[Rd] fortan common block

[Simon Urbanek]

AFAIK it's all much simpler that you think. Technically common blocks are just FORTRAN's slightly complicated way to declare and access global variables. They have nothing to do with IPC - each process is loading SOs in its own virtual memory, so other processes are irrelevant (the actual implementation is usually COW which has the same effect but is more memory-efficient -- but this doesn't change the semantics). The confusion may come from the fact that in Fortran literature "shared" has a different meaning (shared between modules of the same program = process) than in IPC (shared across processes).

Cheers,
Simon


On May 9, 2011, at 10:26 AM, Paul Gilbert wrote:

> Thanks to everyone that replied to this (some offline). I should have been a bit clearer about my question. I did realize that it does work sometimes. My worry is whether it can be expected to work reliably. In this respect, I am aware of three specific reasons for concern: i/ if common data is swapped out does it get swapped back in ok (e.g. the data segment does not get compressed because it should be all zero or NA);  ii/ would R release memory (garbage collect) between different calls to the fortran; and iii/ does another R process using the same package get its own copy of the dll/.so data segment.
> 
> I have been relatively well convinced about the first two i/ swap treats this as user data and preserves it (though some OSes might prevent writing to the code segment); and ii/ R does not garbage collect the dll/.so so the data segment is safely (or otherwise) under fortran's control.
> 
> I am less certain about the third. This seems to depend on sharing the data segment of the shared object within a process but not sharing it between processes. That is, each process has its own data block even though different processes might or might not share the code block. Googling turned up discussions about using common blocks for inter-process communications, but I did not find the conclusion. It seems pretty clear that for the common block to be reliable in the sense I have in mind, inter-process communication would be prevented.
> 
> I am a bit concerned that the answer may be compiler and/or OS dependent.  This is well outside of my expertise, so assurances would be greatly appreciated.
> 
> Paul
> 
>> -----Original Message-----
>> From: b.rowlingson at googlemail.com [mailto:b.rowlingson at googlemail.com]
>> On Behalf Of Barry Rowlingson
>> Sent: May 8, 2011 3:44 AM
>> To: Paul Gilbert
>> Cc: r-devel at r-project. org
>> Subject: Re: [Rd] fortan common block
>> 
>> On Fri, May 6, 2011 at 4:04 PM, Paul Gilbert
>> <pgilbert at bank-banque-canada.ca> wrote:
>>> Is it possible in R to call a fortran routine that sets variables in
>> a common block and expect the values to persist when a call is made
>> from R to a second routine that uses the common block?
>>> 
>>> If not (as I suspect),  is it possible to use a common block in a
>> group of routines that are used together, for as long the routines do
>> not return to R?
>> 
>> Simple test seems to confirm it. Here's some 'tran with a setter and a
>> getter:
>> 
>> comchk.f:
>> 
>>      subroutine bar(n)
>>      common /c/ i
>>      i=n
>>      end
>> 
>>      subroutine geti(j)
>>      common /c/ i
>>      j = i
>>      end
>> 
>> R CMD SHLIB comchk.f
>>> dyn.load("comchk.so")
>> 
>>> .Fortran("bar",as.integer(9))
>> [[1]]
>> [1] 9
>> 
>>> .Fortran("geti",as.integer(-1))
>> [[1]]
>> [1] 9
>> 
>> Barry
> ====================================================================================
> 
> La version française suit le texte anglais.
> 
> ------------------------------------------------------------------------------------
> 
> This email may contain privileged and/or confidential in...{{dropped:26}}
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
>