A question to all you DLM experts...

[Randall Skelton <rhskelto(at)atm.ox.ac.uk>]

Hi all,

There is something subtle that I'm missing here... I have a trivial DLM
called 'array from nothing' because it is supposed to create an array in C
using IDL temporary memory and then return it to IDL using VarCopy.  The
problem is, when I initially define my new array, it works fine, but that
variable name seems to be defunct for re-assignment?  What exactly does,
'Array has a corrupted descriptor' mean?

-------------
IDL> ok=arrayfromnothing(a)
% Loaded DLM: MYIDL.
IDL> print, a
      0.00000      1.00000      4.00000      9.00000      16.0000
      25.0000      36.0000      49.0000      64.0000      81.0000
      100.000      121.000      144.000      169.000      196.000
      225.000      256.000      289.000      324.000      361.000
IDL> help, a
A               FLOAT     = Array[10, 2]
IDL> a=fltarr(10,2)
% Array has a corrupted descriptor: A.
% Execution halted at:  $MAIN$                 
IDL> help, a
A               UNDEFINED = <Undefined>
-------------

Below is the DLM code.  Any comments?

Cheers,
Randall

-------------
/* function: arrayFromNothing */
IDL_VPTR IDL_CDECL arrayFromNothing(int argc,
                    IDL_VPTR argv[], char *argk) {
    /* Called in IDL as:
     *     ret = arrayFromNothing(new_array)
     */
  
    /* general index */
    int i;
    
    /* local pointer */
    float *ptr;
    
    /* IDL specific */
    IDL_MEMINT dim[2];   
    IDL_VPTR tmp;
             
    /* set the correct dimension for the new array */
    dim[0]=10; 
    dim[1]=2;
             
    /* Make Sure we can write to it, free anything already associated */
    IDL_StoreScalarZero(argv[0], IDL_TYP_FLOAT);
    
    /* make a temporary IDL array with the same memory address as ptr */
    ptr=(float *)IDL_MakeTempArray(IDL_TYP_FLOAT,2,dim,IDL_ARR_INI_ZERO,&tmp);
    
    /* fill the array */
    for(i=0;i<100;i++) ptr[i]=i*i;
    
    /* copy the IDL_VPTR tmp to passed arg */
    IDL_VarCopy(tmp,argv[0]);
    
    /* return 1 signifying no errors in execution */
    return (IDL_GettmpLong(1));
}