[Winpcap-users] pcap_fopen_offline not exported?

[Gianluca Varenni]

I can definitely try to include it in the next version of WinPcap (4.1 
branch, not in the 4.0.x stable). Can you send me a sample that I can 
include in the developer's pack for it?

Have a nice day
GV

----- Original Message ----- 
From: "Okraszewski, Marcin" <Marcin.Okraszewski at pl.compuware.com>
To: <winpcap-users at winpcap.org>
Sent: Monday, July 28, 2008 6:45 AM
Subject: RE: [Winpcap-users] pcap_fopen_offline not exported?


> So I have it working now. I have slightly modified WinPcap to make the
> API expose the pcap_fopen_offline():
>
> In pcap.h I have added:
> -------------------------------------------------------
> #ifndef LIBPCAP_EXPORTS
> #define pcap_fopen_offline(f,b) \
>        pcap_hopen_offline(_get_osfhandle(_fileno(f)), b)
> #endif
>
> pcap_t  *pcap_hopen_offline(intptr_t, char *);
> -------------------------------------------------------
>
>
> Then in savefile.c:
> -------------------------------------------------------
> pcap_t* pcap_hopen_offline(intptr_t osfd, char *errbuf)
> {
>    int fd;
>    FILE *file;
>
>    fd = _open_osfhandle(osfd, _O_RDONLY);
>    if ( fd < 0 ) {
>        strerror_s(errbuf, PCAP_ERRBUF_SIZE, errno);
> return NULL;
>    }
>
>    file = _fdopen(fd, "rb");
>    if ( file == NULL ) {
>        strerror_s(errbuf, PCAP_ERRBUF_SIZE, errno);
>        return NULL;
>    }
>    return pcap_fopen_offline(file, errbuf);
> }
> -------------------------------------------------------
>
>
> And finally I have added the pcap_hopen_offline to exports in wpcap.def.
>
>
> Thank you all for input in this matter. Is there any chance this kind of
> solution to be included in WinPcap?
>
> Thanks,
> Marcin
>
>
>
>
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> From: winpcap-users-bounces at winpcap.org
> [mailto:winpcap-users-bounces at winpcap.org] On Behalf Of Bryan Kadzban
> Sent: Saturday, July 26, 2008 3:06 PM
> To: winpcap-users at winpcap.org
> Subject: Re: [Winpcap-users] pcap_fopen_offline not exported?
>
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> Hash: RIPEMD160
>
> Fish wrote:
>> But the point is, the exported function's prototype could include
>> either a regular file descriptions (int fd) or a Win32 HANDLE
>
> HANDLE, yes.  File descriptor, no; an integer file descriptor is a CRT
> library construction on win32 (likely an index into a table of HANDLEs,
> possibly plus a few other things, that's private to the run-time
> library), and therefore can't be passed from one CRT to another.  At
> best, it'll return errors when you try to use it (it may also crash); at
> worst, it'll perform operations on some completely unrelated file.
>
>> but you must still be careful not to have two different modules
>> reading/writing to the same CRT file.
>
> Not just the same CRT file.  You can't use the HANDLE that you passed
> in to _open_osfhandle() *at all*, according to the docs that Guy Harris
> just posted.  Operations like changing the file pointer that's held in
> that HANDLE will break the assumptions made by the CRT's file operation
> functions.
>
>> Now of course if both modules were instead always using the Win32
>> file HANDLES to do all their file accessing with (and NOT the C/C++
>> Runtime handle), then again there would NOT be any problem.
>
> I don't know that for sure.  If you opened a file twice, and got two
> HANDLEs to it, then you would probably be correct.  If you opened the
> file once, passed the HANDLE off to other code, and both pieces of code
> used it (directly), I think the bits of code would probably step on each
> other.  Because I believe the file pointer is held in the HANDLE -- at
> least unless you're doing overlapped I/O.  That *might* be safe, because
> the file position pointers are held in the OVERLAPPED struct.  But there
> may be other bits of information that are held in the HANDLE that can't
> be shared between two pieces of code.
>
> But using DuplicateHandle (on a normal file, not a socket) should be
> fine.
>
>> The only time a problem comes into play is when two different C/C++
>> Runtimes try accessing the same underlying Win32 File object at the
>> same time using their own CRT file descriptions or buffered FILE*
>> pointers. THAT is the only thing that is dangerous.
>
> Not using overlapped I/O on a normal Win32 HANDLE is dangerous as well.
> So is using the HANDLE at the same time as a FILE* or a file descriptor
> that was opened from that HANDLE.  This is not an exhaustive list,
> either; there may be other unsafe operations.
>
>> But as long as only ONE module is ultimately responsible for doing
>> ALL of the read/writing, then there's nothing preventing you from
>> simply changing the prototype of the exported function's parameters
>> to use 'int fd' or HANDLE instead of the existing/dangerous FILE*
>> pointer.
>
> HANDLE, yes.  'int fd', no; see the first paragraph.  ;-)
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