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Re: Static Arrays Inside Functions

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"F. Russell" <fr@random.info> writes:
> On Mon, 29 Apr 2019 01:17:21 +0000, Scott wrote:
>> Most programmers would quickly intuit that object code must
>> necessarily contain all the information it needs for all of its
>> initializations.
> 
> The C language requires more than just simple intuition.  Save
> that for BASIC or even .NET.
>
> Arrays in C, if not explicitly allocated with malloc(), are
> placed on the stack.  But a static array within a function
> has to stored differently and most likely that storage location
> is the data segment of the final executable.  Does the linker
> which joins the function object file with the final executable
> include them in the data segment?

That makes it sound like there's a default way to store array
objects.  There isn't.

A named array object defined outside any function, or defined
inside a function with the "static" keyword, has static storage
duration (it's created at program startup and exists until the
program terminates).  Any other named array object (defined inside
a function without "static") has automatic storage duration; its
lifetime ends at the end of the smallest block that encloses its
definition.  The latter is typically implemented via a "stack",
though the standard doesn't use that term.

A static object with an initialization typically has a bit-for-bit
copy in the executable file, and when the executable is loaded
during program startup, the memory region into which that copy is
loaded is the object.  A static object without an initialization is
initialized to zero; rather than wasting space in the executable
file with a bunch of zero bytes, it typically just specifies the
length; the runtime system takes care of creating and zeroing the
object during program startup.  None of this is specified by the
C standard, which only says how the program must behave.

(I'm glossing over the fact that "zero" for pointer and
floating-point types is not required to be all-bits-zero.  In most
implementations it is, and compilers are free to take advantage
of that.  In a hypothetical implementation where the null pointer
is represented as 0xFFFFFFFF, a static array of pointers with no
explicit initialization might require more work to set it up.)

> My question concerns whether or not these values are "ready to go"
> at run time.  That is, upon a first call to the function, are
> these values immediately accessible or is there an initial
> setup required.  This may make a difference if the array is
> very large.

If a function definition contains a static array definition, that
object will be "ready to go" before the function is ever called --
in principle, even before main() is called.  There may be an initial
setup, but it happens before any of your program code is executed.

> An answer likely requires knowledge of, on GNU/Linux, the
> ELF file format and the linker/loader process.

That knowledge is required if you want to know the details of how the
behavior is implemented, but the behavior itself is specified by the C
standard.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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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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