linking C++ functions in a C program

[abasili]

Hi everyone, I'm trying to compile a C++ function and then call it from
a C program.
Since Google is my friend I've ended up to this link which seems very clear:

http://www.parashift.com/c++-faq-lit...c-and-cpp.html

Unfortunately it does not work.
Here is what I'm doing:

-----------------------------------------
//main.c

#include "mylib.h"

int main (int argc, char *argv[]){
foo();
return 0;
}
-----------------------------------------
//mylib.h

#indef __cplusplus
extern "C"{
#endif

void foo();

#ifdef __cplusplus
}
#endif
-----------------------------------------
//mylib.cc

#include <iostream>
#include "mylib.h"

using namespace std; // don't know what for!

void foo(){
cout<<"Hello World!\n";
}
-----------------------------------------
Thanks a lot for any suggestion.

Al

[Phil Carmody]

Pawel Dziepak <pd******@quarn os.orgwrites:

CBFalconer wrote:

>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons. C
function names are not. So the C system can't link the C++ code.

7.5/3 states that: "Every implementation shall provide for linkage to
functions written in the C programming language" by allowing things like
extern "C"
7.5/6 tells us that is is possible to define function with C-style
linkage in C++ code.
7.5/9 informs us that implementation can allow to link objects from
other languages to C++ and from C++ to other languages.

Concluding, C++ standard gives possibility to link C++ functions from C
code (with some naming restrictions).

But if you're imposing restrictions, you're no longer "In general".

Phil
--
We must respect the other fellow's religion, but only in the sense and to the
extent that we respect his theory that his wife is beautiful and his children
smart. -- Henry Louis Mencken (1880-1956), American editor and critic

[CBFalconer]

jameskuyper wrote:

CBFalconer wrote:

.... snip ...

>

>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons.

True, but extern "C" turns that off. The only part of his program
compiled with the C++ compiler is a function declared with extern
"C".

They are not compiled with the C++ compiler, they should be
compiled with the C compiler. The object code may be linked with
the C++ linker. The <extern "C"term is only in the .h file that
describes the access to the C code required. In general C++ can
access C code, but C cannot access C++ code.

Remember, the languages are different.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

[CBFalconer]

Pawel Dziepak wrote:

CBFalconer wrote:

>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons. C
function names are not. So the C system can't link the C++ code.

7.5/3 states that: "Every implementation shall provide for linkage
to functions written in the C programming language" by allowing
things like extern "C"
7.5/6 tells us that is is possible to define function with C-style
linkage in C++ code.
7.5/9 informs us that implementation can allow to link objects from
other languages to C++ and from C++ to other languages.

Concluding, C++ standard gives possibility to link C++ functions
from C code (with some naming restrictions).

You are quoting from, I believe, the C++ standard. My point is
that C cannot call to C++ code, but C++ can call C code. The idea
is to prepare C object code and allow the C++ system to call it
correctly, by using the extern "C" identification _in the header
file for the C code_.

The C++ standard is meaningless in c.l.c. Here, we use the C std.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

[CBFalconer]

Keith Thompson wrote:

CBFalconer <cb********@yah oo.comwrites:

.... snip ...

>

>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons. C
function names are not. So the C system can't link the C++ code.

Chuck, we've had this discussion before. Yes, you can call C++
functions from C, and yes, you can call C functions from C++. C++
defines the mechanism for doing this in both directions.

Last time we discussed this, I pointed you to the C++ FAQ that
explains this; it's already been cited in this thread. I even
demonstrated how to do it and posted the output. For some reason you
still didn't believe me, and now you're claiming yet again that it
can't be done.

Please consider the possibility that you might be wrong.

I did. That's why I used the "normally altered" phrase.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

[Pawel Dziepak]

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CBFalconer wrote:

You are quoting from, I believe, the C++ standard. My point is
that C cannot call to C++ code, but C++ can call C code. The idea
is to prepare C object code and allow the C++ system to call it
correctly, by using the extern "C" identification _in the header
file for the C code_.

You don't have to use extern "C" in the header file for the C code. C
compilers assume that all functions symbols are in the C style.
extern "C" used in C++ code tell compiler that it has to create C style
symbol for that object (function/variable/whatever), so C code will be
able to call it.

The C++ standard is meaningless in c.l.c. Here, we use the C std.

This discussion is about C *and* C++.

Pawel Dziepak
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[Keith Thompson]

CBFalconer <cb********@yah oo.comwrites:

Keith Thompson wrote:

>CBFalconer <cb********@yah oo.comwrites:

... snip ...

>>

>>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons. C
function names are not. So the C system can't link the C++ code.

Chuck, we've had this discussion before. Yes, you can call C++
functions from C, and yes, you can call C functions from C++. C++
defines the mechanism for doing this in both directions.

Last time we discussed this, I pointed you to the C++ FAQ that
explains this; it's already been cited in this thread. I even
demonstrated how to do it and posted the output. For some reason you
still didn't believe me, and now you're claiming yet again that it
can't be done.

Please consider the possibility that you might be wrong.

I did. That's why I used the "normally altered" phrase.

Please consider the possibility that you might *still* be wrong.

One more time, it is possible to call C functions from C++ code, and
to call C++ functions from C code. The C++ standard defines the
mechanisms for doing this in both directions. Your statement above
that "In general, you just can't do that" is wrong.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

James Kuyper <ja*********@ve rizon.netwrites :

Flash Gordon wrote:

>CBFalconer wrote, On 06/11/08 21:54:

If you're responding to what he wrote at that time, why don't you post
it as a response to the message in which he actually wrote it, on the
thread in which he wrote it? It's a little confusing to post it as a
response to a message I wrote, in a thread where he's said some
incorrect things about C/C++ linking, but he hasn't actually said
these particular incorrect things (at least, not yet):

>>describes the access to the C code required. In general C++ can
access C code, but C cannot access C++ code.

James, either I'm confused or you are.

Here's the entire content of what Flash posted, excluding headers and
signature:

| CBFalconer wrote, On 06/11/08 21:54:
|
| <snip>
|
| describes the access to the C code required. In general C++ can
| access C code, but C cannot access C++ code.
| >
| Remember, the languages are different.
|
| Remember, it has been pointed out to you many times and with references
| that you are WRONG. C++ defines how C code can call C++ code and you
| have been shown how it works. Since the theory says it should work and
| practice shows that it works why keep insisting that it does not?
| Neither theory nor practice are on your side.

Flash's article was a direct followup to CBFalconer's article, and
CBFalconer did write exactly the quoted incorrect words:

describes the access to the C code required. In general C++ can
access C code, but C cannot access C++ code.

Remember, the languages are different.

Did the "06/11/08" date format throw you off? It's DD/MM/YY, not
MM/DD/YY -- i.e., it's today, not 5 months ago. (MM/DD/YY is more
common in the US, DD/MM/YY in the UK; that's why I use YYYY-MM-DD
whenever possible.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[James Kuyper]

CBFalconer wrote:

jameskuyper wrote:

>CBFalconer wrote:

... snip ...

>>In general, you just can't do that. C++ function names are
normally altered by the C++ compiler for various reasons.

True, but extern "C" turns that off. The only part of his program
compiled with the C++ compiler is a function declared with extern
"C".

They are not compiled with the C++ compiler, they should be
compiled with the C compiler. ...

The purpose of the exercise was to find out how to call code compiled by
C++ from code compiled in C. How would compiling them with C serve that
purpose?

... The object code may be linked with
the C++ linker. The <extern "C"term is only in the .h file that
describes the access to the C code required. In general C++ can
access C code, but C cannot access C++ code.

Remember, the languages are different.

Yes, they are different languages - so what?. C++ defines features that
allow C++ code to call C code, and to define C++ functions that can be
called from C.

The program below contains a function named bar() compiled by C++ with
"C" language linkage. I know from previous discussions with you that you
have said some confusing things about whether you consider such a
function to be a C function or a C++ function.

Rather than repeat my previous failed attempts to get a straight answer
out of you about that question, I've written a program that contains one
function named foo() which is compiled in C, and is therefore
unambiguously a C function. It contains another function named baz()
compiled in C++ that has "C++" language linkage, so it is unambiguously
a C++ function. foo() calls bar() which calls baz(). Therefore, no
matter how you classify bar(), this program contains a C function
calling a C++ function.

foobar.h:
=============== =========
#ifndef FOOBAR_H
#define FOOBAR_H

#ifdef __cplusplus
extern "C" {
#endif

void foo(int);
void bar(int);

#ifdef __cplusplus
}
#endif
#endif
foo.c:
=============== =============== =====
#include <stdio.h>
#include "foobar.h"

void foo(int i) {
puts("About to call bar()");
bar(i);
puts("Finished calling bar()");
}

main.C
=============== =============== =============== =====
#include <iostream>
#include "foobar.h"

int main()
{
foo(42);
}

static void baz(int i)
{
std::cout << "the number is:" << i << std::endl;
}

void bar(int i)
{
baz(i);
}

I built the complete program as follows:

~/testprog/CfromC++(63) gcc $CFLAGS -c foo.c
In file included from foo.c:2:

~/testprog/CfromC++(64) !g++
g++ $CXXFLAGS main.C foo.o -o foobar

~/testprog/CfromC++(65) printenv CFLAGS
-std=c99 -pedantic -Wall -Wpointer-arith -Wcast-align -Wwrite-strings
-Wstrict-prototypes -Wmissing-prototypes

~/testprog/CfromC++(66) printenv CXXFLAGS
-std=c++98 -pedantic -Wall -Wpointer-arith -Wcast-align
-fno-enforce-eh-specs -ffor-scope -fno-gnu-keywords
-fno-nonansi-builtins -Wctor-dtor-privacy -Wnon-virtual-dtor
-Wold-style-cast -Woverloaded-virtual -Wsign-promo

~/testprog/CfromC++(67) foobar
About to call bar()
the number is:42
Finished calling bar()

If, as you say, C cannot access C++ code, then what exactly is the above
program doing when foo() calls bar() which calls baz()?

[James Kuyper]

Keith Thompson wrote:

James Kuyper <ja*********@ve rizon.netwrites :

>Flash Gordon wrote:

>>CBFalconer wrote, On 06/11/08 21:54:

If you're responding to what he wrote at that time, why don't you post
it as a response to the message in which he actually wrote it, on the
thread in which he wrote it? It's a little confusing to post it as a
response to a message I wrote, in a thread where he's said some
incorrect things about C/C++ linking, but he hasn't actually said
these particular incorrect things (at least, not yet):

>>>describes the access to the C code required. In general C++ can
access C code, but C cannot access C++ code.

James, either I'm confused or you are.

I am the one who was confused, but it's my news server's fault, not
mine. My news server got Flash's message before it got CBFalconer's
message, and my newsreader displayed Flash's message as a response to my
message.

Another part of the confusion is the notation 06/11/08. I normally use
an ISO format time: 2008-11-06. It is one of the two permitted date
format on the project where I work, but I was a fan of that format
before I ever joined this project. As a result, I misread 06/11/08 as
referring to 2006-11-08.

My message reflected irritation that wasn't caused entirely by my
misinterpretati on of this one message from Flash. I knew from previous
experience that CBFalconer doesn't believe that C++ functions can be
called from C, so ever since he entered this thread I've been itching to
show him a counter-example. However, until this particular message, he
had failed to state his position in a way that made my counter-example
directly relevant. I was getting a bit frustrated waiting for him to say
something I could respond to. Then Flash's message showed up quoting
CBFalconer saying something directly relevant - I could finally post my
counter-example! However, I then noticed that the message Flash referred
to was not displayed by my newsreader, and that Flash's message referred
to a date which I misinterpreted as being from 2 years ago, so I felt a
bit disappointed.

I apologize for the confusion I passed on, and for the irritation I
expressed.

[James Kuyper]

CBFalconer wrote:

Pawel Dziepak wrote:

>CBFalconer wrote:

....

You are quoting from, I believe, the C++ standard. My point is
that C cannot call to C++ code, but C++ can call C code.

Yes, and that point is incorrect, as demonstrated by a response I just
posted in another branch of this discussion.

... The idea
is to prepare C object code and allow the C++ system to call it
correctly, by using the extern "C" identification _in the header
file for the C code_.

Actually, the extern "C" should only appear in the C++ code. You can put
it in C code only by using conditional compilation to ensure that it
doesn't survive translation phase 4.

The C++ standard is meaningless in c.l.c. Here, we use the C std.

Since it is the C++ standard that defines how to link C code to C++
code, this discussion should have been posted to a C++ group. However,
just because the C++ standard is off-topic here doesn't mean that it
doesn't exist, and it doesn't justify making statements here be which
would be true only if it didn't exist. The C++ standard does indeed
define how to create a function compiled in C++, the body of which can
make full use of all C++ features, but callable from C.