[PATCH v2] gdb::handle_eintr, remove need to specify return type
Pedro Alves
pedro@palves.net
Mon Oct 26 14:00:34 GMT 2020
On 10/16/20 9:51 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <pedro@palves.net> writes:
>
> Pedro> This eliminates the need to specify the return type when using
> Pedro> handle_eintr. We let the compiler deduce it for us.
>
> Thanks for doing this. This is closer to how I imagined this wrapper
> being written.
>
> Pedro> +template <typename ErrorValType,
> Pedro> + typename Fun,
> Pedro> + typename... Args>
> Pedro> +inline auto
> Pedro> +handle_eintr (ErrorValType errval, const Fun &f, const Args &... args)
> Pedro> + -> decltype (f(args...))
>
> It seems to me that errval and ErrorValType are unchanging properties of
> the function being wrapped. And, normally they are -1 / int.
>
> Also is there ever a case where the return type isn't the same as
> ErrorValType?
I don't think so. But I don't think it really matters -- I look at
it as ErrorValType being the type of the value used to compare with
the wrapped function's return type.
Like, with:
ssize_t read(int fildes, void *buf, size_t nbyte);
we normally write:
ssize_t res = read (fildes, buf, nbyte);
if (res == -1)
instead of
ssize_t res = read (fildes, buf, nbyte);
if (res == (ssize_t) -1)
So ErrorValType is the type of the "-1" expression. It just
has to be convertible to the return type.
If we get rid of the ErrorValType template parameter, then we
could do instead:
template<typename Fun, typename... Args>
using return_type = decltype (std::declval<Fun> () (std::declval<Args> ()...));
template <typename Fun, typename... Args>
inline auto
handle_eintr (return_type<Fun, Args...> errval,
const Fun &f, const Args &... args)
-> decltype (f (args...))
{
decltype (f (args...)) ret;
do
{
errno = 0;
ret = f (args...);
}
while (ret == errval && errno == EINTR);
return ret;
}
That works for me too, though it looks a little more complicated,
I think.
Let me know which version you prefer.
> So maybe instead of requiring these to all be redundantly specified, the
> template could use a helper template class that specifies these things
> (defaulting to the usual), and then one would write:
>
> pid_t pid = gdb::handle_eintr<::waitpid> (...normal waitpid args);
>
> I'm not sure it's really worth implementing this, but it's closer to
> what I was picturing initially.
That thought initially crossed my mind too, but IMHO it's not worth
it. You have to write the helper template class, so it
doesn't look like a net win over writing a plain wrapper like:
int
my_waitpid (int pid, int *status, int flags)
{
return gdb::handle_eintr (-1, ::waitpid, pid, status, flags);
}
and using my_waitpid throughout. I.e., this way we also only
write the -1 once.
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