[Bug libc/15854] strtod should avoid calling strlen

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

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

--- Comment #2 from emogenet at gmail dot com ---
You are correct, the call tp strlen in strtod is not a problem. I
incrorectly assumed
it was calling strlen on the whole buffer because sscanf does exhibit the
problem I
describe, but as it turns out, the problem is inherent to sscanf, and
strtod works fine.

As a matter of fact, I just tested glibc's strtod on a very large ASCII
mmap'd buffer
just now, an it works fine, no quadratic behavior.

Apologies for not testing this better before reporting the bug. Please feel
free to close.

   - Emmanuel



On Tue, Aug 20, 2013 at 9:11 AM, neleai at seznam dot cz <
sourceware-bugzilla@sourceware.org> wrote:

> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
>
> --- Comment #1 from Ondrej Bilka <neleai at seznam dot cz> ---
> On Tue, Aug 20, 2013 at 02:12:32AM +0000, emogenet at gmail dot com wrote:
> > 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 is not that big problem, strtod only uses strlen in following context
>
>   decimal = _NL_CURRENT (LC_NUMERIC, DECIMAL_POINT); // which is "."
>   decimal_len = strlen (decimal); // which is 1
>
>
> > 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).
> >
> Do you have testcase to demonstrate quadratic behavior? It is possible
> that end is determined by other ineffective means.
>
> > 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
> >
> Not relevant for us as these are windows problems.
>
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