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Re: Static Arrays Inside Functions

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On 27/04/2019 22:14, Keith Thompson wrote:
> Bart <bc@freeuk.com> writes:
>> On 27/04/2019 18:52, F. Russell wrote:
>>> I need to include a lookup table, via an array, within a library function
>>> that will be linked to other C programs:
>>>
>>> double some_function(...)
>>> {
>>> static double[100] = {55.55, 66.66, ...}
> 
> You forgot to provide a name for the array.
> 
> static double arr[100] = {55.55, 66.66, /* ... */};
> 
> The "static" keyword means that the object is created and initialized
> once, during program startup.  Defining it inside a function means that
> the name is visible only within the function, but the object exists and
> can be accessed indirectly from anywhere (for example if the function
> saves a pointer value).
> 
>>> ...
>>> }
>>>
>>> I am not sure when this array will actually be initialized.
>>>
>>> Will the array be initialized during compile time so that the
>>> function code contains the values?
>>>
>>> Or will the array be initialized during the first call to the function
>>> and thereafter remain static?
>>
>> At compile-time.
> 
> The compiler generates code that results, at run-time, in the
> object being allocated and initialized before main() is invoked.
> Typically a copy of the data will be included in the executable,
> loaded at program startup along with the code.
> 
>> (Otherwise it wouldn't retain the same values on subsequent calls.)
> 
> In principle, the compiler could create an implicit flag so that
> the initializion occurs exactly once, when the function is called
> for the first time.  That would probably be conforming as long as
> the visible behavior is the same, but I can't think of any good
> reason to do it that way.
> 

C++ handles function-local statics that way, but their initialisers 
don't need to be known at compile time as they do in C.  It is not as 
simple as just having a flag, since this needs to work even if the 
function is called from multiple threads at about the same time.

The only advantage you might have in C for initialising this way is if 
you had a function that might not be called at all during a run of the 
program, and a static variable that was so big the initialisation time 
was significant.  I doubt if that is common enough for compilers to 
handle it.

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Static Arrays Inside Functions "F. Russell" <fr@random.info> - 2019-04-27 17:52 +0000
  Re: Static Arrays Inside Functions Bart <bc@freeuk.com> - 2019-04-27 19:03 +0100
    Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-27 13:14 -0700
      Re: Static Arrays Inside Functions David Brown <david.brown@hesbynett.no> - 2019-04-28 00:10 +0200
  Re: Static Arrays Inside Functions Siri Cruise <chine.bleu@yahoo.com> - 2019-04-27 13:40 -0700
  Re: Static Arrays Inside Functions nobody@example.org (Scott) - 2019-04-28 17:26 +0000
    Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-28 11:29 -0700
      Re: Static Arrays Inside Functions nobody@example.org (Scott) - 2019-04-29 01:17 +0000
        Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-28 20:09 -0700
        Re: Static Arrays Inside Functions "F. Russell" <fr@random.info> - 2019-04-29 10:33 +0000
          Re: Static Arrays Inside Functions Bart <bc@freeuk.com> - 2019-04-29 11:49 +0100
          Re: Static Arrays Inside Functions Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-04-29 04:50 -0700
          Re: Static Arrays Inside Functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-04-29 17:36 +0100
          Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-29 15:24 -0700
      Re: Static Arrays Inside Functions Ike Naar <ike@iceland.freeshell.org> - 2019-04-29 05:50 +0000
        Re: Static Arrays Inside Functions Ike Naar <ike@iceland.freeshell.org> - 2019-04-29 05:51 +0000
        Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-29 00:04 -0700
          Re: Static Arrays Inside Functions Ike Naar <ike@iceland.freeshell.org> - 2019-04-29 15:06 +0000
    Re: Static Arrays Inside Functions Mateusz Viste <mateusz@wont.tell> - 2019-04-28 19:20 +0000
      Re: Static Arrays Inside Functions Keith Thompson <kst-u@mib.org> - 2019-04-28 13:50 -0700
  Re: Static Arrays Inside Functions James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-04-29 00:44 -0400

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