Union Sizes

[Michael Ritter]

Does anyone know if unions have a size restriction?
Here is the curiosity that I am having:

Union DataBuf
byteBuf(3000000) as BYTE
quadBuf(375000) as QUAD
End Union

'-----------------------------------------
Function AccessUnion(Buf as DataBuf) as Long
'just try to manipulate data in Buf
Dim l&

For l& = 0 to 7
Buf.byteBuf(l&) = 1
Next l&
End Function

'-----------------------------------------
Function PbMain() as Long
Dim Buf as DataBuf, l&

l& = AccessUnion(Buf) 'get stack overflow
End Function

I just typed the above out (look for typos when compiling)
because I am not sitting at my computer. The curiosity is
that it does not generate a Run Time Error if the values
for the union are less than one meg or so, ie:

Union DataBuf
byteBuf(900000) as BYTE
quadBuf(112500) as QUAD
End Union

Thanks for any help.

Cheers,

Michael


------------------

[Lance Edmonds]

LOCAL variables are stored on the stack, so you are limited to UDT and UNION sizes to something less than the 1Mb stack PowerBASIC allocates (the absolute maximum size depends on the stack usage).

For larger sizes, make the UDT/UNION STATIC or GLOBAL instead.

Arrays within UDT's and UNIONS are also limited to 16Mb each, with a total maximum UDT/UNION size of 64Mb.

Please note that the same stack rules apply to LOCAL ASCIIZ and Fixed-length strings.

I hope this helps!


------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Michael Mattias]

Union DataBuf
  byteBuf(900000) as BYTE
  quadBuf(112500) as QUAD
End Union

True, local (stack) variables are limited by the size of the stack, but there are at least two ways to get around that and create LOCAL variables of large size:
Create a one-element array, as in:

FUNCTION Foo() AS LONG
  REDIM X (0) AS DataBuf
  REDIM  szX (0) AS LOCAL ASCIIZ * 2000000

It appears (not documented) in this case PB uses the stack only for the array descriptor, the data residing in a far heap


2. Do your own allocation, and access the data with pointers:

FUNCTION Foo() AS LONG
  DIM pFoo AS DataBuf PTR
  LOCAL hMem AS LONG
  hMem = GlobalAlloc (%GMEM_ZEROINIT OR %GMEM_MOVEABLE, SIZEOF(dataBuf))
  pFoo = GlobalLock (hMem)

   @pFoo.Bytebuff(100) = 3?
   @pFoo.QuadBuf(100000) = 123456789&&

   ...
   GlobalUnlock hMem
   GlobalFree hMem

END FUNCTION

NOTE: Microsoft reccomends using the Heapxxx functions in lieu of the Globalxxx functions except where the Globalxxx functions are required (DDE, Clipboard, etc).

MCM

[Lance Edmonds]

Good point, however considering the original code shows the UDT being defined in PBMAIN, that is a lot of effort to go to when making it STATIC will do the job nicely (since LOCAL vars in PBMAIN will perform identically to STATIC variables in PBMAIN since PBMAIN is not reentrant.

------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Michael Ritter]

Thanks for both replies. That sounds like the problem.
Will test it out as soon as I get to my computer.

Cheers,

Michael

------------------

[Michael Mattias]

..however considering the original code shows the UDT being defined in PBMAIN, that is a lot of effort to go to when making it STATIC will do the job nicely (since LOCAL vars in PBMAIN will perform identically to STATIC variables in PBMAIN since PBMAIN is not reentrant.

My point was more general - that is, sometimes you can't use a STATIC or a GLOBAL, such as when writing thread-safe or 'must-support-reentrancy' procedures.

MCM