Re: wtap_dump_file_seek() and _tell()

[Guy Harris <guy () alum mit edu>]

On Mar 3, 2013, at 11:10 AM, Ed Beroset <beroset () mindspring com> wrote:

> According to svn, version 36318 (March 2011) added, among other things, the following lines to the wiretap/wtap-int.h 
> file:
>
> extern gint64 wtap_dump_file_seek(wtap_dumper *wdh, gint64 offset, int whence, int *err);
> extern gint64 wtap_dump_file_tell(wtap_dumper *wdh);
>
> However, unlike most of the corresponding functions which are implemented in wiretap/file_access.c these two have no 
> implementations.

Gerald?  Planned but never implemented?

> In 18 places within the code involving seven files (5view.c, k12,c, lanalyzer.c, netmon.c, netscaler.c, netxray.c, 
> visual.c) either an ftell or fseek is used which does an implicit conversion from WFILE_T to struct FILE * which is 
> an incompatibility with C++.  I could have added casts to each location, but it seems that the neater way to do this 
> would be to actually implement these function.  I understand that seek() and tell() won't work every time (e.g. pipes 
> or stdin),

"pipes or terminals" - if stdout (as this is for dumping, it'd be stdout rather than stdin) were a file, *seek() and 
*tell() would work, and if stdout were a pipe (a terminal would be silly, unless you *like* reading random binary byte 
streams as they come out on your terminal or terminal emulator or the file format being written is a text file format) 
certain file formats simply can't be written (unless we do the dumping in a two-pass process or otherwise compute 
certain information that appears earlier in the file beforehand).

More importantly, you *also* wouldn't be able to write them out if they're gzipped, and we *do* support writing out 
gzipped files.

> but these places in the code are doing it anyway.  How should we best resolve this? Should I implement the functions?

Might as well.  They'd belong with the others in file_access.c, and should fail if wdh->compressed is set.  (We already 
have wtap_dump_can_compress(), which checks whether writing the file format requires a seek, so no Wireshark code 
should be trying to write out a compressed file in any of those formats.)

We should probably define a new WTAP_ERR_CANT_SEEK_COMPRESSED value and return it as the error value if a seek is 
attempted on a compressed stream.

file_tell() should probably also have an "int *err" argument, as ftell() can fail.

For now, you can probably just cast the "offset" arguments to int, as the SEEK_SET seeks are to a location in the 
beginning of the file, and the SEEK_CUR seeks have short offsets.  We'd bury fseeko()/whatever the Windows equivalent 
is/etc. stuff in those routines if we ever need to fully support 64-bit offsets.
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