I have written a DLL that I load into a Python program. Everything works fine (DLL is loaded via ctypes, functions can be called and are executed). Only the string handling is giving me a bit of a headache. The string in D is always complete garbage. I have written Python modules in D (with C wrappers) before and got them to work. But the DLL in D seems to be a completely different beast. Does anyone have experience with it?

Python uses ctypes, e.g.

myDLL = CDLL("myDLL") / WinDLL("myDLL")
myDLL.printThis(c_char_p("Hello world")) / myDLL.printThis(create_string_buffer("Hello world"))

The D side looks like like this:

export void printThis(ref char[] str) {
   printf("%s\n", str); // prints "Hello World"
   writeln(str); // prints garbage
}

OR

export void printThis(char* str) {
    printf("%s\n", str); // prints garbage
    writeln(str); // prints garbage
}

What am I doing wrong / missing here? I guess it has something to do with the pointers.

Thanks!

prin

On Tuesday, 19 February 2013 at 16:23:45 UTC, Chris wrote:
> I have written a DLL that I load into a Python program. Everything works fine (DLL is loaded via ctypes, functions can be called and are executed). Only the string handling is giving me a bit of a headache. The string in D is always complete garbage. I have written Python modules in D (with C wrappers) before and got them to work. But the DLL in D seems to be a completely different beast. Does anyone have experience with it?
>
> Python uses ctypes, e.g.
>
> myDLL = CDLL("myDLL") / WinDLL("myDLL")
> myDLL.printThis(c_char_p("Hello world")) / myDLL.printThis(create_string_buffer("Hello world"))
>
> The D side looks like like this:
>
> export void printThis(ref char[] str) {
>    printf("%s\n", str); // prints "Hello World"
>    writeln(str); // prints garbage
> }
>
> OR
>
> export void printThis(char* str) {
>     printf("%s\n", str); // prints garbage
>     writeln(str); // prints garbage
> }
>
> What am I doing wrong / missing here? I guess it has something to do with the pointers.
>
> Thanks!
>
> prin

D doesn't use null termination for it's strings, strings are immutable(char)[]. You can form a D slice from a pointer by going
slice = ptr[0..length]
where length is the length of the array the pointer represents.
You can't just take a c style string and expect writeln to work with it.

Also, I think you should have extern(C) in the function definition.

> D doesn't use null termination for it's strings, strings are immutable(char)[]. You can form a D slice from a pointer by going
> slice = ptr[0..length]
> where length is the length of the array the pointer represents.
> You can't just take a c style string and expect writeln to work with it.

You can use std.conv.to to convert a C string to a D string.

On Tuesday, 19 February 2013 at 19:06:47 UTC, jerro wrote:
>> D doesn't use null termination for it's strings, strings are immutable(char)[]. You can form a D slice from a pointer by going
>> slice = ptr[0..length]
>> where length is the length of the array the pointer represents.
>> You can't just take a c style string and expect writeln to work with it.
>
> You can use std.conv.to to convert a C string to a D string.

Good to know.

If you do know the length beforehand though, slicing will O(1) as opposed to O(n)

On Tuesday, 19 February 2013 at 17:40:03 UTC, John Colvin wrote:
>
> D doesn't use null termination for it's strings, strings are immutable(char)[]. You can form a D slice from a pointer by going
> slice = ptr[0..length]
> where length is the length of the array the pointer represents.
> You can't just take a c style string and expect writeln to work with it.


I had tried to!string() and the slicing already but to no avail. I think it has something to do with pointer and the stack. Here is what I get

export void printThis(ref char[] str) {
	printf("Incoming printf: %s\n", str);
	writeln(str);
	writeln(str.ptr);
	writeln(str[0..5]);
	writeln(to!string(str.ptr));
	writeln(to!string(str.ptr[0..5]));
	writeln(to!string(str)); // Error!
}


OUTPUT:

Incoming printf: Hello world
☺
27FBCC
☺
☺
☺
Traceback (most recent call last):
  File "loaddll.py", line 6, in <module>
    lib.printThis(c_char_p("Hello world"))
WindowsError: exception: access violation reading 0x00284000

If I use writefln("%s\n", str) I get the full garbage which starts with

☺   ☺   0³' ░ï^ ♦   ☺       (←5     8¹' l¹' x ' x ' 1þ→↔A±ôj    ░³' .ö→↔☺◄  D0 ►
ô→↔ æ"☻­C←↔Jô→↔È1←↔ ß"☻¶)"☻    °;5     +Æ→↔á╚h P®h P®h `³'     ►   P³' 0³' ░ï^ p
³'     ☺   a↨»wxY#☻êS→↔D0 ►P®h         ☺◄      ░<$☻    á╚h ►Q→↔á╚h         xY#☻
   ░<$☻    D0 ►            \¬▲    P®h         ─²' ¬Ø♫▲á╚h P®h     á╚h ☺   ☺
  P®h EÑ♫▲á╚h ☺   ☺    æ"☻╚Ui Ó("☻hj] └Ui ╣¢♫▲Ô╚♫▲─²' ­½] ­½] └Ui   '  æ"☻Ó("☻
  fÉ"☻☺   4É"☻    ÞJh (←5     á*♥▲­½] └Ui     H▒        ▲─Ui ♥               p■]
     ıÏ♫▲êTi     ­½] ú/5   ' (Ô$☻+¯♫▲ê♀"☻ú/5     └Ui (←5 2┘♫▲­½] êTi ­&^

3◄▲­½] ­&^ ­&^ (Ô$☻Ó┌5 ED◄▲(Ô$☻­&^ ­&^ ☺   ú/5ttt®/5 ¤I◄tttú/5 ☺☺  ­&^ ­&^ ☺
 ' ☺ tttú/5 ▬☻nt­&^ ☺   ¨Rtttú/5 ☺     ' ðÚmttt§ç♥tttú/5 ☺     ' |3 ↔☺   ê '
 ú/5       ttt                              Âü☺           !☻  +e▲Q    ╚¬$Q    +e
▲Q    m◄ ↔☻   É/5 ÿ 5 í9┐w         Ó²~        X ' ×⌂P¢─ ' U▬ ↔±þÿj    ö ' ¬3╚v Ó
²~È ' ‗×┐w Ó²~M∟ºw         Ó²~            á '         ıq├w®&>     ý ' ┼×┐w¶‼ ↔ Ó
²~                ¶‼ ↔ Ó²~    Actx    ☺   ♀3  ▄               ¶   ☺      4   |☺
 ☺                       ☻   N´&→ÿ☻  D   Ó☻  `☻      ║q2¾@♣  J   î♣  ▲♥      [IY
  2      ♥      ═Û╬2Ó♂  B   $♀  6♥      ╚_P8\☼  ^   ╝☼  h♥      D♣(▒$‼  V   |‼
ÿ♥  ►              ý   ☻   ☺   |☺  ð§  ☺   ☻   L↨  á  ☺   ♥   ý▲  î♫  ☺   ♦   x-
  ¶♥  ☻   ♣   î0  ÿ   ☻   ♠   $1  ╠   ☻      ­1  ­   ☺             Ó2  (   ☻   ♂
 3  ♦   ☺   SsHd,   ☺   ☺   ☺   ♠   î   ☺   ÿ§  ,   ^   ^
                ☻   $   8       C : \ W i n d o w s \ W i n S x s \     N´&→∟☺
D   d☺  `☻  ☺   ║q2¾─♥  J   ►♦  ▲♥  ☻   [IY-0  2   d   ♥  ♥   ═Û╬2d
  B   ¿
  ^   @♫  h♥  ♣   D♣(▒¿◄  V    ↕  ÿ♥  ♠   M i c r o s o f t . W i n d o w s . S
y s t e m C o m p a t i b l e     l   ☺   ♀☺  ð☺  ☻   ,   ▄☻  º¹Lí$♦╩☺☺

ETC. ETC......

> Also, I think you should have extern(C) in the function definition.

I based the DLL on this how-to: http://dlang.org/dll.html

I tried extern (C) which has some advantages. However, it still doesn't produce the desired result (tried slicing too).

extern(C) {
  export void synthesize(ref char[] str) {
	printf("Incoming printf: %s\n", &str);
	writefln("writefln %s", &str);
	writefln("writefln %s", to!string(&str));
	writefln("writefln %s", to!string(str));
  }
}

OUTPUT:

Incoming printf: Hello from Python
writefln 2366A34
writefln 2366A34
Traceback (most recent call last):
  File "loaddll.py", line 4, in <module>
    lib.printThis(c_char_p("Hello from Python"))
WindowsError: [Error -532414463] Windows Error 0xE0440001

On Wednesday, 20 February 2013 at 12:48:53 UTC, Chris wrote:
> I tried extern (C) which has some advantages. However, it still doesn't produce the desired result (tried slicing too).
>
> extern(C) {
>   export void synthesize(ref char[] str) {
> 	printf("Incoming printf: %s\n", &str);
> 	writefln("writefln %s", &str);
> 	writefln("writefln %s", to!string(&str));
> 	writefln("writefln %s", to!string(str));
>   }
> }
>
> OUTPUT:
>
> Incoming printf: Hello from Python
> writefln 2366A34
> writefln 2366A34
> Traceback (most recent call last):
>   File "loaddll.py", line 4, in <module>
>     lib.printThis(c_char_p("Hello from Python"))
> WindowsError: [Error -532414463] Windows Error 0xE0440001

Your function is being called from Python, correct? Then in addition to the extern(C), the argument needs to be a char*, not a D array or a reference to one.

On Wednesday, 20 February 2013 at 14:05:40 UTC, Mike Parker wrote:
> Your function is being called from Python, correct? Then in addition to the extern(C), the argument needs to be a char*, not a D array or a reference to one.

Correct, and it works _now_*! The lines

printf("Incoming printf: %s\n", str);
writefln("writefln %s", to!string(str));

Print now:

Incoming printf: Hello from Python
writefln Hello from Python


So the correct signature is

extern (C) {export void printThis(char* str);}

Thanks you guys! I'm so glad I don't have to write a C-wrapper!

*("_now_" because I tried char* with extern (C) before as I would in my other Python modules, but it didn't work for some reason. Must have overlooked something. Mea culpa!).

Chris:

> extern (C) {export void printThis(char* str);}

Maybe the D wiki should contain info to save some time and experiments to people.

Possible alternative syntax:

extern(C) export void printThis(char* str);

Also, think if you want some const:

extern(C) export void printThis(const(char)* str);

Bye,
bearophile

On Wednesday, 20 February 2013 at 14:28:06 UTC, Chris wrote:
> On Wednesday, 20 February 2013 at 14:05:40 UTC, Mike Parker wrote:
>> Your function is being called from Python, correct? Then in addition to the extern(C), the argument needs to be a char*, not a D array or a reference to one.
>
> Correct, and it works _now_*! The lines
>
> printf("Incoming printf: %s\n", str);
> writefln("writefln %s", to!string(str));
>
> Print now:
>
> Incoming printf: Hello from Python
> writefln Hello from Python
>
>
> So the correct signature is
>
> extern (C) {export void printThis(char* str);}
>
> Thanks you guys! I'm so glad I don't have to write a C-wrapper!
>
> *("_now_" because I tried char* with extern (C) before as I would in my other Python modules, but it didn't work for some reason. Must have overlooked something. Mea culpa!).

yeah, although extern(C) doesn't have to have any braces.

I've never had to use export before, but then that could be a .so/.dylib vs .dll thing