| reading gziped CDFs [message #34120] |
Wed, 26 February 2003 20:50  |
erikb
Messages: 1
Registered: February 2003
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Junior Member |
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Hi all,
I've got a question that google can't answer, so I'm posing it to you:
I have gziped CDF files that I would like to open without having to
manually unzip them for space considerations. I know that IDL can
handle opening gziped files with the /compress keyword to the open
routines, but that doesn't help with a CDF. I see that my ver. of IDL
(5.4) has a CDF_compression command, but it seems that this is an
internal compression method within the CDF structure. So... is there a
way to get IDL to understand that I have a gzipped CDF? I'm thinking I
need to kludge CDF_open somehow. I would only need to read these files,
I would never write/append to them.
Thanks for any ideas
-Erik
p.s. I suppose an alternative is to write wrapper open/close functions
that would SPAWN a gunzip to a temp file and then delete it on close.
Anyone know a fast way of making unique temp filenames? (I'm on a linux
system, btw.)
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| Re: reading gziped CDFs [message #34249 is a reply to message #34120] |
Thu, 27 February 2003 10:43  |
K. Bowman
Messages: 330
Registered: May 2000
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Senior Member |
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In article <b3lhp2$1j8e$1@nntp6.u.washington.edu>,
"Rick Towler" <rtowler@u.washington.edu> wrote:
> Ken suggested using mktemp() to create unique file
> names although that is a C library function and isn't native to IDL.
Actually, on my BSD-based system (OS X) mktemp has a command line
interface. Since he was thinking of spawning a gunzip command, he could
easily spawn a mktemp command first to generate a filename.
It would be safer, but not as portable as using the system clock.
Regards, Ken
bowman> mktemp /tmp/idl.XXXXX
/tmp/idl.9OIp9
bowman> ll /tmp
total 0
-rw------- 1 bowman wheel 0 Feb 27 12:36 idl.9OIp9
MKTEMP(1) System General Commands Manual
MKTEMP(1)
NAME
mktemp - make temporary file name (unique)
SYNOPSIS
mktemp [-d] [-q] [-t prefix] [-u] template ...
mktemp [-d] [-q] [-u] -t prefix
DESCRIPTION
The mktemp utility takes each of the given file name templates and
over-writes a portion of it to create a file name. This file name is
unique and suitable for use by the application. The template may be any
file name with some number of `Xs' appended to it, for example
/tmp/temp.XXXX. The trailing `Xs' are replaced with the current process
number and/or a unique letter combination. The number of unique file
names mktemp can return depends on the number of `Xs' provided; six `Xs'
will result in mktemp testing roughly 26 ** 6 combinations.
If mktemp can successfully generate a unique file name, the file is
created with mode 0600 (unless the -u flag is given) and the filename is
printed to standard output.
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| Re: reading gziped CDFs [message #34254 is a reply to message #34120] |
Thu, 27 February 2003 10:40 |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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"erikb" <erikb@bu.edu> wrote in message news:b3k5k4$cva$1@news3.bu.edu...
> Hi all,
>
> I've got a question that google can't answer, so I'm posing it to
> you:
>
> I have gziped CDF files that I would like to open without having to
> manually unzip them for space considerations. I know that IDL can
> handle opening gziped files with the /compress keyword to the open
> routines, but that doesn't help with a CDF. I see that my ver. of
> IDL (5.4) has a CDF_compression command, but it seems that this is
> an internal compression method within the CDF structure.
Yes. The CDF format supports compression of each variable
internally. I haven't used this feature for a long time, but I see
from the IDL documentation that it allows a number of compression
methods including GZIP, so it should be pretty effective. Is there any
possibility of (re)writing your files using CDF compression? Then you
might not need to gzip them.
--
Mark Hadfield "Ka puwaha te tai nei, Hoea tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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| Re: reading gziped CDFs [message #34256 is a reply to message #34120] |
Thu, 27 February 2003 09:26 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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"erikb" wrote ...
> I have gziped CDF files that I would like to open without having to
> manually unzip them for space considerations. I know that IDL can
> handle opening gziped files with the /compress keyword to the open
> routines, but that doesn't help with a CDF. I see that my ver. of IDL
> (5.4) has a CDF_compression command, but it seems that this is an
> internal compression method within the CDF structure.
> So... is there a way to get IDL to understand that I have
> a gzipped CDF?
No.
> I'm thinking I need to kludge CDF_open somehow.
Since (net)CDF functionality is provided via .dlms your ability to kludge
anything will be rather limited unless you roll your own.
I think your only option is the unzip -> use -> zip approach you have
already thought of. Ken suggested using mktemp() to create unique file
names although that is a C library function and isn't native to IDL. An
alternative would be to use the last 8 or so digits of systime(/seconds) to
create your filename. This approach has limits (you only get 1 unique name
per second) but I think it would be fine for what you want to do.
-Rick
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| Re: reading gziped CDFs [message #34267 is a reply to message #34120] |
Thu, 27 February 2003 04:22 |
Kenneth P. Bowman
Messages: 585
Registered: May 2000
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Senior Member |
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In article <b3k5k4$cva$1@news3.bu.edu>, erikb <erikb@bu.edu> wrote:
> p.s. I suppose an alternative is to write wrapper open/close functions
> that would SPAWN a gunzip to a temp file and then delete it on close.
> Anyone know a fast way of making unique temp filenames? (I'm on a linux
> system, btw.)
Try the mktemp command.
Ken Bowman
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