Re: [Vala] Strange Behaviour?

[Thomas Jollans <t jollybox de>]

On 06/27/2012 09:12 AM, Peter Cartwright wrote:
> Hello Vala-ites!
>
> I'm working with the Mysql package and I'm stuck with something.
>
> row1 = results.fetch_row ();
> row2 = results.fetch_row ();
>
> row1 and row2 are unique. No problem.
>
> But..
>
> fields1 = field_results.fetch_fields ();
> fields2 = field_results.fetch_fields ();
>
> fields1 seems to interfere with fields2. Are they pointing to the same
> memory space?
> Is this expected behaviour?

That sounds reasonable - I don't know the MySQL API, but I had  a quick 
look at the C API docs and it they don't say that the user has to clean 
up the returned pointers, which implies that the library takes care of 
it, which means it's only safe to use the result immediately after 
obtaining it.

The usual way to use an API like this is to go through the results in a 
loop and save the data you need in your own data structures.

T

> In the code example below (with output) you can see that a reference kept
> to the first fetch_fields() (temp) get overwritten (garbled?) by subsequent
> fetch_fields().
>
> Code Example:
> --------------------------------
>
> var row = results.fetch_row ();
> var table = row[0];
> var field_results = _DB.db.list_fields (table);
> var fields = field_results.fetch_fields ();
> var temp = fields;
> stdout.printf("1)\n");
> stdout.printf(" %s\n", table);
> stdout.printf(" %s\n", fields[0].name);
> stdout.printf("temp: %s\n", temp[0].name);
>
> row = results.fetch_row ();
> table = row[0];
> field_results = _DB.db.list_fields (table);
> fields = field_results.fetch_fields ();
> stdout.printf("2)\n");
> stdout.printf(" %s\n", table);
> stdout.printf(" %s\n", fields[0].name);
> stdout.printf("temp: %s\n", temp[0].name);
>
> row = results.fetch_row ();
> table = row[0];
> field_results = _DB.db.list_fields (table);
> fields = field_results.fetch_fields ();
> stdout.printf("3)\n");
> stdout.printf(" %s\n", table);
> stdout.printf(" %s\n", fields[0].name);
> stdout.printf("temp: %s\n", temp[0].name);
>
> row = results.fetch_row ();
> table = row[0];
> field_results = _DB.db.list_fields (table);
> fields = field_results.fetch_fields ();
> stdout.printf("4)\n");
> stdout.printf(" %s\n", table);
> stdout.printf(" %s\n", fields[0].name);
> stdout.printf("temp: %s\n", temp[0].name);
>
> row = results.fetch_row ();
> table = row[0];
> field_results = _DB.db.list_fields (table);
> fields = field_results.fetch_fields ();
> stdout.printf("5)\n");
> stdout.printf(" %s\n", table);
> stdout.printf(" %s\n", fields[0].name);
> stdout.printf("temp: %s\n", temp[0].name);
>
> Output:
> ---------------------------
>
> 1)
>   AreaAccessAuthorisation
>   idAreaAccess
> temp: idAreaAccess
> 2)
>   AreaClosedTimes
>   idAreaClosedTimes
> temp: idAreaAccess
> 3)
>   AreaOpenAndClosedTimes
>   idAreaOpenTimes
> temp: idAreaOpenTimes
> 4)
>   AreaOpenTimes
>   idAreaOpenTimes
> temp: idAreaOpenTimes
> 5)
>   Areas
>   idAreas
> temp: 
>
> cheers,
> Peter
>
>
>
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