Re: [PATCH 5/7] dlopen.3: Remove unneeded cast

Michael Kerrisk (man-pages)
2020-09-06T13:03:10Z

Hello Alex,

On 9/5/20 5:14 PM, Alejandro Colomar wrote:
> Casting `void *` to `double (*cosine)(double)` is already done
> implicitly.
> I had doubts about this one, but `gcc -Wall -Wextra` didn't complain
> about it.
> Explicitly casting can silence warnings when mistakes are made, so it's
> better to remove those casts when possible.
>
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
> ---
> man3/dlopen.3 | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/man3/dlopen.3 b/man3/dlopen.3
> index 8e18f70c0..2de358ea3 100644
> --- a/man3/dlopen.3
> +++ b/man3/dlopen.3
> @@ -581,7 +581,7 @@ main(void)
>
> dlerror(); /* Clear any existing error */
>
> - cosine = (double (*)(double)) dlsym(handle, "cos");
> + cosine = dlsym(handle, "cos");
>
> /* According to the ISO C standard, casting between function
> pointers and 'void *', as done above, produces undefined results.

This cast really is needed. See the comment just below, and also try
compiling the code with your patch applied:

cc -pedantic -Wall prog.c
d.c: In function ‘main’:
d.c:21:19: warning: ISO C forbids assignment between function pointer and ‘void *’ [-Wpedantic]
21 | cosine = dlsym(handle, "cos");
| ^

Thanks,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/