• Jan Allersma (33/33) Apr 27 2023 Hello,
• Mike Parker (10/13) Apr 27 2023 That's a compilation error, not a linker problem. You need to
• Jan Allersma (9/11) Apr 27 2023 Declaring the function does fix the compiler problem. However, I
• Steven Schveighoffer (6/21) Apr 27 2023 You may have to link the library second. Sometimes linkers are
• Jan Allersma (6/8) Apr 28 2023 Hi Steve,
• Jan Allersma (26/29) Apr 28 2023 I used to `dub run` to solve the problem, nothing special. My

Jan Allersma <janallersma gmail.com> writes:

Hello,

I see some examples on the internet on how to call C(++) in D. 
But I want to do it the other way around. Because of (probably) 
linkage issues, this way seems easier. I have this D code:

```
import std.stdio;

int maint() {
   writeln("Returning some random stuff...");
   return 10;
}

extern (C++) int foo() {
   return maint();
}

```

And this CPP source file:

```
#include <iostream>

int main() {
   std::cout << "deede" << foo() << std::endl;
}
```

This is some gibberish, but the point is that foo needs to be 
called from the D source file.

In Bash I compile it and get a response:
```
$ g++ ../build/libshared-api.so main.cpp
main.cpp: In function ‘int main()’:
main.cpp:4:27: error: ‘foo’ was not declared in this scope
     4 |   std::cout << "deede" << foo() << std::endl;
       |
```
Apparently foo isn't found from the CPP source file. Anyone some 
ideas on how to solve this? :)

Mike Parker <aldacron gmail.com> writes:

On Thursday, 27 April 2023 at 20:32:24 UTC, Jan Allersma wrote:

 ```
 Apparently foo isn't found from the CPP source file. Anyone 
 some ideas on how to solve this? :)

That's a compilation error, not a linker problem. You need to 
tell the compiler about the function with a prototype:

```C++
#include <iostream>

int foo();

int main() {
   std::cout << "deede" << foo() << std::endl;
}
```

Jan Allersma <janallersma gmail.com> writes:

On Thursday, 27 April 2023 at 21:05:00 UTC, Mike Parker wrote:
 That's a compilation error, not a linker problem. You need to 
 tell the compiler about the function with a prototype:

Declaring the function does fix the compiler problem. However, I 
do get a linker error once I compile that:

```bash
$ g++ ../build/libshared-api.so main.cpp
/usr/bin/ld: /tmp/ccNMKxEE.o: in function `main':
main.cpp:(.text+0x2a): undefined reference to `foo()'
collect2: error: ld returned 1 exit status
```

Steven Schveighoffer <schveiguy gmail.com> writes:

On 4/27/23 5:29 PM, Jan Allersma wrote:
 On Thursday, 27 April 2023 at 21:05:00 UTC, Mike Parker wrote:
 That's a compilation error, not a linker problem. You need to tell the 
 compiler about the function with a prototype:

 
 Declaring the function does fix the compiler problem. However, I do get 
 a linker error once I compile that:
 
 ```bash
 $ g++ ../build/libshared-api.so main.cpp
 /usr/bin/ld: /tmp/ccNMKxEE.o: in function `main':
 main.cpp:(.text+0x2a): undefined reference to `foo()'
 collect2: error: ld returned 1 exit status
 ```
 
 

You may have to link the library second. Sometimes linkers are 
particular about object order.

Also, please include your commands for building the D library so 
reproduction is possible.

-Steve

Jan Allersma <janallersma gmail.com> writes:

On Thursday, 27 April 2023 at 21:34:17 UTC, Steven Schveighoffer 
wrote:

 You may have to link the library second. Sometimes linkers are 
 particular about object order.

Hi Steve,

Your suggestion worked! Reversing the order solved the problem. I 
have another problem that I'm facing, but that is something 
hardly related to this so I'll open a new thread. Thanks :)

Jan Allersma <janallersma gmail.com> writes:

On Thursday, 27 April 2023 at 21:34:17 UTC, Steven Schveighoffer 
wrote:
 Also, please include your commands for building the D library 
 so reproduction is possible.

 -Steve

I used to `dub run` to solve the problem, nothing special. My 
`dub.json` isn't that interesting either, but I'll show it anyway 
for the sake of reproduction purposes:

```json
{
         "authors": [
                 "Jan Allersma"
         ],
         "copyright": "Copyright © 2023, Jan Allersma",
         "dependencies": {
                 "d2sqlite3": "~>1.0.0",
                 "secured": "~>2.0.2"
         },
         "description": "A minimal D application.",
         "license": "proprietary",
         "name": "shared-api",
         "targetType": "dynamicLibrary",
         "targetPath": "../build"
}
```

For people that use a Linux machine and don't have experience 
with using shared libraries just like me: Don't forget to add 
your shared library to a library path (e.g. `/usr/local/lib`). 
And just to be sure, I also executed `ldconfig` in Bash.