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

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │ Interface                                                            │ Attribute     │ Value          │
       ├──────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │ nan(), nanf(), nanl()                                                │ Thread safety │ MT-Safe locale │
       └──────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘

       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.

       glibc 2.1.  C99, POSIX.1-2001.

       Math library (libm, -lm)

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

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

Linux man-pages 6.9.1                              2024-05-02                                             nan(3)

       C11, POSIX.1-2008.

       See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854.

#include<math.h>doublenan(constchar*tagp);floatnanf(constchar*tagp);longdoublenanl(constchar*tagp);

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

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