Thank you Isaiah,
that worked perfectly.
On Wed, Feb 19, 2014 at 4:28 AM, Isaiah Norton <[email protected]>wrote:
> 1. Its not clear what an appropriately defined bits type is. Should I
>> determine the size of the c struct and create a bits type with that many
>> bytes? If so how do I access the fields?
>>
>
> Use `immutable`. it should look like this definition linked below, except
> with `type` -> `immutable`:
>
> https://gist.github.com/ihnorton/9081076#file-libmysql_h-jl-L1359-L1379
>
> Search for `isbits` in the manual, which will let you check whether a
> given type is C-compatible.
> (this documentation is a bit scattered and can certainly be improved).
>
> 2. What is jl_value_t ? does that also get translated to struct?
>>
>
> No. It is possible to pass a pointer to a jl_value_t, but that is not
> necessary for mysql.
>
> jl_value_t is a struct containing a type tag element. Much of the Julia C
> code is built around checking
> `((jl_value_t*)v)->type`, casting to a more specific type, and then
> branching appropriately.
>
> 3. Will you please look at the small piece of code in the first message
>> and let me know why the value of 'somefield' is garbled? i would be
>> grateful if you can show me the right way to do it.
>>
>
> There are a number of examples of passing pointer-to-struct in Julia
> packages. A relatively simple one is
> Nettle.jl (https://github.com/staticfloat/Nettle.jl) and a complex one is
> PyCall.
>
> Regarding the code you sent, see a simplified version:
> https://gist.github.com/ihnorton/9081625
>
> I haven't followed the objref calls through in the original, but they are
> not necessary. probably not corruption so much as looking at the wrong
> thing. Note that `print(val)` or just `val` at the REPL will display the
> address for any Ptr value automatically.
>
> Best,
> Isaiah
>
>
> On Tue, Feb 18, 2014 at 4:57 PM, Sharmila Gopirajan <
> [email protected]> wrote:
>
>> This is from the latest documentation
>>
>> http://docs.julialang.org/en/latest/manual/calling-c-and-fortran-code/
>>
>> *Note that no C header files are used anywhere in the process. Currently,
>> it is not possible to pass structs and other non-primitive types from Julia
>> to C libraries. However, C functions that generate and use opaque structs
>> types by passing around pointers to them can return such values to Julia as
>> Ptr{Void}, which can then be passed to other C functions as Ptr{Void}.
>> Memory allocation and deallocation of such objects must be handled by calls
>> to the appropriate cleanup routines in the libraries being used, just like
>> in any C program.*
>>
>> i'm extending the MySQL package that you mentioned. In the documentation,
>> structs are mentioned only under Type Correspondence as
>> struct T* (where T represents an appropriately defined bits type)
>> Ptr{T}(call using &variable_name in the parameter list)
>> struct T (where T represents an appropriately defined bits type) T (call
>> using &variable_name in the parameter list) jl_value_t* (any Julia Type)
>> Ptr{Any}
>> 1. Its not clear what an appropriately defined bits type is. Should I
>> determine the size of the c struct and create a bits type with that many
>> bytes? If so how do I access the fields?
>> 2. What is jl_value_t ? does that also get translated to struct?
>> 3. Will you please look at the small piece of code in the first message
>> and let me know why the value of 'somefield' is garbled? i would be
>> grateful if you can show me the right way to do it.
>>
>> Thank you,
>>
>> regards,
>> Sharmi
>>
>>
>>
>>
>>
>>
>> On Wed, Feb 19, 2014 at 1:06 AM, Jameson Nash <[email protected]> wrote:
>>
>>> Where is that statement in the manual? It should be updated/removed. The
>>> unsafe* functions hide an object from the gc and should generally not be
>>> used. There is a section in the manual about how to pass struct a to/from
>>> C. There's also many packages you can reference for various strategies for
>>> doing this (including a MySQL wrapper, iirc)
>>>
>>>
>>> On Tuesday, February 18, 2014, Kevin Squire <[email protected]>
>>> wrote:
>>>
>>>>
>>>> On Tue, Feb 18, 2014 at 10:54 AM, Sharmila Gopirajan Sivakumar <
>>>> [email protected]> wrote:
>>>>
>>>>> I'm working on the mysql client lib for Julia. I'm using the C api
>>>>> that support statement mode. To retrieve data after executing the query,
>>>>> it needs C structs (MYSQL_BIND) to be created and passed to C. Each field
>>>>> of the row retrieved from db will be stored in the corresponding struct
>>>>> and
>>>>> I should read the value from the struct.
>>>>>
>>>>> It is unfortunate that I did not notice the line in Julia
>>>>> documentation stating "Currently, it is not possible to pass structs and
>>>>> other non-primitive types from Julia to C libraries." and spent an
>>>>> inordinate amount of time trying to debug what I did wrong.
>>>>>
>>>>> This particular struct is defined in mysql/mysql.h. What are the
>>>>> options I have?
>>>>>
>>>>
>>>> One option is the StrPack.jl package, which serializes/deserializes
>>>> Julia types into/from C structs.
>>>>
>>>> There is ongoing work to make Julia types bitwise identical to C
>>>> structs (see https://github.com/JuliaLang/julia/pull/2818), but this
>>>> won't be generally available until at least v0.4.
>>>>
>>>>
>>>>> 1. Is there a julia call that can read the header files and create
>>>>> the struct or an equivalent feature?
>>>>>
>>>>
>>>> There's no Julia call for this, but the Clang.jl package also might be
>>>> useful. I believe it has some struct support.
>>>>
>>>> Hope this helps.
>>>>
>>>> Cheers,
>>>> Kevin
>>>>
>>>>
>>
>
