[PATCH] D39464: Define fs::allocate_file which preallocates disk blocks.

[Kamil Rytarowski via llvm-commits]

On 13.11.2017 19:21, Rafael Avila de Espindola wrote:
> Kamil Rytarowski <n54 at gmx.com> writes:
> 
>> On 13.11.2017 18:19, Rafael Avila de Espindola wrote:
>>> Kamil Rytarowski via Phabricator <reviews at reviews.llvm.org> writes:
>>>
>>>> krytarowski added inline comments.
>>>>
>>>>
>>>> ================
>>>> Comment at: llvm/lib/Support/Unix/Path.inc:436
>>>>        return std::error_code(Err, std::generic_category());
>>>> +    return make_error_code(errc::function_not_supported);
>>>>    }
>>>> ----------------
>>>> NetBSD needs `ftruncate`(2) as a fallback for `posix_fallocate`(2).
>>>>
>>>> In the default setup `posix_fallocate`() returns EOPNOTSUPP.
>>>
>>> Will ftruncate allocate space on netbsd? Note that it is OK for
>>> allocate_file to fail. The user then has to use a buffer and write(2)
>>> instead of mmap for output.
>>>
>>> Cheers,
>>> Rafael
>>>
>>
>> ftruncate(2)/NetBSD as an extension extends area and makes it
>> zero-filled. It's equivalent to lseek(2) + write(2).
> 
> Cool, so an mmap of the file after a ftruncate will never crash because
> of disk full, correct?
> 
> If so we could use use ftruncate on netbsd as a special case, similar to
> the special case we have for OS X.
> 
> Cheers,
> Rafael
> 

I'm not that sure. Could you please provide a reproducer of a problem to
double check?

I was trying to create a program that takes the whole space with
ftruncate(2) and mmap(2) over it works. I cannot take more space than
available, as I'm getting ENOSPC during ftruncate(2).

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