[Gambas-user] Minor bug with READ & WRITE commands?

[Jussi Lahtinen]

I agree.
But there should be warning message in documentation of read and write commands.


Jussi


On Mon, Jan 19, 2009 at 7:11 AM, nando <nando_f at ...951...> wrote:
> If something like..
>  WRITE #pResult + (ii * 8), CINT(.x)
> casts it as int, then no problem
>
>
> ---------- Original Message -----------
> From: Benoit Minisini <gambas at ...1...>
> To: nando_f at ...951..., mailing list for gambas users
> <gambas-user at lists.sourceforge.net>
> Sent: Mon, 19 Jan 2009 02:01:51 +0100
> Subject: Re: [Gambas-user] Minor bug with READ & WRITE commands?
>
>> On vendredi 16 janvier 2009, nando wrote:
>> > Benoit,
>> >
>> > Question regarding the line:
>> > > > WRITE #pResult + (ii * 8), .x
>> >
>> > does WRITE see it casted as variant or int ?
>> > -Fernando
>> >
>>
>> You exactly got the point. :-)
>>
>> As fResult is an Object[] array, fResult[ii].x is a Variant.
>>
>> Why? Because, in Gambas, the datatype of an operator or a function is never
>> more precise than the datatype of the arguments, unless the operator or
>> function always returns the same datatype. Read that sentence at least twice.
>>
>> So, as fResult is Object[], then fResult[ii] is Object, and fResult[ii].x is
>> always a Variant, as the interpreter cannot know that fResult[ii] is always a
>> Class1.
>>
>> As READ and WRITE receive a Variant, they read/write a byte before the value
>> in memory.
>>
>> So what is written at the pointer is not two bytes &H64+&H00 (the 100 value)
>> but three bytes &H04+&H64+&H00 (&H04 meaning Integer).
>>
>> As the memory address is computed at each loop, the third byte is replaced by
>> the first byte of the next write. The last written byte is outside of the
>> array. It does not crash because memory allocation are often rounded by the C
>> library.
>>
>> When reading, what happens? The GetAStruct C function receives:
>>
>> &H04+&H64+ &H04+&HC8+ &H04+&H64 +...+ &H04+&HC8+&H00
>>
>> And modifies it:
>>
>> &H0E+&H64+ &H05+&HC8+ ...
>>
>> Then READ wants to read a Variant, and so interprets the first byte, &H0E, as
>> a datatype that means...a Class, something that is cannot understand, and so
>> you get an error message. But in other situations, you can crash the
>> interpreter, because READ can write in memory more than expected!
>>
>> What is the solution? You have to tell Gambas which is the real datatype to
>> read and write.
>>
>> The first solutiion is what you did: using a local variable with the accurate
>> datatype.
>>
>> The second solution is not having Object as intermediate expression datatype.
>> Instead of:
>>
>>   WITH fResult[ii]
>>
>> Do that:
>>
>>   DIM hClass AS Class1
>>
>>   hClass = fResult[ii]
>>   WITH hClass
>>
>> Then the interpreter will know at runtime that hClass is a Class1, and so that
>> hClass.x is always an integer.
>>
>> So, all that is not really a bug, but I admit it is a bit weird, and not
>> beginner-friendly.
>>
>> In Gambas 3, I'm currently thinking about a better, or at least clearer
>> syntax. It will be a mix of:
>>
>> - True memory streams, i.e. a class that inherits Stream and that points at a
>> memory emplacement.
>>
>> - Classic Basic memory access function. PEEK, POKE ? :-) And/Or MkInt$(),
>> MkLong$()...
>>
>> - Explicitely specifying the datatype in READ and/or WRITE, or having
>> ReadInteger(), ReadFloat(), WriteInteger()... methods in the Stream class, or
>> as subroutines.
>>
>> People, tell me what you think about that. I'm going to bed now.
>>
>> --
>> Benoit Minisini
>>
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> ------- End of Original Message -------
>
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