Re: Sprintf weirdness
Eloy Paris <
peloy@...>
2010-06-02 04:31:27 GMT
Hi Ian,
On 06/02/2010 12:01 AM, Ian Schorr wrote:
> Hello list,
>
> This isn't exactly a Wireshark-specific question, but it's coming up
> while I'm working on a dissector, and I'm sure someone here will know
> the answer, so... =)
>
> I'm trying to use sprintf() to append to an existing string with some
> formatted text. Obviously there's several ways to do this, but
> sprintf() seemed to be most efficient for the way I'm doing things. I
> end up appending this string to the Info Column later, but that seems
> irrelevant.
>
> For example, I have 4 variables:
> - string mystring, with value "LOCK"
> - guint32 last_fh_hash, with value "2056735708"
> - guint64 file_offset, with value 0
> - guint64 lock_length, with value 10
>
> The weird thing is that when I do this:
>
> sprintf (mystring, "%s FH: 0x%08x Offset: %lu Length: %lu",
> mystring,last_fh_hash,file_offset,lock_length);
>
> ...then "mystring" becomes "LOCK FH: 0x7a974bdc Offset: 0 Length: 0".
> Length is WRONG. It is wrong in a very consistent way.
>
> But if I do this:
>
> sprintf (mystring, "%s FH: 0x%08x", mystring,last_fh_hash);
> sprintf (mystring, "%s Offset: %u", mystring,file_offset);
> sprintf (mystring, "%s Length: %u", mystring,lock_length);
>
> Then the resulting value of mystring is correct. "LOCK FH: 0x7a974bdc
> Offset: 0 Length: 10". In fact, if I flip the positions of
> "file_offset" and "lock_length" then things are fine, regardless of
> their values.
>
> It's difficult to reproduce or debug. I have a number of similar
> statements scattered throughout code and each has varying degrees of
> strangeness. Some work properly. In some cases the values are
> actually flipped (one variable printed one place, the other in
> another). In some cases the values are actually empty. I can't
> imagine it has anything to do with the way the string was declared or
> memory allocated in the first place, it looks like sprintf() is simply
> writing out the wrong values to memory for some reason.
>
> Anybody have any thoughts on why that might be? I'm assuming I've
> done something silly, though having a tough time guessing where.
>
> I haven't tested yet to see if this is something specific to the dev
> platform I'm using. At the moment that's Windows.
Just a suggestion: why don't you try to use the PRIxxx macros defined in
stdint.h? I believe they're pretty portable since they are specified by
ISO C99.
To use them in the specific example you used, you'd do something like:
#include <stdint.h>
...
sprintf (mystring, "%s FH: 0x%08" PRIx32 " Offset: %" PRIu64 " Length:
%" PRIu64, mystring,last_fh_hash,file_offset,lock_length);
I think these macros are the best portable way of handling these
variables that are represented by different basic integer types
depending on the platform. Worth giving it a try...
Cheers,
Eloy Paris.-
netexpect.org
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