Hi Mike,
Thanks for following up.
On Thu, Jan 2, 2014 at 5:06 PM, Mike Frysinger <vapier@xxxxxxxxxx> wrote:
> On Wednesday 01 January 2014 20:46:59 Michael Kerrisk (man-pages) wrote:
>> --- a/man3/dlopen.3
>> +++ b/man3/dlopen.3
>>
>> /* According to the C99 standard, casting 'void *' to a function pointer
>> as shown above is forbidden. POSIX.1-2003 and POSIX.2008 followed
>> C99's requirement, and proposed the following workaround:
>
> i don't want to be pedantic, buuuuut:
> - you want to say "ISO C" rather than "C99"
Yes, good point. Fixed now.
> - the spec says that it is "undefined behavior" rather than "forbidden"
Likewise; fixed now.
> i wonder about deleting the "followed C99's requirement" part. they aren't
> really following the requirement since (at least 2008) says that it must be
> supported.
Yes, the language could be better. I changed it to "[they] accepted
this state of affairs"
> the "POSIX.2008" probably should be "POSIX.1-2008". or maybe just change it
> to "POSIX.1" ?
POSIX.1-2008.
>> + *(void **) (&cosine) = dlsym(handle, "cos");
>> +
>> + This (clumsy) cast conforms with C99 and will avoid any compiler
>> + warnings.
>
> might it be worth suggesting a union too ?
> double (*cosine)(double);
> union {
> void *vptr;
> double (*ptr)(double);
> } cast;
> cast.vptr = dlsym(...);
> cosine = cast.double;
I think I'll not worry about this.
> i think this also avoids aliasing warnings and works nicely in C++ ? or i
> could be making this up entirely :).
>
>> The 2013 Technical Corrigendum to POSIX.1-2008 (a.k.a. POSIX.1-2013)
>> improved matters by requiring that conforming implementations support
>> casting 'void *' to a function pointer.
>
> how about adding a comment inside of the man page itself linking to the POSIX
> pages ? making it visible to end users might be too much, but for people
> maintaining the comments (i.e. you) would be good.
> http://pubs.opengroup.org/onlinepubs/009695399/functions/dlsym.html#tag_03_112_08
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html#tag_16_96_07
Yes, good idea.
>> Nevertheless, some compilers
>> (e.g., gcc with the '-pedantic' option) may complain about the cast
>> used in this program. */
>
> three things:
> - i think the ISO C standard is required to at least emit a warning
Yes, but it's not quite clear to me if the POSIX.1-2013 is meaning
that compilers should not issue a warning in this case. Maybe I'm
misunderstanding something, though.
> - i was going to suggest the -pedantic thing :)
> - seriously, you're using "e.g." after chastising me ? :D
I don't recall chastising you as such... But, in running text using
"e.g." and "i.e." often does not work well, I find (and most style
manuals agree). As I said, mostly the only place you'll find these
forms in the man pages is in parenthetical asides, as is done here.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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