hi all ,
anyone could tell me the useage of the keyworkd "extern" in c?
and
the difference between c & cplusplus?
in the C99Rationalv5.10.pdf it lists 4 cases. while the first
common cases will incur the vc compiler to complain.
Jack Klein said:The extern keyword specifies that the identifier being declared has
external linkage. This allows an object or function to be defined in
one translation unit (basically, source file and everything it
includes) and referred to by name from code in other translation
units.
C++ is off-topic here, is down the hall to the
right.
As to what Visual C++ might complain about, it might make a difference
whether you are using it as a C or C++ compiler, as it handles both
languages.
The document you reference is not necessarily one that many people
have.
Even for those who do, like I do, your reference is far too vague.
The document is well over 200 pages, and the word 'extern' appears
many times. I spent a brief time looking in both the PDF and printed
versions I have, and did not find the section you are referring to.
Post again and include the specific section number from the document.
That is, if you are indeed compiling C code and not C++. And for
readers here who do not have a copy of the rationale, copy and paste
the code sample you are referring to, and copy and paste the compiler
or linker error messages as well.
Jack Klein said:The extern keyword specifies that the identifier being declared has
external linkage. This allows an object or function to be defined in
one translation unit (basically, source file and everything it
includes) and referred to by name from code in other translation
units.
C++ is off-topic here, is down the hall to the
right.
As to what Visual C++ might complain about, it might make a difference
whether you are using it as a C or C++ compiler, as it handles both
languages.
The document you reference is not necessarily one that many people
have.
Even for those who do, like I do, your reference is far too vague.
The document is well over 200 pages, and the word 'extern' appears
many times. I spent a brief time looking in both the PDF and printed
versions I have, and did not find the section you are referring to.
Post again and include the specific section number from the document.
That is, if you are indeed compiling C code and not C++. And for
readers here who do not have a copy of the rationale, copy and paste
the code sample you are referring to, and copy and paste the compiler
or linker error messages as well.
In said:what's the difference when the keyword "extern" appears and when it doest appear?
for instance,
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
extern int var1; | int var1;
_____________________________________________________________
extern int var1 = 0x0bad; | int var1 = 0x0bad;
_____________________________________________________________
extern void func(); | void func();
Assuming all declarations have file scope:
declaration, external linkage tentative external definition
external definition external definition
declaration, external linkage declaration, external linkage
As you can see, the only context where extern makes any difference is
in object declarations that are not intended to be definitions. Otherwise
it can be omitted in file scope declarations, because it is the default.
Dan
external definition external definition
thanks, as you stated,
I remember in C the defintion of an object can ONLY occur ONLY ONCE.
but if I write extern int var1 = 0x0bad ; in one.c,
and write extern int var1 = 0xdead; in another.c
the vc c compiler will happy accept it.
since you think
how to explain it?
thanks
Jack Klein said:There are a lot of technical details about different models of linking
that is used by the linkers of different implementations on different
platforms. Some of them will allow multiple definitions of an
external symbol, as long as no more than one of them has an
initializer.
The C standard requires that any object or function with external
linkage that is not actually referenced in the program must have
exactly 0 or 1 definitions somewhere in the program. Any object
or function with external linkage that is actually referenced in a
program must have exactly 1 definition, no more and no less.
A violation of this requirement causes undefined behavior, and
compilers are never required to diagnose undefined behavior.
Any compiler should be able to compile the two source files with two
external definitions of 'var1', both with initializers. The error
should come when the two object files are linked together to make the
program.
I don't know of any version of Microsoft's Visual C++, or any other C
compiler for that matter, that will not generate an error if those two
source files are put into a single program.
Here is what I got from Visual C++ 6.0:
--------------------Configuration: multi - Win32
Debug--------------------
Linking...
multi2.obj : error LNK2005: _var1 already defined in multi1.obj
multi___Win32_Debug/multi.exe : fatal error LNK1169: one or more
multiply defined symbols found
Error executing link.exe.
multi.exe - 2 error(s), 0 warning(s)
And here is what I got from the Visual C++ 2005 Express Edition Beta:
------ Build started: Project: multi, Configuration: Debug Win32
------
Compiling...
multi2.c
Linking...
multi1.obj : error LNK2005: _var1 already defined in multi2.obj
Debug/multi.exe : fatal error LNK1169: one or more multiply defined
symbols found
Build log was saved at "file://c:\Program Files\Microsoft Visual
Studio 8\projects\multi\multi\Debug\BuildLog.htm"
multi - 2 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped
==========
Jack Klein said:There are a lot of technical details about different models of linking
that is used by the linkers of different implementations on different
platforms. Some of them will allow multiple definitions of an
external symbol, as long as no more than one of them has an
initializer.
The C standard requires that any object or function with external
linkage that is not actually referenced in the program must have
exactly 0 or 1 definitions somewhere in the program. Any object
or function with external linkage that is actually referenced in a
program must have exactly 1 definition, no more and no less.
A violation of this requirement causes undefined behavior, and
compilers are never required to diagnose undefined behavior.
Any compiler should be able to compile the two source files with two
external definitions of 'var1', both with initializers. The error
should come when the two object files are linked together to make the
program.
I don't know of any version of Microsoft's Visual C++, or any other C
compiler for that matter, that will not generate an error if those two
source files are put into a single program.
Here is what I got from Visual C++ 6.0:
--------------------Configuration: multi - Win32
Debug--------------------
Linking...
multi2.obj : error LNK2005: _var1 already defined in multi1.obj
multi___Win32_Debug/multi.exe : fatal error LNK1169: one or more
multiply defined symbols found
Error executing link.exe.
multi.exe - 2 error(s), 0 warning(s)
And here is what I got from the Visual C++ 2005 Express Edition Beta:
------ Build started: Project: multi, Configuration: Debug Win32
------
Compiling...
multi2.c
Linking...
multi1.obj : error LNK2005: _var1 already defined in multi2.obj
Debug/multi.exe : fatal error LNK1169: one or more multiply defined
symbols found
Build log was saved at "file://c:\Program Files\Microsoft Visual
Studio 8\projects\multi\multi\Debug\BuildLog.htm"
multi - 2 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped
==========
In said:if int var1; defined in one.c, and extern int var1=23; defined in
another.c.
vc will accept it.
vc will also accept
1) int var1 in one.c and int var1 in another.c
2) int var1 in one.c and int var1 = 23 in another.c
it seems to me that in a program an object can have mutiple tentative
definitions, but only one external definition.
Assuming all declarations have file scope, there are two external
definitions for var1 in all your examples.
Only fools try to figure out how the language works from the behaviour of
their compilers. If multiple definitions of an object cause undefined
behaviour, any implementation is free to silently accept them. And,
according to Annex J.2 (I'm too lazy to search the chapter and verse):
- An identifier with external linkage is used, but in the program
there does not exist exactly one external definition for the
identifier, or the identifier is not used and there exist
multiple external definitions for the identifier (6.9).
So, VC is free to silently accept as many external definitions for var1
as you provide. But the behaviour of your program is undefined, if it
provides more than one.
A tentative definition becomes a genuine definition at the end of the
translation unit, if not superseded by another definition for the same
object. This is why it is called "tentative" definition. So, your
conclusion is valid only for one translation unit (C source file plus
whatever stuff is included via #include directives), not for a complete
program containing multiple definitions for the same object in multiple
translation units.
Dan
ooze said:.... snip ...
till now, I don't know what's right usage of the keyword. I don't
want to write a program depending on a specific compiler. if I can
write codes according to C Standard, I think it would be easy to
port to other OS, compiler.
CBFalconer said:I suggest you follow this simple rule in each .c .h file pair:
1. Mark all file scope variables as static, unless:
1a. You want to access them from some other file, when you
don't mark them static, and you do list them in the
associated .h file as extern.
2. Mark all functions as static, unless:
2a. You want to call them from some other file, when you
don't mark them as static, and you do put their
prototypes in the .h file. No extern needed.
Note that this is for files whose storage is primarily associated
with the .c file. Think of the .h file as a means of exporting
things that you want other modules to know about and access.
Similarly, where you put #defines and the ilk is dependent on who
needs to know. By confining them to the .c file you make their
scope local.
In said:your method is good only for writting code by yourself. you can not
tell whether the code written by others is right or wrong, so it is
good if I could have a copy of the C Standard which giving the rules
what is right, what is undefined, what's wrong.
Anything wrong with K&R2?
Dan
ooze said:(e-mail address removed) (Dan Pop) wrote
I didn't say anything wrong about K&R. I was talking about how to
judge whether the code write by others is right or not, Sir.
Anyone can post the usage of extern from the C Standard, don't
state them yourself.
In said:I didn't say anything wrong about K&R. I was talking about how to
judge whether the code write by others is right or not, Sir.
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