NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/24

Text File | 1996-08-06 | 5.1 KB | 130 lines

Newsgroups: comp.std.c
Path: netcom.com!ahicks
From: [e-mail] (Aaron Hicks at Netcom)
Subject: SUMMARY: static and extern, and a variable be both ?
Message-ID: <[e-mail]>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
Date: Thu, 4 Jan 1996 17:08:48 GMT
Sender: [e-mail]

Folks,

I have summarized the answers I got about my problem with the static and
extern keywords below.  But first I would like to thank the following folks
for being so kind as to enlighten me as to how this works.

[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]

SUMMARY:

  If a variable is declared static inside a function this means to keep it
in memory along with the globals and to allocate this space at run time.  If
a global is declared as static ( as I was doing ) it is hidden from all other
modules ( .c files ).  So a static in a function can be an extern for another
module but a global static can not be seen from other modules.

  I have to admit that in hind sight a static global is kind of redundant in
the context of what we wanted to do with this variable.

  After this message I have included the responces I got from the group.  I
have removed the header and most of the signatures of each message and any
part that included my original post.

Once again Thank You all for you help

Aaron Hicks
-------------------------------------------------------------------------
[e-mail] wrote

    When an object (variable, array) is declared with global scope
(not declared within a function), it is permanently allocated and
available for external linking. When you declare a global variable
static, you are limiting access to it to the code in that
translation unit (source file).

    I think you're confusing the meaning of static within
functions, where it means "allocate at compile time", and the meaning
of static for global variables: "make available only to this source
file". All global variables are allocated at compile time, so the
modifier of static is used to change the object's scope.

    It's confusing, but it makes sense after you've used it a
while.

-------------------------------------------------------------------------
[e-mail] wrote

You are confused.
static *inside a function* causes only one copy of a variable to be
created.

static *outside* a function is specifically to prevent other modules
(other .c files) from being allowed to see the variable.

-s

------------------------------------------------------------------------
[e-mail] wrote

When you declare something static to a module, that means only the
functions in that module can reference the variable.  Only functions in
the raster.c module can refer to bit_fail_array_table and have it
defined.  When you declare bit_fail_array_table to be extern in
raster_analysis.c, it expects another module to actually define it,
which causes storage to ba allocated.  Since the variable in raster.c is
only visible within raster.c, raster_analysis.c cannot see it, hence the
message about the variable being undefined.  When you took off the
static declaration in raster.c, this made the variable a system-wide
global, visible to anybody who has an external declaration for it.

Hope this helps.

--
Joe Gilliatt
[e-mail]

------------------------------------------------------------------------
[e-mail] wrote

If bit_fail_array_table is at file scope (outside of any function)
then it has static extent (i.e. it lives for the entire program), no
matter what.  In this context, the static modifier means that the
variable name is invisible outside of the file.  This is a problem if
the variable is declared extern in another file.  The solution is
simply to not declare it static in the defining file.  But it's good
practice to explicitly initialize it, even if it is to be initialized
to zero, just to indicate that it is the defining instance.

If a variable has block scope (inside a function), it can never be
visible outside the block it appears in, and then the static modifier
indicates that it has static (as opposed to automatic) extent.

        Matthew Saltzman
        Clemson University Math Sciences
        [e-mail]

-----------------------------------------------------------------------------
[e-mail] wrote

   The purpose of using the 'static' keyword on a global variable is *precisely*
to prevent it from being accessed outside of the translation unit, i.e. other
source files.  Using static within a function has another purpose in that
the variable so declared will retain its value.  You must remember that
local variables within functions are generally given temporary storage on the
stack, which is lost when the function returns.  Declaring a local variable
as static will cause it to be stored in memory alongside global variables
even though it still retains local scope.  Using 'static' on global variables
only makes its scope or 'viewability' limited.  In short, get rid of the
'static'.  Hope this helps.

R.Weathersby

P.S. I hope I interpreted your message correctly.