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POHow to get the upper-/lower machine-word of a double according to IEEE 754 (ansi-c)?
 

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  1. POHow to get the upper-/lower machine-word of a double according to IEEE 754 (ansi-c)?
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    <p>i want to use the sqrt implementation of <a href="http://www.netlib.org/fdlibm/" rel="nofollow">fdlibm</a>.<br> This implementation defines (according to the endianess) some macros for accessing the <strong>lower/upper 32-bit of a double</strong>) in the following way (here: only the little-endian-version):</p> <pre><code>#define __HI(x) *(1+(int*)&amp;x) #define __LO(x) *(int*)&amp;x #define __HIp(x) *(1+(int*)x) #define __LOp(x) *(int*)x </code></pre> <p>The readme of flibm is saying the following (a little bit shortened)</p> <pre><code>Each double precision floating-point number must be in IEEE 754 double format, and that each number can be retrieved as two 32-bit integers through the using of pointer bashing as in the example below: Example: let y = 2.0 double fp number y: 2.0 IEEE double format: 0x4000000000000000 Referencing y as two integers: *(int*)&amp;y,*(1+(int*)&amp;y) = {0x40000000,0x0} (on sparc) {0x0,0x40000000} (on 386) Note: Four macros are defined in fdlibm.h to handle this kind of retrieving: __HI(x) the high part of a double x (sign,exponent,the first 21 significant bits) __LO(x) the least 32 significant bits of x __HIp(x) same as __HI except that the argument is a pointer to a double __LOp(x) same as __LO except that the argument is a pointer to a double If the behavior of pointer bashing is undefined, one may hack on the macro in fdlibm.h. </code></pre> <p><em>I want to use this implementation and these macros with the <a href="http://www.cprover.org/cbmc/" rel="nofollow">cbmc</a> model checker, which should be conformable with ansi-c</em>.<br> I don't know exactly whats wrong, but the following example shows that these macros aren't working (little-endian was chosen, 32-bit machine-word was chosen):</p> <pre><code>temp=24376533834232348.000000l (0100001101010101101001101001010100000100000000101101110010000111) high=0 (00000000000000000000000000000000) low=67296391 (00000100000000101101110010000111) </code></pre> <p>Both seem to be wrong. High seems to be empty for every value of temp.</p> <p><strong>Any new ideas for accessing the both 32-words with ansi-c?</strong></p> <p><strong>UPDATE:</strong> Thanks for all your answers and comments. All of your proposals worked for me. For the moment i decided to use "R.."s version and marked this as favorite answer because it seems to be the most robust in my tool regarding endianness.</p>
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    1. COOut of curiosity, what is `sizeof(int)` on your platform? `LO` looks like it's getting you the right value.
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    2. COsizeof(int) = 4
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