sscanf(3) return value doesn't count %100c assignments

sscanf(3) return value doesn't count %100c assignments

[David Wuertele]

Comparing musl-0.9.14 with glibc-2.13, I find sscanf(3) behaves
differently.  In Glibc, sscanf() returns the same assignment counts when
using %Nc compared with using %s, but in Mulsl, sscanf returns different
assignment counts.

For example, take the following two instructions:

  sscanf (string, "%d %s", &number, remainder);
  sscanf (string, "%d %100c", &number, remainder);

If each of these makes two assignments, they should both return 2.
Glibc works this way.  But even though with Musl they both make two
assignments, Musl sscanf() returns 2 for the %s and it returns 1 for
the %100c version.

Here is a test program:

  #include <stdio.h>
  #include <strings.h>

  int main ()
  {
    int number = 0;
    char *string = "1234 five six seven";
    char remainder[100];
    bzero (remainder, sizeof(remainder));
    int args = sscanf (string, "%d %s", &number, remainder);
    fprintf (stderr,
             "format=%%s string=\"%s\" args=%d number=%d remainder=%s\n",
             string, args, number, remainder);
    args = sscanf (string, "%d %100c", &number, remainder);
    fprintf (stderr,
             "format=%%100c string=\"%s\" args=%d number=%d remainder=%s\n",
             string, args, number, remainder);

    return 0;
  }

Here is how I compile them:

  arm-tegra452-linux-gnueabi-gcc --static test.c -o test-glibc
  arm-linux-musleabishf-gcc -static test.c -o test-musl

Here is what I see when I execute them:

  # ./test-musl
  format=%s string="1234 five six" args=2 number=1234 remainder=five
  format=%100c string="1234 five six" args=1 number=1234 remainder=five six
  # ./test-glibc
  format=%s string="1234 five six" args=2 number=1234 remainder=five
  format=%100c string="1234 five six" args=2 number=1234 remainder=five six

This seems like a bug to me.  I tried to read through
musl-0.9.14/src/stdio/vfscanf.c to troubleshoot this, but I couldn't find the
source of the difference.  Can anyone give me pointers to build a musl-libc
that has a sscanf() that is compatible with glibc's sscanf()?

Thanks,
Dave

Re: sscanf(3) return value doesn't count %100c assignments

[Rich Felker]

On Fri, Nov 15, 2013 at 08:23:42PM +0000, David Wuertele wrote:
> Comparing musl-0.9.14 with glibc-2.13, I find sscanf(3) behaves
> differently.  In Glibc, sscanf() returns the same assignment counts when
> using %Nc compared with using %s, but in Mulsl, sscanf returns different
> assignment counts.
> 
> For example, take the following two instructions:
> 
>   sscanf (string, "%d %s", &number, remainder);
>   sscanf (string, "%d %100c", &number, remainder);
> 
> If each of these makes two assignments, they should both return 2.
> Glibc works this way.  But even though with Musl they both make two
> assignments, Musl sscanf() returns 2 for the %s and it returns 1 for
> the %100c version.

musl's sscanf returns 1 because only the %d was matched. %100c
requires _exactly_ 100 characters; anything shorter is a matching
failure. See C99 7.19.6.2 The fscanf function, paragraph 12:

    12 The conversion specifiers and their meanings are:

    ....

    c

    Matches a sequence of characters of exactly the number specified
    by the field width (1 if no field width is present in the
    directive).

What you're seeing is a known bug in glibc:

    https://sourceware.org/bugzilla/show_bug.cgi?id=12701

It's an old WONTFIX from the days when Ulrich Drepper was maintainer.

Rich

Re: sscanf(3) return value doesn't count %100c assignments

[David Wuertele]

Rich Felker <dalias <at> aerifal.cx> writes:

> What you're seeing is a known bug in glibc:
> 
>     https://sourceware.org/bugzilla/show_bug.cgi?id=12701
> 
> It's an old WONTFIX from the days when Ulrich Drepper was maintainer.

Thanks for the quick reply, and for linking to the bug.
Reading that bug was very entertaining!

Dave