Re: [Wireshark-commits] rev 36513: /trunk/wiretap/ /trunk/wiretap/: file_access.c file_wrappers.c file_wrappers.h wtap-int.h wtap.c

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

On Apr 8, 2011, at 9:13 AM, Stephen Fisher wrote:

> This revision broke builds on FreeBSD 8.2-RELEASE (64-bit):
>
> file_access.c:924: warning: dereferencing 'void *' pointer
> file_access.c:924: error: request for member '_file' in something not a 
> structure or union
> file_access.c: In function 'wtap_dump_file_write':
> file_access.c:1084: warning: dereferencing 'void *' pointer
> file_access.c:1084: error: request for member '_flags' in something not 
> a structure or union
> gmake[2]: *** [libwiretap_la-file_access.lo] Error 1
>
> It is referring to these two lines:
>
> 924: fd = fileno(wdh->fh);
> 1084: if (ferror(wdh->fh))
>
> "wdh" is a wtap_dumper struct, which contains (wtap-int.h):
>
> struct wtap_dumper {
>       WFILE_T                 fh;
>
> "WFILE_T" is defined in my case to be a gzFile:
>
> #ifdef HAVE_LIBZ
> #include <zlib.h>
> #define WFILE_T       gzFile
> #else /* No zLib */
> #define WFILE_T       FILE *
> #endif /* HAVE_LIBZ */
>
> And "gzFile" is (from /usr/include/zlib.h):
>
> typedef voidp gzFile;

...and *this* is why C supports incomplete structure declarations such as

        struct gz_file;

so you can do things such as

        typedev gz_file *gzFile;

(or whatever you want for the structure name), so that you can have a pointer that points to a specific type, so type 
checking can be done on it, but keep the contents of the structure opaque, so you can change them from release to 
release without having to worry about breaking source or binary compatibility.  If the zlib guys had done that, the 
compiler would have complained about passing a WFILE_T to something expecting a FILE *, rather than saying "void *, OK, 
I'll assume whoever wrote this knows what they're doing, whatever".  They may, however, have had to deal with pre-ANSI 
compilers that don't support that (I don't think it was *introduced* in ANSI C, but, prior to ANSI C, you didn't *have* 
to support it).

It compiles if fileno() is a function - which it is on OS X; fileno_unlocked() is a macro that directly reaches inside 
a FILE *, but fileno() is a function.  I guess FreeBSD, or, at least, recent versions of FreeBSD, do it differently.

For now, I've made WFILE_T just be a "void *", and casted it to the appropriate type in all cases.
___________________________________________________________________________
Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org>
Archives:    http://www.wireshark.org/lists/wireshark-dev
Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev
             mailto:wireshark-dev-request () wireshark org?subject=unsubscribe