MESSAGE
| DATE | 2010-02-13 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop - side discussing on C++ variable declarations
|
----- Forwarded message from Ruben Safir -----
X-Original-To: mrbrklyn-at-panix.com
Delivered-To: mrbrklyn-at-panix.com
Date: Sat, 13 Feb 2010 21:19:51 -0500
From: Ruben Safir
To: mrbrklyn-at-panix.com
Subject: [ruben-at-mrbrklyn.com: [nylug-talk] C++ Workshop - side discussing
on C++ variable declarations]
User-Agent: Mutt/1.5.20 (2009-06-14)
----- Forwarded message from Ruben Safir -----
Date: Thu, 11 Feb 2010 17:27:59 -0500
From: Ruben Safir
To: nylug-talk-at-nylug.org
Subject: [nylug-talk] C++ Workshop - side discussing on C++ variable
declarations
User-Agent: Mutt/1.5.20 (2009-06-14)
Anyone have any constructive insight on this conversation I had with a
friend?
(16:35:10) NYLXS:
variables declared outside of main are global scope
(16:35:14) billy31337:
right
(16:35:21) NYLXS:
is that to the file or to the entire program?
(16:35:26) billy31337:
entire program
(16:35:36) billy31337:
unless they are declared 'static'
(16:35:46) NYLXS:
So I have a real question now...forget static for a moment
(16:36:01) billy31337:
then they are of 'internal linkage' and invisible outside the file
(16:36:07) billy31337:
ok
(16:36:23) NYLXS:
variables declared in a header file file are also then global in scope?
(16:36:28) billy31337:
yes
(16:36:29) NYLXS:
that was the real question
(16:36:42) NYLXS:
wow
(16:37:05) billy31337:
but you have to be careful to never DEFINE anything in a header.. only
DECLARE.
(16:37:16) billy31337:
for variables, the difference is the 'extern' keyword.
(16:37:46) NYLXS:
not enough information
(16:38:07) billy31337:
foo.h:
extern int foo; // declare
foo.cpp:
int foo; // define
(16:38:12) NYLXS:
how do to define an vairable? You difine a function
(16:38:38) billy31337:
any global declared in a header has an 'extern' in front of it.;
(16:38:39) NYLXS:
well that is depressing
(16:39:05) billy31337:
and there's a corresponding cpp file somewhere with the variable defined
(same thing without the extern)
(16:39:23) billy31337:
why is that depressing
(16:39:24) billy31337:
?
(16:39:56) NYLXS:
Because it means I didn't understand a fundemenatal thing and damn books
aren't clear on this
16:40
(16:40:11) billy31337:
totally correct point.
(16:40:11) NYLXS:
Lets start from the beginning.
(16:40:21) billy31337:
they almost NEVER cover physical design.
(16:40:36) billy31337:
that is, what goes in headers, what goes in source.. linker problems,
etc.
(16:40:48) NYLXS:
what is the difference between declaring and defining a variable
(16:41:38) billy31337:
ok. a _declaration_ is a description of a variable (usually in a
header) so that code which wants to access it knows how to link to it
and how big it is.
(16:41:41) billy31337:
and what type it is.
(16:42:32) billy31337:
a _definition_ is in a .c file, and actually allocates the storage for
the thing in its resultant .o file
(16:43:05) NYLXS:
define link in this context
(16:43:32) billy31337:
link is the final construction of the executable from one or more object
files.
(16:44:18) NYLXS:
why wouldn't the storage issue not take place just at run time?
(16:44:20) billy31337:
the linker is the program which looks into the .o files, trying to
satisfy their dependencies by looking into the other .o files to find
matching symbols.
(16:44:37) billy31337:
because this isn't java.
(16:44:49) billy31337:
and the storage is in the binary.
(16:44:58) NYLXS:
But my variable is not going to be in an external library or o file
16:45
(16:46:14) NYLXS:
The object file creation is a substep of the compiler that can be dumped
to the hard drive as an object file
(16:48:44) NYLXS:
something else bothers me about this
(16:49:11) billy31337:
ld
(16:49:14) billy31337:
ld is a linker
(16:49:34) billy31337:
gcc invokes it for you, but sometimes you just want to use ld directly.
(16:49:42) NYLXS:
why use the syntax twice, ones in the header and once in the library
source file
16:50
(16:50:02) billy31337:
because it's C.
(16:50:37) NYLXS:
If I don't define it in the header then my program will still compile
and work
(16:50:47) NYLXS:
s/dfine/decalre/
(16:50:50) billy31337:
declare in header
(16:50:51) billy31337:
right
(16:50:53) billy31337:
yes
(16:51:13) billy31337:
but outside source files won't be able to access the variable.
(16:51:25) billy31337:
imagine an executable made of 1000 .o files.
(16:51:37) billy31337:
they all have to agree on the name and type of your int.
(16:51:47) billy31337:
that's what the declaration is for.
(16:52:11) NYLXS:
Its a debugging issue?
(16:52:17) billy31337:
not really.
(16:52:28) billy31337:
if someone calls sizeof(foo) it has to know its type
(16:52:54) NYLXS:
and it can't do that without the declaration?
(16:52:59) NYLXS:
I think it can
(16:53:16) billy31337:
or the code generated by an expression like "shrinkFoo(&foo)" has to
know where to get foo's pointer address.
(16:53:21) billy31337:
it can't.
(16:53:27) billy31337:
without the declaration it can't.
(16:53:46) billy31337:
C will complain that it doesn't know what foo is.
(16:54:18) NYLXS:
even though you defined it in your library source file as
(16:54:23) NYLXS:
int 1 = 100
(16:54:25) NYLXS:
;
(16:54:43) NYLXS:
int I = 100;
(16:54:45) billy31337:
but that library source isn't available to your application code.
(16:54:50) billy31337:
at compile time.
16:55
(16:55:21) NYLXS:
what?
(16:55:29) NYLXS:
If I create a function
(16:56:00) NYLXS:
func(int){ int I = 100; sizeof(I); }
(16:56:07) NYLXS:
that won't work?
(16:56:32) billy31337:
that will work.
(16:56:39) billy31337:
because 'I' is local
(16:56:58) NYLXS:
int I = 100;
(16:57:13) NYLXS:
func(int){
(16:57:26) billy31337:
the extern thing is only important for communicating about globals
ACROSS object files.
(16:57:27) NYLXS:
wait
(16:57:52) NYLXS:
To make them GLOBAL
(16:58:05) billy31337:
they're global anyway
(16:58:29) NYLXS:
Ok - I'm confused
(16:58:37) billy31337:
but several C files might want to use the same global.
(16:58:44) billy31337:
that global is only defined in one place.
(16:58:51) NYLXS:
one sec ... I want to look up extern
(16:58:53) billy31337:
this is necessary.
(16:59:00) NYLXS:
defined or declared?
(16:59:10) billy31337:
defined. one c file defines it.
(16:59:48) billy31337:
you have to understand what the linker is.
17:00
(17:00:13) billy31337:
write a c program, and run 'nm' on it.
(17:01:48) NYLXS:
http://www.cppreference.com/wiki/keywords/extern
(17:01:51) billy31337:
http://www-zeuthen.desy.de/apewww/APE/software/nlibc/html/errno_8h-source.html
<== look at line 473 for the definition of 'errno'
(17:02:05) billy31337:
nevermind.. let's look at your link
(17:04:03) NYLXS:
This got more complicated than I anticipated very fast.
(17:04:47) NYLXS:
You declare a variable (optinally in a header file) and define it
17:05
(17:05:04) NYLXS:
If I define it in the header file
(17:05:38) NYLXS:
int abc; that is a dfinition and although it shouldn't be done in a
header, if you do so it has global scope
(17:06:18) billy31337:
yes. and it will be given storage in every C file that includes that
header, which is bad.
(17:06:49) NYLXS:
Go it
(17:06:57) billy31337:
because when the linker goes to build your app by combining these object
files, they'll all have an 'int abc' and the linker won't be able to
tell which is the right one.
(17:07:10) NYLXS:
extern int abc; will declare it without defining it
(17:07:16) billy31337:
right.
(17:07:48) billy31337:
it will leave a 'hole' in your object file to be filled in at link time
for every instance of 'abc'.
(17:07:52) NYLXS:
then only the library sources that define it will create space for it
(17:07:58) billy31337:
the linker's only job is to go through object files filling in holes.
(17:08:10) NYLXS:
one sec = phone
17:15
(17:19:59) NYLXS:
OK - so in a header file, if you want a real declaration you have to
have the extern keyword
17:20
(17:20:44) NYLXS:
unless otherwise your actually defineing it in the header file which
causes more overhead
(17:20:53) NYLXS:
and should be avoided
--
http://www.mrbrklyn.com - Interesting Stuff
http://www.nylxs.com - Leadership Development in Free Software
So many immigrant groups have swept through our town that Brooklyn, like Atlantis, reaches mythological proportions in the mind of the world - RI Safir 1998
http://fairuse.nylxs.com DRM is THEFT - We are the STAKEHOLDERS - RI Safir 2002
"Yeah - I write Free Software...so SUE ME"
"The tremendous problem we face is that we are becoming sharecroppers to our own cultural heritage -- we need the ability to participate in our own society."
"> I'm an engineer. I choose the best tool for the job, politics be damned.<
You must be a stupid engineer then, because politcs and technology have been attached at the hip since the 1st dynasty in Ancient Egypt. I guess you missed that one."
? Copyright for the Digital Millennium
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----- End forwarded message -----
--
http://www.mrbrklyn.com - Interesting Stuff
http://www.nylxs.com - Leadership Development in Free Software
So many immigrant groups have swept through our town that Brooklyn, like Atlantis, reaches mythological proportions in the mind of the world - RI Safir 1998
http://fairuse.nylxs.com DRM is THEFT - We are the STAKEHOLDERS - RI Safir 2002
"Yeah - I write Free Software...so SUE ME"
"The tremendous problem we face is that we are becoming sharecroppers to our own cultural heritage -- we need the ability to participate in our own society."
"> I'm an engineer. I choose the best tool for the job, politics be damned.<
You must be a stupid engineer then, because politcs and technology have been attached at the hip since the 1st dynasty in Ancient Egypt. I guess you missed that one."
? Copyright for the Digital Millennium
----- End forwarded message -----
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