what is it for?

[poison.summer]

Hello, I know what ifdef is used for, but I am not sure what these
lines are for for compilation.

#ifdef __cplusplus
extern "C" {
#endif

Thanks a lot!

[Walter Roberson]

In article <11**********************@g14g2000cwa.googlegroups .com>,
<po***********@gmail.com> wrote:

Hello, I know what ifdef is used for, but I am not sure what these
lines are for for compilation. #ifdef __cplusplus
extern "C" {
#endif

Those lines are for use with C++. Parts of C++ are close enough to
parts of C to make it practical to use the C code within C++, except
that C++ has to be told to that the C code will be using C calling
conventions instead of C++ calling conventions. The #if you show
will have no effect on the code when processed by a C compiler, but
when processed by a C++ compiler then __cplusplus will be true,
causing extern "C" { } to be placed around the entire block of C
code -- which is the necessary signal to C++ about the calling conventions.
--
"No one has the right to destroy another person's belief by
demanding empirical evidence." -- Ann Landers

[Kenneth Brody]

po***********@gmail.com wrote:

Hello, I know what ifdef is used for, but I am not sure what these
lines are for for compilation.

#ifdef __cplusplus
extern "C" {
#endif

Note that there will be a corresponding

#ifdef __cplusplus
}
#endif

somewhere further down the file.

This tells a C++ compiler that the following is C code, rather than C++.

If you need more information on how this works, you'll need to ask over
in comp.lang.c++ rather than here.

--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer.h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:Th*************@gmail.com>

[Keith Thompson]

Kenneth Brody <ke******@spamcop.net> writes:

po***********@gmail.com wrote:

Hello, I know what ifdef is used for, but I am not sure what these
lines are for for compilation.

#ifdef __cplusplus
extern "C" {
#endif
Note that there will be a corresponding

#ifdef __cplusplus
}
#endif

somewhere further down the file.

This tells a C++ compiler that the following is C code, rather than C++.

I don't think that's *quite* accurate. As I understand it, the code
is still C++ (it's being processed by a C++ compiler, after all); for
example, it can't use "class" as an ordinary identifier.
If you need more information on how this works, you'll need to ask over
in comp.lang.c++ rather than here.

Indeed -- but first check the comp.lang.c++ FAQ and/or any decent C++
textbook.

--
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.

[Old Wolf]

Kenneth Brody wrote:

po***********@gmail.com wrote:

Hello, I know what ifdef is used for, but I am not sure what these
lines are for for compilation.

#ifdef __cplusplus
extern "C" {
#endif

This tells a C++ compiler that the following is C code, rather than C++.

Actually it doesn't. If you are using a C compiler, the code will
be treated as C, and if you are using a C++ compiler, the code
will be treated as C++.

This directive tells a C++ compiler to generate code that's
compatible with its C ABI, so that C and C++ object files can
subsequently be linked together. Examples of this include
disabling name mangling, and maybe different calling
conventions and structure padding.

Finally, nothing in the C standard prevents a C compiler from
defining __cplusplus, although only the DS9000 would do so.

I've attempted to set follow-ups to comp.lang.c++, because this
stuff is all part of C++ rather than C. I haven't done this before
so I hope it turns out well...

[Jordan Abel]

On 2005-12-05, Old Wolf <ol*****@inspire.net.nz> wrote:

Finally, nothing in the C standard prevents a C compiler from
defining __cplusplus, although only the DS9000 would do so.
6.10.8 p5
|The implementation shall not predefine the macro __cplusplus, nor shall
|it define it in any standard header.
I've attempted to set follow-ups to comp.lang.c++, because this
stuff is all part of C++ rather than C. I haven't done this before
so I hope it turns out well...

It did, but I chose to ignore it because I'm replying to an aspect of
your post that only applies to C.

[Keith Thompson]

"Old Wolf" <ol*****@inspire.net.nz> writes:
[...]

Finally, nothing in the C standard prevents a C compiler from
defining __cplusplus, although only the DS9000 would do so.
Unless you count C99 6.10.7p5:

The implementation shall not predefine the macro __cplusplus, nor
shall it define it in any standard header.

(An aside: when I did a copy-and-paste from n1124.pdf, the macro name
showed up as "_ _cplusplus", which might make it difficult to find; I
searched for just "cplusplus".)
I've attempted to set follow-ups to comp.lang.c++, because this
stuff is all part of C++ rather than C. I haven't done this before
so I hope it turns out well...

It did, but I overrode the "Followup-To:" header for this article.

--
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.

[Eric Sosman]

Old Wolf wrote On 12/05/05 17:12,:

[...]
Finally, nothing in the C standard prevents a C compiler from
defining __cplusplus, although only the DS9000 would do so.
Section 6.10.8, paragraph 5:

The implementation shall not predefine the
macro __cplusplus, nor shall it define it in
any standard header.
I've attempted to set follow-ups to comp.lang.c++, [...]

You did it correctly, but I added c.l.c. back
because the misteak should not stand unchallenged.

--
Er*********@sun.com

[pete]

Eric Sosman wrote:

misteak

.... so now I'm still hungry.

;)

--
pete

[Mark McIntyre]

On Mon, 05 Dec 2005 22:49:43 GMT, in comp.lang.c , Keith Thompson
<ks***@mib.org> wrote:

"Old Wolf" <ol*****@inspire.net.nz> writes:
[...]

Finally, nothing in the C standard prevents a C compiler from
defining __cplusplus, although only the DS9000 would do so.

Unless you count C99 6.10.7p5:

The implementation shall not predefine the macro __cplusplus, nor
shall it define it in any standard header.

(An aside: when I did a copy-and-paste from n1124.pdf, the macro name
showed up as "_ _cplusplus", which might make it difficult to find; I
searched for just "cplusplus".)

I've attempted to set follow-ups to comp.lang.c++, because this
stuff is all part of C++ rather than C. I haven't done this before
so I hope it turns out well...

It did, but I overrode the "Followup-To:" header for this article.

Is there an echo in here?
:-)

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