[Bug libc/15854] New: strtod should avoid calling strlen

["emogenet at gmail dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

            Bug ID: 15854
           Summary: strtod should avoid calling strlen
           Product: glibc
           Version: 2.18
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: emogenet at gmail dot com
                CC: drepper.fsp at gmail dot com

Problem : glibc's strtod seem to systematically call strlen on its input.

To the layman that I am, there doesn't seem to be any legitimate reason why it
should: it seems that strtod should simply consume its input one char at a time
until it reaches a char that marks the end of a valid FP number ASCII rep. and
should therefore work on a non-zero terminated buffer, as long said buffer ends
with a char that terminates the parsing.

This internal call to strlen makes it essentially impossible to call strtod
on a no zero terminated buffer, and there seems to be no other way to otherwise
access the non-trivial code that converts an ASCII buffer to a FP number.

This makes it in particular painful to call strtod on a very large mmap'd
buffer of ASCII floats : strlen will plow through the entire file for every
call to strtod, making things highly inefficient (it is also not guaranteed
not to crash).

To work around this shortcoming, one ends up having to figure out the end of
the FP ASCII string, "by hand", copy the result to a zero terminated buffer,
and then call strtod on that.

This is both inefficient and clunky.

See this article for a good description of the issue:

http://www.ryanjuckett.com/programming/c-cplusplus/25-optimizing-atof-and-strtod

Here's another instance of the problem:

http://stackoverflow.com/questions/2033845/any-one-know-how-to-convert-a-huge-char-array-to-float-very-huge-array-perform

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