.\" Copyright (c) 2007-2014 Daniel Adler <dadler AT uni-goettingen DOT de>, 
.\"                         Tassilo Philipp <tphilipp AT potion-studios DOT com>
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.Dd $Mdocdate$
.Dt dyncallback 3
.Sh NAME
.Nm dyncallback
.Nd callback interface of dyncall
.Sh SYNOPSIS
.In dyncall_callback.h
.Ft DCCallback *
.Fn dcbNewCallback "const char * signature" "DCCallbackHandler * funcptr" "void * userdata"
.Ft void
.Fn dcbInitCallback "DCCallback * pcb" "const char * signature" "DCCallbackHandler * funcptr" "void * userdata"
.Ft void
.Fn dcbFreeCallback "DCCallback * pcb"
.Ft void
.Fn dcbGetUserData "DCCallback * pcb"
.Sh DESCRIPTION
The
.Nm
dyncall library has an interface to create callback objects, that can be passed
to functions as callback arguments. In other words, a pointer to the callback
object can be "called", directly. The callback handler then allows iterating
dynamically over the arguments once called back.
.Pp
.Fn dcbNewCallback
creates a new callback object, where
.Ar signature
is a signature string describing the function to be called back (see manual for
format). This is needed for
.Nm
dyncallback to correctly prepare the arguments passed in by the function that
calls the callback handler. Note that the handler doesn't return the value
specified in the signature, directly, but simply 'i' or 'f' depending on whether
it is a integral or floating point type. The return value itself is stored
where the handler's 3rd parameter points to (see example).
.Ar funcptr
is a pointer to the
.Nm
dyncallback callback handler (see below), and
.Ar userdata
a pointer to arbitrary user data you want to use in the callback handler.
Use the returned pointer as callback argument in functions requiring a callback
function pointer.
.Pp
.Fn dcbInitCallback
(re)initialize the callback object.
.Pp
.Fn dcbFreeCallback
destroys and frees the callback handler.
.Pp
.Fn dcbGetUserData
returns a pointer to the userdata passed to the callback object on creation or
initialization.
.Pp
Declaration of a dyncallback handler (following function pointer definition in
dyncallback/dyncall_callback.h):
.Bd -literal -offset indent
char cbHandler(DCCallback* cb,
               DCArgs*     args,
               DCValue*    result,
               void*       userdata);
.Ed
.Pp
.Ar cb is a pointer to the DCCallback object in use
.Nm
result is a pointer to a DCValue object in order to store the callback's
return value (output, to be set by handler). Finally,
.Ar userdata is a pointer to some user defined data that can be
set when creating the callback object.
The handler itself returns a signature character (see manual for format)
specifying the data type used for
.Ar result .
.Sh EXAMPLE
Let's say, we want to create a callback object and call it. For simplicity, this
example will omit passing it as a function pointer to a function (e.g. compar
in qsort(), etc.) and demonstrate calling it, directly. First, we need to define
our callback handler - the following handler illustrates how to access the passed-
in arguments:
.Bd -literal -offset indent
char cbHandler(DCCallback* cb,
               DCArgs*     args,
               DCValue*    result,
               void*       userdata)
{
  int* ud = (int*)userdata;
  int       arg1 = dcbArgInt     (args);
  float     arg2 = dcbArgFloat   (args);
  short     arg3 = dcbArgShort   (args);
  double    arg4 = dcbArgDouble  (args);
  long long arg5 = dcbArgLongLong(args);

  // .. do something ..

  result->s = 1244;
  return 'i';
}
.Ed
.Pp
Note that the return value of the handler is a signature character, not the
actual return value, itself, and note that the actual return value is of type
short.
Now, let's call it through a DCCallback object:
.Bd -literal -offset indent
  DCCallback* cb;
  short result = 0;
  int userdata = 1337;
  cb = dcbNewCallback("ifsdl)s", &cbHandler, &userdata);
  result = ((short(*)(int, float, short, double, long long))cb)
    (123, 23.f, 3, 1.82, 9909ll);
  dcbFreeCallback(cb);
.Ed
.Sh SEE ALSO
.Xr dyncall 3 ,
.Xr dynload 3
and the dyncall manual (available in PDF format) for a way more detailed documentation of this
library.
.Sh AUTHORS
.An "Daniel Adler" Aq dadler@uni-goettingen.de
.An "Tassilo Philipp" Aq tphilipp@potion-studios.com