[Jprogramming] j807 cd - pass array arguments

Mon Nov 19 11:52:04 UTC 2018

Surprising that & would crash, though possible.
When you use &, the safe way is to apply memu to the value that you are 
assigning to the name that is going to go into cd. So if you use
ret =. (sqrdll, 'sqr > i i *i &i') cd (#y);y;res
you should have written
res =. memu (#y)#4 NB. make sure it's integer
Likewise if you are going to write over y you could use & but you would need to execute (y =. memu y). If you know that y is not going to be modified by the DLL you don't need to modify y.
If the function-call letter doesn't match the actual type of the data, the data is copied. Best if you keep the sizes of integers the same. On J64, J's integers are long long.
If the crashing code is short feel free to post it here, or send it to me privately, for debugging help.
Henry Rich

On 11/19/2018 12:17 AM, Ben Gorte wrote:
> When trying & , I apparently do not know what I am doing, because it
> immediately crashes my session.
>> However, I do feel meanwhile that I get an increasing number of copies of
> my (large) dataset in memory, so perhaps I should find out what I should be
> doing. As I understand, & can help to avoid copying the data.
>> The basic situation is that I pass three arrays to my DLL function:
> - a one-dimensional array with N integers,
> - another one with N bytes (literals),
> - and a 2D one with N x 2 integers.
> N is up to of few hundred millions (but larger would be even better). The
> first array is modified by the DLL; the others are 'read-only'.
>> Any suggestions are appreciated. In my function, the integers can be int or
> long - what would be preferred? If they are the same size in J and C, does
> it avoid (or reduce) copying?
>> Thanks again,
> Ben
>> On Mon, Nov 19, 2018 at 1:58 PM bill lam <bbill.lam at gmail.com> wrote:
>>> "The memory is unaliased." refers to the memory as seen by
>> the callee of cd. Any changes made by callee won't be written
>> back to the original memory. That's why the value of names
>> remained unchanged after cd call. Unless you had taken
>> the address of name and passed the address to cd.
>>>> The release note also mentioned & but use it only if you
>> know what you are doing.
>>>> 2018年11月19日, Ben Gorte написал(а):
>>> Dear Henry, Bill, Linda,
>>>>>> Thanks for your responses, I am getting the point/ers.
>>>>>> Perhaps in
>>>>> https://code.jsoftware.com/wiki/Guides/DLLs/Calling_DLLs#J807_Incompatibilities
>>> , the phrase: "The DLL can read or write this memory." is a bit
>> confusing,
>>> although it also says "The memory is unaliased." That means "copied if
>>> required", and I thought it wouldn't be...
>>>>>> Anyway, all is well again,
>>>>>> Thanks,
>>> Ben
>>>>>>>>>>>>>>>>>> On Mon, Nov 19, 2018 at 1:15 AM Henry Rich <henryhrich at gmail.com> wrote:
>>>>>>> What Bill is saying is that starting in 807, cd makes an copy of any
>>>> argument to cd that might be shared by another noun. So, the value of
>>>> 'res' is put into a temporary block and passed to the DLL. The
>>>> temporary block is put into the boxed result of cd. You need to look
>> in
>>>> that boxed result, not in the name res, which has not been modified.
>>>>>>>> This change was made in 807 because that release has a much more
>>>> aggressive policy of sharing nouns and parts of nouns without making
>>>> separate copies; for example
>>>>>>>> ({. name)
>>>>>>>> actually gives you a pointer to the first item of name, without
>>>> recopying it, and
>>>>>>>> (i. 3)
>>>>>>>> is liable to give you a pointer to unchangeable internal data. With all
>>>> these pointers flying around we can no longer trust the user to know
>>>> when it is safe to pass a pointer into a DLL that might modify the
>> data.
>>>> Henry Rich
>>>>>>>> On 11/18/2018 7:55 AM, bill lam wrote:
>>>>> j807 must get from result of cd, like this
>>>>> 0 3{ (sqrdll, 'sqr i i *i *i') cd (#y);y;res
>>>>>>>>>> On Sun, Nov 18, 2018, 2:22 PM Ben Gorte <bgorte at gmail.com wrote:
>>>>>>>>>>> Dear All,
>>>>>>>>>>>> Long ago I made quite a lot of c-functions that I am calling from J
>>>> using
>>>>>> the cd mechanism (load 'dll') in linux.
>>>>>>>>>>>> They stopped working in j64-807. I see in the release notes that
>> indeed
>>>>>> something has changed, but I cannot quite relate that to my case
>> (but
>>>> maybe
>>>>>> I should - I'm not sure I entirely understand the story).
>>>>>>>>>>>> In the simplest form it looks like this:
>>>>>> First the c-code, sqr.c :
>>>>>> int sqr (int n, int a[], int b[])
>>>>>> {
>>>>>> int i, sumsqr=0;
>>>>>> for (i=0; i<n; i++)
>>>>>> sumsqr += b[i] = a[i] * a[i];
>>>>>> return sumsqr;
>>>>>> }
>>>>>>>>>>>> which I compile with
>>>>>> gcc -O3 -shared -fPIC -o sql.so -Wl,--no-as-needed sqr.c
>>>>>> (omitting -O3 does not make a difference).
>>>>>>>>>>>> In J I go like:
>>>>>>>>>>>> load 'dll'
>>>>>>>>>>>> sqrdll =: jpath '~user/ext/sqr.so '
>>>>>>>>>>>>>>>>>> sqr =: 3 : 0
>>>>>>>>>>>> res =. (#y)#4 NB. make sure it's integer
>>>>>>>>>>>> ret =. (sqrdll, 'sqr > i i *i *i') cd (#y);y;res
>>>>>>>>>>>> ret, res
>>>>>>>>>>>> )
>>>>>>>>>>>>>>>>>> Up until j806 I would get:
>>>>>>>>>>>> sqr i.10
>>>>>>>>>>>> 285 0 1 4 9 16 25 36 49 64 81
>>>>>>>>>>>>>>>>>> But in j807:
>>>>>>>>>>>> sqr i.10
>>>>>>>>>>>> 285 4 4 4 4 4 4 4 4 4 4
>>>>>>>>>>>>>>>>>>>>>>>> So my DLL 'reads' an array alright, but seems to be not 'writing'
>> it.
>>>>>>>>>>>> For completeness:
>>>>>> JVERSION
>>>>>> Engine: j807/j64/linux
>>>>>> Release: commercial/2018-10-04T15:00:41
>>>>>> Library: 8.07.20
>>>>>> Qt IDE: 1.7.7/5.5.1
>>>>>> Platform: Linux 64
>>>>>> Installer: J807 install
>>>>>> InstallPath: /home/ben/j64-807
>>>>>> Contact: www.jsoftware.com
>>>>>>>>>>>> Perhaps I'm overlooking something trivial; any help is highly
>>>> appreciated!
>>>>>> Ben
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