[ET Trac] [Einstein Toolkit] #1377: Cacus could/should provide M_PI

[Einstein Toolkit]

#1377: Cacus could/should provide M_PI
--------------------------+-------------------------------------------------
  Reporter:  sbrandt      |       Owner:                     
      Type:  enhancement  |      Status:  new                
  Priority:  minor        |   Milestone:                     
 Component:  Cactus       |     Version:  development version
Resolution:               |    Keywords:  GRHydro M_PI       
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Comment (by hinder):

 Confusingly, on another page in the "Mathematics" section of the GNU C
 Library manual, it is more specific:

 https://www.gnu.org/software/libc/manual/html_node/Trig-Functions.html:

 > The math library normally defines M_PI to a double approximation of pi.
 If strict ISO and/or POSIX compliance are requested this constant is not
 defined, but you can easily define it yourself:

 So M_PI is not a part of POSIX, and requiring POSIX would not solve this
 problem.  Apparently, M_PI was removed in C99; it was present in Unix98.

 According to http://stackoverflow.com/questions/5007925/using-m-pi-
 with-c89-standard, the only way to get M_PI is to use compiler-specific
 non-standard methods.  Given this, we have several options:

 1. The user defines _GNU_SOURCE or _BSD_SOURCE in their compilation flags,
 or in some other way arranges that M_PI is available.
 2. The thorn writer sets these macros in the source file that needs M_PI,
 or defines M_PI in their source files.
 3. Cactus arranges that M_PI is available by using either of the above
 methods or defining M_PI in the same way as the GNU C Library if it is not
 defined already.
 4. Cactus defines CCTK_PI in the same way as the GNU C Library.

 I prefer 3, as it uses a standard name that many people expect.  The only
 case where this might cause a problem is if somebody is trying to import
 some external code to a Cactus thorn, and this thorn expects that there is
 no M_PI defined, and uses it itself. I think this is very unlikely.

 On a related note, I have created a page https://docs.einsteintoolkit.org
 /et-docs/Compiling_the_Einstein_Toolkit where we can start to collect
 policies and knowledge about compilation flags and optionlists.  Erik,
 perhaps you could run through and correct/extend it?

-- 
Ticket URL: <https://trac.einsteintoolkit.org/ticket/1377#comment:7>
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