| 241 |
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| 242 | <p>See also the answer to the <a href="#undef_ref1">next question</a>.</p></dd>
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| 243 |
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| 244 | <dt id="undef_ref1">4. I get undefined reference to <code>__gmp_get_memory_functions</code>.</dt>
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| 245 |
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| 246 | <dd><p>Note: this was mainly a problem when upgrading from
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| 247 | <cite><acronym>GMP</acronym></cite> 4.1.4 to a later version,
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| 248 | but information given below may still be useful in other cases,
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| 249 | when several <cite><acronym>GMP</acronym></cite> libraries are
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| 250 | installed on the same machine.</p>
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| 251 |
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| 252 | <p>If you get such an error, in particular when running
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| 253 | <samp>make check</samp>, then this probably means that you are using
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| 254 | the header file from <cite><acronym>GMP</acronym></cite> 4.2.x but the
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| 255 | <cite><acronym>GMP</acronym></cite> 4.1.4 library. This can happen if
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| 256 | several <cite><acronym>GMP</acronym></cite> versions are installed on
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| 257 | your machine (<abbr>e.g.</abbr>, one provided by the system in
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| 258 | <samp>/usr/{include,lib}</samp> and a new one installed by the owner or
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| 259 | administrator of the machine in <samp>/usr/local/{include,lib}</samp>)
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| 260 | and your include and library search paths are inconsistent. On various
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| 261 | <acronym>GNU</acronym>/Linux machines, this is unfortunately the case
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| 262 | by default (<samp>/usr/local/include</samp> is in the default include
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| 263 | search path, but <samp>/usr/local/lib</samp> is <em>not</em> in the
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| 264 | default library search path). Typical errors are:
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| 265 | <samp class="block-code">undefined reference to `__gmp_get_memory_functions'</samp>
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| 266 | in <samp>make check</samp>. The best solution is to add
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| 267 | <samp>/usr/local/include</samp> to your <var class="env">C_INCLUDE_PATH</var>
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| 268 | environment variable and to add <samp>/usr/local/lib</samp> to your
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| 269 | <var class="env">LIBRARY_PATH</var> and <var class="env">LD_LIBRARY_PATH</var>
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| 270 | environment variables (and/or <var class="env">LD_RUN_PATH</var>).
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| 271 | Alternatively, you can use <samp>--with-gmp*</samp> configure options,
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| 272 | <abbr>e.g.</abbr> <samp>--with-gmp=/usr/local</samp>, but <strong>this is
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| 273 | not guaranteed to work</strong> (in particular with <samp>gcc</samp> and
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| 274 | system directories such as <samp>/usr</samp> or <samp>/usr/local</samp>),
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| 275 | and other software that uses <cite><acronym>GMP</acronym></cite> and/or
|
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| 276 | <cite><acronym>MPFR</acronym></cite> will need correct paths too;
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| 277 | environment variables allow you to set them in a global way.</p>
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| 278 | <p>Other information can be given in the <samp>INSTALL</samp> file and
|
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| 279 | <samp>ld</samp> manual. Please look at them for more details. See also
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| 280 | the <a href="#undef_ref2">next question</a>.</p></dd>
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| 281 |
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| 282 | <dt id="undef_ref2">5. When I link my program with
|
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| 283 | <cite><acronym>MPFR</acronym></cite>, I get undefined reference
|
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| 284 | to <code>__gmpXXXX</code>.</dt>
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| 285 |
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| 286 | <dd><p>Link your program with <cite><acronym>GMP</acronym></cite>. Assuming
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| 287 | that your program is <samp>foo.c</samp>, you should link it using:
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| 288 | <samp class="block-code">cc link.c -lmpfr -lgmp</samp>
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| 289 | <cite><acronym>MPFR</acronym></cite> library reference (<samp>-lmpfr</samp>)
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| 290 | should be before <cite><acronym>GMP</acronym></cite>'s one
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| 291 | (<samp>-lgmp</samp>). Another solution is, with <acronym>GNU</acronym>
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| 292 | <samp>ld</samp>, to give all the libraries inside a group:
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| 293 | <samp class="block-code">gcc link.c -Wl,--start-group libgmp.a libmpfr.a -Wl,--end-group</samp>
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| 294 | See <samp>INSTALL</samp> file and <samp>ld</samp> manual for more
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| 295 | details.</p>
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| 296 | <p>If you used correct link options, but still get an error, this may mean
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| 297 | that your include and library search paths are inconsistent. Please see the
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| 298 | <a href="#undef_ref1">previous question</a>.</p></dd>
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| 299 |
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| 300 | <dt id="crash_high_prec">6. My program crashes with high precisions.</dt>
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| 301 |
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| 302 | <dd><p>Your stack size limit may be too small; indeed, by default,
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|---|
| 303 | <cite><acronym>GMP</acronym></cite> 4.1.4 and below allocates all
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| 304 | temporary results on the stack, and in very high precisions, this
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| 305 | limit may be reached. You can solve this problem in different ways:</p>
|
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| 306 | <ul>
|
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| 307 | <li><p>You can upgrade to <cite><acronym>GMP</acronym></cite> 4.2 (or above),
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| 308 | which now makes temporary allocations on the stack only when they are
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| 309 | small.</p></li>
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| 310 | <li><p>You can increase the stack size limit with the <samp>limit</samp>,
|
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| 311 | <samp>unlimit</samp> or <samp>ulimit</samp> command, depending on your
|
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| 312 | shell. This may fail on some systems, where the maximum stack size cannot
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| 313 | be increased above some value.</p></li>
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| 314 | <li><p>You can rebuild both <cite><acronym>GMP</acronym></cite> and
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| 315 | <cite><acronym>MPFR</acronym></cite> to use another allocation method.</p></li>
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| 316 | </ul></dd>
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| 317 |
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| 318 | <dt id="accuracy">7. Though I have increased the precision, the results
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| 319 | are not more accurate.</dt>
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| 320 |
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| 321 | <dd><p>The reason may be the use of C floating-point numbers. If you want
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| 322 | to store a floating-point constant to a <code>mpfr_t</code>, you should use
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| 323 | <code>mpfr_set_str</code> (or one of the <cite><acronym>MPFR</acronym></cite>
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| 324 | constant functions, such as <code>mpfr_const_pi</code> for π) instead
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| 325 | of <code>mpfr_set_d</code> or <code>mpfr_set_ld</code>. Otherwise the
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| 326 | floating-point constant will be first converted into a reduced-precision
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| 327 | (<abbr>e.g.</abbr>, 53-bit) binary number before
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| 328 | <cite><acronym>MPFR</acronym></cite> can work with it. This is the case
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| 329 | in particular for most exact decimal numbers, such as 0.17, which are
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| 330 | not exactly representable in binary.</p>
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| 331 | <p>Also remember that <cite><acronym>MPFR</acronym></cite> does not track
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| 332 | the accuracy of the results: copying a value <var>x</var> to <var>y</var>
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| 333 | with <code>mpfr_set (y, x, MPFR_RNDN)</code> where the variable <var>y</var>
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| 334 | is more precise than the variable <var>x</var> will not make it more
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| 335 | accurate; the (binary) value will remain unchanged.</p></dd>
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| 336 |
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| 337 | <dt id="detect_mpfr">8. How can I detect <cite><acronym>MPFR</acronym></cite>
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| 338 | installation using <cite>autoconf</cite> or <cite>pkg-config</cite>?</dt>
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| 339 |
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| 340 | <dd><p>The <cite><acronym>MPFR</acronym></cite> team does not currently
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| 341 | recommend any <cite>autoconf</cite> code, but a section will later
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| 342 | be added to the <cite><acronym>MPFR</acronym></cite> manual.
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| 343 | Limited <cite>pkg-config</cite> support has been added for
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| 344 | <cite><acronym>MPFR</acronym></cite> 4.0.0; example:</p>
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| 345 | <pre style="margin-left: 2em">cc myprogram.c $(pkg-config --cflags --libs mpfr)</pre></dd>
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| 346 |
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| 347 | <dt id="cite">9. How to cite <cite><acronym>MPFR</acronym></cite> in a
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| 348 | scientific publication?</dt>
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| 349 |
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| 350 | <dd><p>To properly cite <cite><acronym>MPFR</acronym></cite> in a scientific
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| 351 | publication, please cite the
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| 352 | <a href="https://doi.org/10.1145/1236463.1236468"><acronym title="Association for Computing Machinery">ACM</acronym>
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| 353 | <acronym title="Transactions on Mathematical Software">TOMS</acronym>
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| 354 | paper</a>
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| 355 | and/or the library web page
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| 356 | <a href="https://www.mpfr.org/">https://www.mpfr.org</a>. If your publication
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| 357 | is related to a particular release of <cite><acronym>MPFR</acronym></cite>,
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| 358 | for example if you report timings, please also indicate the release number
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| 359 | for future reference.</p></dd>
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| 360 |
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| 361 | <dt id="fpic">10. When I build <cite><acronym>MPFR</acronym></cite>, I get
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| 362 | an error asking me to recompile with <samp>-fPIC</samp>.</dt>
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| 363 |
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| 364 | <dd><p>A typical error looks like:</p>
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| 365 | <p><tt>/usr/bin/ld: <em>/path/to/</em>libgmp.a(realloc.o): relocation
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| 366 | R_X86_64_32 against `.rodata.str1.1' can not be used when making a
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| 367 | shared object; recompile with -fPIC<br />
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| 368 | <em>/path/to/</em>libgmp.a: could not read symbols: Bad value<br />
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| 369 | collect2: ld returned 1 exit status</tt></p>
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| 370 | <p>The probable reason is that you tried to build
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| 371 | <cite><acronym>MPFR</acronym></cite> with the shared library enabled (this
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| 372 | is the default), while only a static <cite><acronym>GMP</acronym></cite>
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| 373 | library could be found. To solve this problem, either rebuild and reinstall
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| 374 | <cite><acronym>GMP</acronym></cite> without the <samp>--disable-shared</samp>
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| 375 | configure option, or configure <cite><acronym>MPFR</acronym></cite> with
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| 376 | <samp>--disable-shared</samp>. If you did this and still get the above
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| 377 | error, the cause may be conflicting <cite><acronym>GMP</acronym></cite>
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| 378 | versions installed on your system; please check that your search path
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| 379 | settings are correct.</p>
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| 380 | <p>Additional note about the last sentence: Under <acronym>GNU</acronym>/Linux
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| 381 | (for instance), the linker takes the first library found in the library search
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| 382 | path, whether it is dynamic or static. The default behavior under darwin is
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| 383 | different, but <cite><acronym>MPFR</acronym></cite> will change it.</p></dd>
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| 384 | <!-- Reference concerning darwin: see MPFR_LD_SEARCH_PATHS_FIRST
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| 385 | in MPFR's configure.{ac,in} and acinclude.m4 -->
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| 386 |
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| 387 | </dl>
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| 388 |
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| 389 | </body>
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| 390 |
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| 391 | </html>
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