[Gambas-user] [Fwd: Gambas3 Pointers example] signal #6

[Demosthenes Koptsis]

i made some examples in c++ and i get the same behavior when i free a
pointer the variable that holds its value is unusable.

ok, sorry i must study about pointers i think i mixed up some things in
the last post... sorry

On Mon, 2011-01-10 at 23:05 +0200, Demosthenes Koptsis wrote:
> On Mon, 2011-01-10 at 21:58 +0200, Jussi Lahtinen wrote:
> > Oh, I forgot to mention, that when you do "Free(pPointer)", Gambas doesn't
> > occupy that memory address for you anymore.
> > And therefore some other process can write something to it, and result is
> > not necessarily human readable.
> > 
> > Jussi
> > 
> 
> Ok, understood.
> But i have put the contents of memory address to a variable.
> 
> sWorkingDirectory = Str@(pFunc)
> 
> if i free pFunc why variable sWorkingDirectory is affected?
> 
> if this is happend i must know it, so to use variables before free any
> pointer.
> 
> 
> > 
> > On Mon, Jan 10, 2011 at 21:54, Jussi Lahtinen <jussi.lahtinen at ...626...>wrote:
> > 
> > >
> > >
> > >> first print gives the correct output
> > >> /home/user/Documents/Development/Gambas3/Pointers7
> > >>
> > >> and the second after Free(pFunc) gives this
> > >> �s&^@^@^@^@^@^@^@^@^@ocuments/Development/Gambas3/Pointers7
> > >>
> > >
> > >
> > > This has something to do how strings are handled in Gambas, I don't think
> > > this is bug.
> > >
> > > Some lines of simplified example code and explanation:
> > >
> > > Dim pPointer As Pointer
> > > Dim hMemory As Stream
> > > Dim s As String
> > >
> > > pPointer = Alloc(4) ' Alloc for four characters, last one means 'end of
> > > string'.
> > > hMemory = Memory pPointer For Write
> > > Write #hMemory, Asc("a") As Byte
> > > Write #hMemory, Asc("b") As Byte
> > > Write #hMemory, Asc("c") As Byte
> > > Write #hMemory, Asc("\0") As Byte
> > >
> > > s = Str@(pPointer) ' This associates address to variable. Ie, in address
> > > pPointer is content of this string.
> > >
> > > Print s ' This will print out; "abc"
> > >
> > > Seek #hMemory, 0
> > > Write #hMemory, Asc("x") As Byte ' These two lines will access to address
> > > pPointer and change first character in there.
> > >
> > > Print s ' Therefore string changes to "xbc". Variable 's' is just
> > > representation of content in some (in this case pPointer) address.
> > >
> 
> Yes this is happens!
> 
> This is behaved like s variable acts as a pointer to pPointer.
> As it beahaves seems s is linked with pPointer.
> I think that s should not point to pPointer but copy its value to s
> variable.
> 
> Result = String@ ( Pointer AS Pointer )
> 
> Result i think is a value, so this value is assigned to s var.
> After assignation s has a value and should not change it if i Free a
> pointer. 
> 
> If i write to pPointer or Free it, the s var is placed in other adress
> normally in RAM.
> 
> As i understand the case,
> 
> pPointer = Alloc(4) allocate a memory address1
> Dim s as string     allocate a memory address2
> 
> if i use 
> s=Str@(pPointer)
> 
> this means
> address1=adress2 ?
> 
> s holds a value, Str@(pPointer) returns a value not its address and
> should not be linked s by addresses as pointers
> 
> Am i wrong?
> 
> 
> For now i will upgrade Gambas to latest revision and try again.
> > >
> > >
> > >>
> > >> b) if i use Free(pBuf)
> > >> i get signal #6
> > >>
> > >
> > > I don't think this should happen... Benoit?
> > >
> > >
> > >
> > >
> > >> here i get correct results
> > >> but if i use Free(pFunc) i get signal #11
> > >>
> > >
> > > This shouldn't happen, it is bug.
> > > However I couldn't reproduce it with Gambas 3 rev 3451,
> > > but I got signal 6 from it.
> > >
> > >
> > > Jussi
> > >
> > >
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-- 
Regards,
Demosthenes