Help with __cplusplus

[Arturo]

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

If you know about this stuff or about a reading to consult please
let me know it.

[osmium]

Arturo writes:

I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

If you know about this stuff or about a reading to consult please
let me know it.

It looks like you have encountered the header of a library function and the
vendor that supplies the compiler uses the same library for a C++ compiler.
This is the norm, nowadays. __cplusplus means something to *your* compiler
(it can't mean anything in the C standard because time only seems to flow in
the forward direction and C preceded C++). The syntax extern"C" { is a way
to defeat name mangling, a property that C++ has and C does not.

I could be wrong but my guess is you are taking the wrong approach to
learning; trying to absorb and have everything make sense as you encounter
it. A great many people on these newsgroups implicitly advocate that but I,
personally, have had very poor luck my entire life learning *anything* that
way. Get fairly fluent in writing simple programs first, then go back and
fill in the holes in your knowledge. What some people call a spiral or
helical approach. It may take several revisits before you finally absorb it
all - in the case of C++ you may very well die first.

Looking at headers is not a good way for a novice to learn much of anything
except that computer programming is a complex business.

[Irrwahn Grausewitz]

eg******@yahoo.com.mx (Arturo) wrote:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...
[flame-shield: warning, partially OT]

__cplusplus is a macro that a C implementation is *not* allowed to
define in standard conforming mode. It's conventionally defined by
C++ compilers. The extern "C" part tells a C++ compiler, that the
following code requires C calling conventions. (Please note: further
questions about C++ should be directed to comp.lang.c++ to avoid
irritations.)
If you know about this stuff or about a reading to consult please
let me know it.

You definitely want to read _The C Programming Language_ by Kernighan
and Ritchie, second edition (K&R2), accompanied by the comp.lang.c-FAQ
(link in signature). BTW, the FAQ provides some more good book
recommendations in section 18.10.

Good luck!

Regards
--
Irrwahn Grausewitz (ir*******@freenet.de)
welcome to clc: http://www.ungerhu.com/jxh/clc.welcome.txt
clc faq-list : http://www.faqs.org/faqs/C-faq/faq/
clc OT guide : http://benpfaff.org/writings/clc/off-topic.html

[Darrell Grainger]

On Wed, 9 Jun 2004, Arturo wrote:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

If you know about this stuff or about a reading to consult please
let me know it.

This is a feature of C++. The code you are looking at is designed for a
compiler that works as a C or C++ compiler. If you set the switches for C
the #ifdef is false and the extern is used. If you set the switches for
C++ the #ifdef is true and it uses the extern as required by C++. If you
are interested in more information ask the guys in comp.lang.c++ what the
'extern "C"' is all about or check out a book on C++.

--
Send e-mail to: darrell at cs dot toronto dot edu
Don't send e-mail to vi************@whitehouse.gov

[Dan Pop]

In <d9**************************@posting.google.com > eg******@yahoo.com.mx (Arturo) writes:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

ALL you need to know in the context of the C language is that a C compiler
is NOT allowed to predefine the __cplusplus macro. Therefore, any time
you see a conditional block starting with #ifdef __cplusplus you can
*safely* ignore it.

If you ever learn C++, such things will make sense when included in a C++
program, but in C programs you can simply treat them as meaningless
comments.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Keith Thompson]

da*****@NOMORESPAMcs.utoronto.ca.com (Darrell Grainger) writes:

On Wed, 9 Jun 2004, Arturo wrote:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

If you know about this stuff or about a reading to consult please
let me know it.

This is a feature of C++. The code you are looking at is designed for a
compiler that works as a C or C++ compiler. If you set the switches for C
the #ifdef is false and the extern is used. If you set the switches for
C++ the #ifdef is true and it uses the extern as required by C++. If you
are interested in more information ask the guys in comp.lang.c++ what the
'extern "C"' is all about or check out a book on C++.

Not necessarily. The headers could be designed to be used by a C
compiler and by a separate C++ compiler, both of which look for header
files in the same place. (But since system headers tend to contain a
lot of implementation-specific code, it's likely that the two
compilers are closely related, even if they aren't a single program.)

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Jack Klein]

On 9 Jun 2004 18:56:55 GMT, Da*****@cern.ch (Dan Pop) wrote in
comp.lang.c:

In <d9**************************@posting.google.com > eg******@yahoo.com.mx (Arturo) writes:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

ALL you need to know in the context of the C language is that a C compiler
is NOT allowed to predefine the __cplusplus macro. Therefore, any time
you see a conditional block starting with #ifdef __cplusplus you can
*safely* ignore it.

This is true for compilers conforming to the C99 standard that you so
love to malign. It is quite false for C90.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Jack Klein]

On Wed, 09 Jun 2004 18:49:08 +0200, Irrwahn Grausewitz
<ir*******@freenet.de> wrote in comp.lang.c:

eg******@yahoo.com.mx (Arturo) wrote:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

[flame-shield: warning, partially OT]

__cplusplus is a macro that a C implementation is *not* allowed to
define in standard conforming mode. It's conventionally defined by
C++ compilers. The extern "C" part tells a C++ compiler, that the
following code requires C calling conventions. (Please note: further
questions about C++ should be directed to comp.lang.c++ to avoid
irritations.)

Close, but no cigar. C99 implementations are not allowed to define
__cplusplus. Earlier versions of the standard had no such
prohibitions.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Neil Kurzman]

Arturo wrote:

Hello Netlanders, I'm starting to learn the 'C' language
and reading some code I found this code:

#ifdef __cplusplus
extern "C" {
#endif

Is there someone who could explain me about this code, because
I see that this "__cplusplus" and "extern "C" {" is a kind of
standard to see the difference between C and C++, but I'm not sure
and I want more Information ...

If you know about this stuff or about a reading to consult please
let me know it.

This specific case tells a C++ compiler that the functions for this
library are C functions.
This compiler calls C and C++ functions differently and needs to be
informed that you are trying to call C functions from C++.
But since this is the C group, It is just trivia.

[Dan Pop]

In <v5********************************@4ax.com> Jack Klein <ja*******@spamcop.net> writes:

On Wed, 09 Jun 2004 18:49:08 +0200, Irrwahn Grausewitz
<ir*******@freenet.de> wrote in comp.lang.c:

eg******@yahoo.com.mx (Arturo) wrote:

>Hello Netlanders, I'm starting to learn the 'C' language
>and reading some code I found this code:
>
>#ifdef __cplusplus
>extern "C" {
>#endif
>
>Is there someone who could explain me about this code, because
>I see that this "__cplusplus" and "extern "C" {" is a kind of
>standard to see the difference between C and C++, but I'm not sure
>and I want more Information ...

[flame-shield: warning, partially OT]

__cplusplus is a macro that a C implementation is *not* allowed to
define in standard conforming mode. It's conventionally defined by
C++ compilers. The extern "C" part tells a C++ compiler, that the
following code requires C calling conventions. (Please note: further
questions about C++ should be directed to comp.lang.c++ to avoid
irritations.)

Close, but no cigar. C99 implementations are not allowed to define
__cplusplus. Earlier versions of the standard had no such
prohibitions.

For implementations of earlier versions of the standard, it was a (huge)
quality of implementation issues.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Dan Pop]

In <a7********************************@4ax.com> Jack Klein <ja*******@spamcop.net> writes:

On 9 Jun 2004 18:56:55 GMT, Da*****@cern.ch (Dan Pop) wrote in
comp.lang.c:

In <d9**************************@posting.google.com > eg******@yahoo.com.mx (Arturo) writes:

>Hello Netlanders, I'm starting to learn the 'C' language
>and reading some code I found this code:
>
>#ifdef __cplusplus
>extern "C" {
>#endif
>
>Is there someone who could explain me about this code, because
>I see that this "__cplusplus" and "extern "C" {" is a kind of
>standard to see the difference between C and C++, but I'm not sure
>and I want more Information ...

ALL you need to know in the context of the C language is that a C compiler
is NOT allowed to predefine the __cplusplus macro. Therefore, any time
you see a conditional block starting with #ifdef __cplusplus you can
*safely* ignore it.

This is true for compilers conforming to the C99 standard that you so
love to malign. It is quite false for C90.

It is quite true for C90 *implementations*, unless you can prove
otherwise.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de