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From: "jim at meyering dot net" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sources.redhat.com
Subject: [Bug libc/13246] New: RFE: strtold: do not include uninitialized bytes when converting "NaN"
Date: Sat, 01 Oct 2011 17:39:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=13246

             Bug #: 13246
           Summary: RFE: strtold: do not include uninitialized bytes when
                    converting "NaN"
           Product: glibc
           Version: 2.14
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: jim@meyering.net
    Classification: Unclassified


strtold ("nan", NULL) appears to return a result that depends on the prior
state of the stack.  That non-determinism caused GNU sort to infloop, but we've
worked around this glibc-specific issue: http://debbugs.gnu.org/9612

Here's a little program to demonstrate the problem.
Note how the results differ with compilation options:

  #include <stdlib.h>
  #include <stdio.h>

  static char *
  fmt_nan (long double x)
  {
    unsigned int i;
    static char buf[33];
    unsigned char const *p = (unsigned char const *) &x;
    for (i = 0; i < sizeof x; i++)
      sprintf (buf + 2*i, "%02x", *p++);
    return buf;
  }

  int
  main ()
  {
    const char *q = "nan";
    long double x = strtold (q, NULL);
    printf ("%s\n", fmt_nan (x));

    x = 0;
    x = strtold (q, NULL);
    printf ("%s\n", fmt_nan (x));

    return 0;
  }

  $ gcc -O0 -Wall -Wextra -W /t/strtold-bogosity.c && ./a.out
  00000000000000c0ff7f400000000000
  00000000000000c0ff7f000000000000
  $ gcc -O1 -Wall -Wextra -W /t/strtold-bogosity.c && ./a.out
  00000000000000c0ff7f000000000000
  00000000000000c0ff7f000000000000

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