note BrowserUk <blockquote><i> Actually, make that 10 separate symbols, as the 5 nan symbols that appear in the output of BrowserUk's qnan.c, under some circumstances, will be suffixed with '0' ('00' in the case of -1.#IND).I wonder why so many symbols are needed ? </i></blockquote> <p>The suffixing of zeros to the symbols is just an artifact of using "%f" printf format, which implicitly is equivalent to "%.6f" and therefore requires that the fractional part be padded with trailing zeros to 6 places. <p>If you adjust the template, you can see this. (The printf from your code above + output):<code> printf("%f %f\n", inf / inf, -(inf / inf)); // -1.#IND00 -1.#IND00 printf("%.4f %.4f\n", inf / inf, -(inf / inf)); // -1.#IND -1.#IND printf("%.10f %.10f\n", inf / inf, -(inf / inf)); // -1.#IND000000 -1.#IND000000 </code> <P>I've found %g more useful:<code> printf("%g %g\n", inf / inf, -(inf / inf)); // -1.#IND -1.#IND </code> <blockquote><i> It's also interesting that,of my Microsoft compilers (6.0, 7.0 and 8.0), none of them produce the same output as BrowserUk's 9.0 compiler. </i></blockquote> <p>The changes are (roughly) in line with the timing of the compiler releases relative to the requirements of the various ANSI C standards C89/C90/C99/C11; and as affected by the updates to the IEEE754 (1987) standard and its revision in 2004. (Roughly:) <blockquote><i> For that program, my 8.0 MS compiler outputs: -1.#IND00 -1.#IND00 but my gcc-4.5.2 produces: -1.#IND00 1.#QNAN0 Which one is doing it correctly ? </i></blockquote> <p>You'll hate this answer. Effectively, <b>they both are!</b>. <P>The problem is, the computer scientists and the mathematicians can't agree on a single [http://en.wikipedia.org/wiki/NaN#Function_definition|Function definition] with respect to FP math. <p>The problem is two-fold: <ol><li>FP math isn't like real math (Real in the mathematical sense). <p>Eg. FP is not always associative: <code> print +( 10.0**20 + 1 ) - 10.0**20;; 0 print +( 10.0**20 - 10.0**20 ) + 1;; 1 </code> </li><li>Mathematicians have conventions for the results of some operations that don't quite work arithmetically. <p>Eg. infinity**0 == 1! <P>These allow their operations on the continuous Real domain to avoid singularity points </li></ol> <p>You pays your money and makes your choice. <blockquote><i>What operations will result in a signalling nan ?</i></blockquote> <p>From what I've been able to discover, <b>none of them!</b> (At least as far as MSC v9 is concerned.) <p>The problem here is that the IEEE-754 <i>allows</i> for traps, but doesn't <i>require them</i>. See [http://en.wikipedia.org/wiki/NaN#Signaling_NaN|Signaling NaNs] <P>My take on what I see coming out of my attempts to produce NaNs with MSC(v9), is that they settled on using -1.#IND as the result of any math operation that is (for want of a better phrase) 'not well defined'. eg. sqrt( -1 ), 0/0; fmod( 1, 0 ) etc. <div class="pmsig"><div class="pmsig-171588"> <hr /> <font size=1 > <div>With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'</div> <div>Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.</div> <div>"Science is about questioning the status quo. Questioning authority". </div> <div>In the absence of evidence, opinion is indistinguishable from prejudice. <p align=right>[http://www.theregister.co.uk/2011/11/29/sas_versus_world_programming/|The start of some sanity?]</p></div> </font> </div></div> 984141 984264