Callbacks in D as void functions
Hi all,
I already posted this to Code Project, but maybe you have a
solution for my problem.
I am trying to write a wrapper for a C API in D. In C I have the
following definition of a callback type:
typedef void (*Callback)(void*);
In a struct all members are of the type Callback. In the main
function each member is assigned a function like this:
Events.OnData = MyDtaCB;
Here the MyDtaCB function has no prototype but is defined as
follows:
void MyDtaCB(void* pVoid){
// Do stuff
}
Now I tried to port this to D. The wiki says that callback
functions should be declared as integer functions, so my D code
looks like this:
extern (C) alias CEventCallback_t = int function(int, int);
extern (C) CEventCallback_t MyDtaCB(void*);
Here's the catch: when I try to assign the MyDtaCB function to a
member of the Events struct, I get a compilation error because
MyDtaCB is a void function and therefore returns no value. Seems
to work in C, though, but I don't know how to handle this in D.
The D tutorial has a function called getCallback() which returns
the void function but itself has the Callback type as return
value. However, in D this would lead to the same error as before
since the void function has no non-empty return value.
Any suggestions how to accomplish this? My thought now was to
screw the function definitions of the C API and implement them in
D. But what's the point in having the API then?
Thanks in advance.
Re: Callbacks in D as void functions
Wsdes: The wiki says that callback functions should be declared as integer functions, What's bad in returning void? Bye, bearophile
Re: Callbacks in D as void functions
On 11/13/14 9:49 AM, Wsdes wrote:
Hi all,
I already posted this to Code Project, but maybe you have a
solution for my problem.
I am trying to write a wrapper for a C API in D. In C I have the
following definition of a callback type:
typedef void (*Callback)(void*);
in D, this would be:
alias Callback = void function(void *);
In a struct all members are of the type Callback. In the main
function each member is assigned a function like this:
Events.OnData = MyDtaCB;
Here the MyDtaCB function has no prototype but is defined as
follows:
void MyDtaCB(void* pVoid){
// Do stuff
}
Now I tried to port this to D. The wiki says that callback
functions should be declared as integer functions, so my D code
looks like this:
extern (C) alias CEventCallback_t = int function(int, int);
extern (C) CEventCallback_t MyDtaCB(void*);
This doesn't look right. MyDtaCB is a function taking void *, that
returns a function pointer.
Can you point at the wiki you are talking about?
Here's the catch: when I try to assign the MyDtaCB function to a
member of the Events struct, I get a compilation error because
MyDtaCB is a void function and therefore returns no value. Seems
to work in C, though, but I don't know how to handle this in D.
C is a lot looser in function pointer types.
Without more context of what you are doing, it's hard to diagnose this
any further.
-Steve
Re: Callbacks in D as void functions
On Thursday, 13 November 2014 at 14:50:00 UTC, Wsdes wrote:
I am trying to write a wrapper for a C API in D. In C I have the
following definition of a callback type:
typedef void (*Callback)(void*);
I would translate this directly to D:
extern(C) alias Callback = void function(void*);
Here the MyDtaCB function has no prototype but is defined as
follows:
void MyDtaCB(void* pVoid){
// Do stuff
}
And don't forget extern(C) on this too:
extern(C) void MyDtaCB(void* pVoid) {
}
And assign it to the struct:
Events.OnData = MyDtaCB;
Unless you have a link to the wiki that talks about ints, maybe
that says something different, but I like to keep my C and D code
that calls it looking pretty much the same when I can.
Re: Callbacks in D as void functions
On Thursday, 13 November 2014 at 15:17:45 UTC, Adam D. Ruppe
wrote:
On Thursday, 13 November 2014 at 14:50:00 UTC, Wsdes wrote:
I am trying to write a wrapper for a C API in D. In C I have
the
following definition of a callback type:
typedef void (*Callback)(void*);
I would translate this directly to D:
extern(C) alias Callback = void function(void*);
Here the MyDtaCB function has no prototype but is defined as
follows:
void MyDtaCB(void* pVoid){
// Do stuff
}
And don't forget extern(C) on this too:
extern(C) void MyDtaCB(void* pVoid) {
}
And assign it to the struct:
Events.OnData = MyDtaCB;
Unless you have a link to the wiki that talks about ints, maybe
that says something different, but I like to keep my C and D
code that calls it looking pretty much the same when I can.
Hi,
thank you everybody for your replies.
First of all, the link to the wiki that has an example of
callbacks in C and D:
http://dlang.org/interfaceToC.html
Secondly, I tried your code and that was exactly what I was
thinking and what I tried before. Then I thought I'd turn to the
wiki example, so that's where the int function came from. In the
meantime I changed that to return void so I gave you my old code
:(
Anyway, I think I got the problem solved. Well, there seems to
never have been any problem as I am taught now. I asked the
developer of the C API this morning if I should try to implement
the callback functions redundantly in D and he said he will have
a look with me later. So now it turns out that I cannot call the
extern callback function because it's not provided within the
library O.O I was already wondering why there are no prototypes
to these callback functions but I assumed they are provided from
another library that I don't have direct access to...
So the solution to my problem finally is to define the function
in my D file and then cast the function pointer to the Callback
type like this:
void MyDtaCB(void* v){
// Do stuff
}
Events.OnData = cast(Callback) MyDtaCB;
At least this one compiles. Now I'm facing other problems with
struct members but I guess I'll figure this out. If not, I'll ask
again so stay tuned for some more unsolved mysteries ;)
Still, thank you all for your effort and time. Keep it up! :)
Re: Callbacks in D as void functions
On Thursday, 13 November 2014 at 15:58:34 UTC, Wsdes wrote:
void MyDtaCB(void* v){
// Do stuff
}
Events.OnData = cast(Callback) MyDtaCB;
this compiles but might crash, the extern(C) is important on a
callback to be used from a C program. I'd really recommend adding
that to the function, even if you cast when assigning to the
struct.
Re: Callbacks in D as void functions
On 11/13/14 10:58 AM, Wsdes wrote: First of all, the link to the wiki that has an example of callbacks in C and D: http://dlang.org/interfaceToC.html Secondly, I tried your code and that was exactly what I was thinking and what I tried before. Then I thought I'd turn to the wiki example, so that's where the int function came from. In the meantime I changed that to return void so I gave you my old code :( The int example in the above wiki page (not technically a wiki, but that's OK) is because the function type in C is ALSO taking ints :) Not all callback functions are the same. Your callback type in D should be the same as the C definition, but with extern(C). Can you tell us the exact prototype of the function you are trying to assign, whether it is a member of a struct, class, or is an inner function, and the exact prototype you declared the Callback type to be? I don't need to see any code of the function. -Steve
Re: Callbacks in D as void functions
On 11/14/2014 12:58 AM, Wsdes wrote:
Anyway, I think I got the problem solved. Well, there seems to never
have been any problem as I am taught now. I asked the developer of the C
API this morning if I should try to implement the callback functions
redundantly in D and he said he will have a look with me later. So now
it turns out that I cannot call the extern callback function because
it's not provided within the library O.O I was already wondering why
there are no prototypes to these callback functions but I assumed they
are provided from another library that I don't have direct access to...
Callbacks wouldn't be implemented in the library. They are intended to
be implemented by the user of the library. That's why they're called
callbacks. The library calls back your function. In C, this is how
event programming is usually handled. You pass a function pointer to the
library and when some event occurs, the library calls your function (the
callback).
So the solution to my problem finally is to define the function in my D
file and then cast the function pointer to the Callback type like this:
void MyDtaCB(void* v){
// Do stuff
}
Events.OnData = cast(Callback) MyDtaCB;
Define it as extern( C ) and you don't need the cast.
extern( C ) MyDtaCB( void* v ) { ... }
Events.OnData = MyDtaCB;
Callbacks intended to be passed to C should *always* be implemented as
extern( C ).
