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.103 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.

Footnotes

(103)

You asked for it, you got it.