Re: Casting the return value of ...

scott@slp53.sl.home (Scott Lurndal) writes:

Kaz Kylheku <433-929-6894@kylheku.com> writes:
>

On 2024-03-28, Kenny McCormack <gazelle@shell.xmission.com> wrote:
>

But here's the thing.  Is the casting of the result of dlsym()
[to a function pointer type] necessary?
Isn't this the same as "casting the return value of malloc()", where you

>
Conversions between function pointers and data pointers are an
extension;  it is not well-defined behavior in ISO C.
>
I also seem to remember something like this, but I cannot trust this
documentation without a chapter-and-verse citation.

>
https://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html
>

Assuming it is true, there you have it;  if you're on POSIX, the compiler
is required to have the extension and it is connected to casting,
in which case the cast is required.

>
I've used this form for the last two decades, with gcc, with no
issues:
>
/**
 * Each shared object that simulates a Data Link Processor (DLP), will
 * contain a single namespace-scope function <b>get_dlp</b> which constructs
 * a DLP object of the specified type (for example, a #c_uniline_dlp,
 * #c_card_reader_dlp, et alia).   <b>get_dlp</b> returns the constructed
 * object as a #c_dlp object to the #channel function, which is then used
 * by the I/O subsystem to request services of the DLP at the MLI level.
 */
typedef c_dlp* (*get_dlp_t)(const char *, uint64_t, c_logger *);
...
    get_dlp_t   sym;
...
>
    sym = (get_dlp_t)dlsym(handle, "get_dlp");
    if (sym == NULL) {
        lp->log("Invalid DLP shared object format: %s\n", dlerror());
        unregister_handle(channel);
        dlclose(handle);
        return 1;
    }
>
>

I think there was a time in the development of GCC when there was
a warning even with the cast.  I don't think it's enabled by default
now?

>
We compile with -Wall -Werror and have never seen any warnings related to
casting the result of dlsym(), and we build with GCC[4..13].

Do you use -pedantic?  Compiling with -pedantic using gcc 8.4.0
gives a warning diagnostic (and a fatal error if -pedantic-errors
is specified in place of -pedantic).