| Re: Memory fragmentation, passing and common blocks [message #1031] |
Sun, 23 May 1993 15:31  |
webb
Messages: 3
Registered: May 1993
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Junior Member |
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mayor@vaxine.larc.nasa.gov writes:
> 2) I've written quite a large IDL application and have stuck with
> the convention of passing variables among all the modules. With the
> exception of a few very small common blocks for a few widget event
> handlers, I've avoided common blocks. Now after all this development,
> I'm realizing that I'm passing huge arrays back and forth and wondering
> if it would have been better to put these in common blocks. So this is
> a two part question:
>
> a) Can passing variables cause memory fragmentation?
>
> b) What exactly do I have to gain or loose if I start putting variables
> in common blocks instead of passing them?
Passing variables does *not* cause memory problems, because IDL passes
variables by reference. In the user's guide, the description of parameter
passing implies that copies are made of actual parameters, then they are
processed, then they are copied back (see section 10-5). However, this does
not really seem to be true. I've tested this by filling up memory with
large arrays till IDL cannot allocate any more space, then attempting to
call a procedure that does not itself need to allocate data -- IDL
successfully calls the routine, implying parameter passing does not cause
data to be copied.
e.g.
declare the routine mtest.pro
pro mtest, arg1
print, arg1(0)
end
then (on a Sun with 64M of virtual memory)
IDL> a = fltarr(2000,2000,/nozero)
IDL> b = a
IDL> c = a
IDL> d = a
% Unable to allocate memory: to make array.
Not enough memory
% Execution halted at $MAIN$ .
IDL> mtest, a
% Compiled module: MTEST.
0.000668437
This section of the User's guide could use some rewriting!
Peter
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