Re: Linking external functions?

[thinkunix via Digitalmars-d-learn]

DLearner via Digitalmars-d-learn wrote:

Wanted to try out linking two source files independantly compiled.

ExtCallee.d source file:
```
extern(C) void ExtCallee() {
    import std.stdio;

    writeln("Entered: ", __FUNCTION__);
    writeln("Exiting: ", __FUNCTION__);
}
```
ExtMain.d source file:
```
void main() {
    import std.stdio;
    extern(C) void ExtCallee();


    writeln("Entered: ", __FUNCTION__);

    ExtCallee();

    writeln("Exiting: ", __FUNCTION__);
}

What is the advantage of using extern(C)?
Since both are D source files, why wouldn't you do just this:

```d
// ExtCallee.d
void ExtCallee()
{
        import std.stdio;

        writeln("Entered:  ", __FUNCTION__);
        writeln("Exiting:  ", __FUNCTION__);
}
```

```d
// ExtMain.d
void main()
{
        import std.stdio;
        import ExtCallee;

        writeln("Entered:  ", __FUNCTION__);

        // I had to scope this to get it to compile
        // If instead I put the function in myfn.d and
        // did "import myfn;" it works without the scope operator.
        ExtCallee.ExtCallee();

        writeln("Exiting:  ", __FUNCTION__);
}
```

Compile with:
$ dmd ExtCallee.d ExtMain.d -of=prog


Without extern(C), the linker mangles names:

$ nm ExtCallee.o | grep ExtCallee
0000000000000000 R _D9ExtCallee12__ModuleInfoZ
0000000000000000 W _D9ExtCalleeQkFZv

$ nm ExtMain.o | grep ExtCallee
                 U _D9ExtCalleeQkFZv


With extern(C), the linker does not mangle names:

$ nm ExtCallee.o | grep ExtCallee
0000000000000000 W ExtCallee
0000000000000000 R _D9ExtCallee12__ModuleInfoZ

$ nm ExtMain.o | grep ExtCallee
                 U ExtCallee


If not calling C code, why use extern(C) for D code?
scot