[Plplot-devel] patch re mktime

[Steve S. <s.s...@im...>]

Dear Plplot-devel,

I attach a patch that immunizes plplot from the peculiarities of C's
mktime function, that arise because mktime requires input in localtime.
The various recipes to calculate offsets from local time to utc, such as
those in example x29, are not foolproof or portable. I have not explored
the equivalent timegm function that I've seen in one of the plplot-devel
threads, but some of them still rely on mktime and then attempt to
adjust the result.

The patch works by converting a broken-down (year, month, day, ...) utc
time to seconds from a robust epoch, and also converting time_t t=0 to
the same epoch so that the difference can be calculated. My
understanding is that t=0 is not part of the C-standard, and certainly
some implementations use different values.

The robust epoch is seconds since 0AD. Actually this isn't a real date,
but this choice is used by NASA NSSDC CDF file standards and the
resulting code, which is what I used in modified form, is robust and has
been in use for many years on numerous platforms.

The result is a mktimeutc routine that replaces mktime and is the
matching partner/inverse for gmtime. I've put the prototypes for the
helper routines in plplotP.h, and the routines in pldtik.c, which is the
only place where mktime is used in plplot. Indeed, it is only found in
pldtfac, which is now tidier in that tdiff doesn't need to be
calculated.

I have NOT modified x29 to use mktimeutc, but I think it might be
helpful for users that need to handle time to have access to it. It's
not really part of the plplot api, but it wouldn't hurt to employ it in
x29, with a small comment so the user can find it, to help users code
their own time-handling routines.

This matter is important to us since we supply a sophisticated analysis
suite, QSAS (http://www.sp.ph.ic.ac.uk/csc-web/QSAS/qsas_welcome.html)
for space plasma physics; time plays an absolutely critical role in the
data handling and analysis, so our product needs to be robust. Up until
now, we have used pgplot for graphics, but we are in the process of
converting to plplot to avoid the porting issues with fortran in a C/C++
environment - especially in an attempt to port to Windows/mingw -, and
to take advantage of ongoing plplot development. I'm sure you'll be
hearing more from us, and I hope we will be able to contribute to the
development in addition to cries for help.

For those who want to look at or try my changes, I attach my pldtik.c
routine. If you uncomment the prototypes near the top of that file and
drop it into the src directory, it should build without any changes to
header files and such.

Comments welcome.

Thanks for the good work.

Steve
-- 
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Professor Steven J Schwartz      Phone: +44-(0)20-7594-7660
Space and Atmospheric Physics    Fax:   +44-(0)20-7594-7772
The Blackett Laboratory          E-mail: s.s...@im...
Imperial College London          Office: Huxley 6M70 
London SW7 2BW, U.K.             Web: http://www.sp.ph.ic.ac.uk/~sjs
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