nan, nanf, nanl - return 'Not a Number'


   #include <math.h>

   double nan(const char *tagp);
   float nanf(const char *tagp);
   long double nanl(const char *tagp);

   Link with -lm.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   nan(), nanf(), nanl():
       _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L


   These functions return a representation (determined by tagp) of a quiet
   NaN.   If  the  implementation  does  not  support  quiet  NaNs,  these
   functions return zero.

   The call nan("char-sequence") is equivalent to:

       strtod("NAN(char-sequence)", NULL);

   Similarly, calls to nanf() and nanl() are equivalent to analogous calls
   to strtof(3) and strtold(3).

   The argument tagp is used  in  an  unspecified  manner.   On  IEEE  754
   systems,  there  are many representations of NaN, and tagp selects one.
   On other systems it may do nothing.


   These functions first appeared in glibc in version 2.1.


   For  an  explanation  of  the  terms  used   in   this   section,   see

   Interface              Attribute      Value          
   nan(), nanf(), nanl()  Thread safety  MT-Safe locale 


   C99,  POSIX.1-2001,  POSIX.1-2008.   See  also IEC 559 and the appendix
   with recommended functions in IEEE 754/IEEE 854.


   isnan(3), strtod(3), math_error(7)


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