How to use nan and inf in C

Question

I have a numerical method that could return nan or inf if there was an error  and for testing purposed I d like to temporarily force it to return nan or inf to ensure the situation is being handled correctly  Is there a reliable  compiler-independent way to create values of nan and inf in C   After googling for about 10 minutes I ve only been able to find compiler dependent solutions

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A compiler independent way  but not processor independent way to get these   int inf   0x7F800000  return   float   amp inf   int nan   0x7F800001  return   float   amp nan    This should work on any processor which uses the IEEE 754 floating point format  which x86 does    UPDATE  Tested and updated

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I usually use   define INFINITY  1e999    or  const double INFINITY   1e999   which works at least in IEEE 754 contexts because the highest representable double value is roughly 1e308  1e309 would work just as well  as would 1e99999  but three nines is sufficient and memorable  Since this is either a double literal  in the  define case  or an actual Inf value  it will remain infinite even if you re using 128-bit     long double     floats

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You can test if your implementation has it    include  lt math h gt   ifdef NAN    NAN is supported     endif  ifdef INFINITY    INFINITY is supported     endif   The existence of INFINITY is guaranteed by C99  or the latest draft at least   and  expands to a constant expression of type float representing positive or unsigned infinity  if available  else to a positive constant of type float that overflows at translation time    NAN may or may not be defined  and  is defined if and only if the implementation supports quiet NaNs for the float type  It expands to a constant expression of type float representing a quiet NaN    Note that if you re comparing floating point values  and do   a   NAN    even then   a    NAN    is false   One way to check for NaN would be    include  lt math h gt  if  isnan a             You can also do  a    a to test if a is NaN   There is also isfinite    isinf    isnormal    and signbit   macros in math h in C99   C99 also has nan functions    include  lt math h gt  double nan const char  tagp   float nanf const char  tagp   long double nanl const char  tagp      Reference  n1256    Docs INFINITY Docs NAN

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There is no compiler independent way of doing this  as neither the C  nor the C    standards say that the floating point math types must support NAN or INF   Edit  I just checked the wording of the C   standard  and it says that these functions  members of the templated class numeric limits    quiet NaN    signalling NaN     wiill return NAN representations  if available   It doesn t expand on what  if available  means  but presumably something like  if the implementation s FP rep supports them   Similarly  there is a function   infinity      which returns a positive INF rep  if available    These are both defined in the  lt limits gt  header - I would guess that the C standard has something similar  probably also  if available   but I don t have a copy of the current C99 standard

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lt inf h gt      IEEE positive infinity        if   GNUC PREREQ 3 3    define INFINITY      builtin inff     else   define INFINITY   HUGE VALF  endif   and   lt bits nan h gt   ifndef  MATH H   error  Never use  lt bits nan h gt  directly  include  lt math h gt  instead    endif      IEEE Not A Number        if   GNUC PREREQ 3 3     define NAN       builtin nanf         elif defined   GNUC      define NAN        extension                                           union   unsigned   l   attribute       mode      SI       float   d                l  0x7fc00000UL      d    else    include  lt endian h gt     if   BYTE ORDER      BIG ENDIAN    define   nan bytes         0x7f  0xc0  0  0     endif   if   BYTE ORDER      LITTLE ENDIAN    define   nan bytes         0  0  0xc0  0x7f     endif  static union   unsigned char   c 4   float   d      nan union       attribute used         nan bytes      define NAN       nan union   d    endif     GCC

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I m also surprised these aren t compile time constants  But I suppose you could create these values easily enough by simply executing an instruction that returns such an invalid result  Dividing by 0  log of 0  tan of 90  that kinda thing

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double a nan   strtod  NaN   NULL   double a inf   strtod  Inf   NULL

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Here is a simple way to define those constants  and I m pretty sure it s portable   const double inf   1 0 0 0  const double nan   0 0 0 0    When I run this code   printf  inf     f n   inf   printf  -inf    f n   -inf   printf  nan     f n   nan   printf  -nan    f n   -nan     I get   inf    inf -inf   -inf nan    -nan -nan   nan

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This works for both float and double   double NAN   0 0 0 0  double POS INF   1 0  0 0  double NEG INF   -1 0 0 0    Edit  As someone already said  the old IEEE standard said that such values should raise traps  But the new compilers almost always switch the traps off and return the  given values because trapping interferes with error handling

[c] How to use nan and inf in C?

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