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"
- the spec says that it is "undefined behavior" rather than "forbidden"
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.
the "POSIX.2008" probably should be "POSIX.1-2008". or maybe just change it
to "POSIX.1" ?
> + *(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 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
> 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
- i was going to suggest the -pedantic thing :)
- seriously, you're using "e.g." after chastising me ? :D
-mike
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