File: gawk.info,  Node: POSIX Floating Point Problems,  Next: Floating point summary,  Prev: Checking for MPFR,  Up: Arbitrary Precision Arithmetic

16.7 Standards Versus Existing Practice
=======================================

Historically, 'awk' has converted any nonnumeric-looking string to the
numeric value zero, when required.  Furthermore, the original definition
of the language and the original POSIX standards specified that 'awk'
only understands decimal numbers (base 10), and not octal (base 8) or
hexadecimal numbers (base 16).

   Changes in the language of the 2001 and 2004 POSIX standards can be
interpreted to imply that 'awk' should support additional features.
These features are:

   * Interpretation of floating-point data values specified in
     hexadecimal notation (e.g., '0xDEADBEEF').  (Note: data values,
     _not_ source code constants.)

   * Support for the special IEEE 754 floating-point values "not a
     number" (NaN), positive infinity ("inf"), and negative infinity
     ("-inf").  In particular, the format for these values is as
     specified by the ISO 1999 C standard, which ignores case and can
     allow implementation-dependent additional characters after the
     'nan' and allow either 'inf' or 'infinity'.

   The first problem is that both of these are clear changes to
historical practice:

   * The 'gawk' maintainer feels that supporting hexadecimal
     floating-point values, in particular, is ugly, and was never
     intended by the original designers to be part of the language.

   * Allowing completely alphabetic strings to have valid numeric values
     is also a very severe departure from historical practice.

   The second problem is that the 'gawk' maintainer feels that this
interpretation of the standard, which required a certain amount of
"language lawyering" to arrive at in the first place, was not even
intended by the standard developers.  In other words, "We see how you
got where you are, but we don't think that that's where you want to be."

   Recognizing these issues, but attempting to provide compatibility
with the earlier versions of the standard, the 2008 POSIX standard added
explicit wording to allow, but not require, that 'awk' support
hexadecimal floating-point values and special values for "not a number"
and infinity.

   Although the 'gawk' maintainer continues to feel that providing those
features is inadvisable, nevertheless, on systems that support IEEE
floating point, it seems reasonable to provide _some_ way to support NaN
and infinity values.  The solution implemented in 'gawk' is as follows:

   * With the '--posix' command-line option, 'gawk' becomes "hands off."
     String values are passed directly to the system library's
     'strtod()' function, and if it successfully returns a numeric
     value, that is what's used.(1)  By definition, the results are not
     portable across different systems.  They are also a little
     surprising:

          $ echo nanny | gawk --posix '{ print $1 + 0 }'
          -| nan
          $ echo 0xDeadBeef | gawk --posix '{ print $1 + 0 }'
          -| 3735928559

   * Without '--posix', 'gawk' interprets the four string values '+inf',
     '-inf', '+nan', and '-nan' specially, producing the corresponding
     special numeric values.  The leading sign acts a signal to 'gawk'
     (and the user) that the value is really numeric.  Hexadecimal
     floating point is not supported (unless you also use
     '--non-decimal-data', which is _not_ recommended).  For example:

          $ echo nanny | gawk '{ print $1 + 0 }'
          -| 0
          $ echo +nan | gawk '{ print $1 + 0 }'
          -| +nan
          $ echo 0xDeadBeef | gawk '{ print $1 + 0 }'
          -| 0

     'gawk' ignores case in the four special values.  Thus, '+nan' and
     '+NaN' are the same.

   Besides handling input, 'gawk' also needs to print "correct" values
on output when a value is either NaN or infinity.  Starting with version
4.2.2, for such values 'gawk' prints one of the four strings just
described: '+inf', '-inf', '+nan', or '-nan'.  Similarly, in POSIX mode,
'gawk' prints the result of the system's C 'printf()' function using the
'%g' format string for the value, whatever that may be.

     NOTE: The sign used for NaN values can vary!  The result depends
     upon both the underlying system architecture and the underlying
     library used to format NaN values.  In particular, it's possible to
     get different results for the same function call depending upon
     whether or not 'gawk' is running in MPFR mode ('-M') or not.
     Caveat Emptor!

   ---------- Footnotes ----------

   (1) You asked for it, you got it.