dyncallback(3)

NAME
dyncallback - callback interface of dyncall

SYNOPSIS
#include <dyncall_callback.h>
DCCallback *
dcbNewCallback(const char * signature, DCCallbackHandler * funcptr, void * userdata);
void
dcbInitCallback(DCCallback * pcb, const char * signature, DCCallbackHandler * funcptr, void * userdata);
void
dcbFreeCallback(DCCallback * pcb);
void
dcbGetUserData(DCCallback * pcb);

DESCRIPTION
The dyncallback 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.

dcbNewCallback() creates a new callback object, where signature is a signature string describing the function to be called back (see manual or dyncall_signature.h for format). This is needed for dyncallback 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 a signature character, specifying the return value's type. The return value itself is stored where the handler's 3rd parameter points to (see below). funcptr is a pointer to the dyncallback dyncallback callback handler (see below), and 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.

dcbInitCallback() (re)initialize the callback object.

dcbFreeCallback() destroys and frees the callback handler.

dcbGetUserData() returns a pointer to the userdata passed to the callback object on creation or initialization.

Declaration of a dyncallback handler (following function pointer definition in dyncallback/dyncall_callback.h):

    char cbHandler(DCCallback* cb,
                   DCArgs*     args,
                   DCValue*    result,
                   void*       userdata);

cb is a pointer to the DCCallback object in use dyncallback result is a pointer to a DCValue object in order to store the callback's return value (output, to be set by handler). Finally, 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 result.

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:

    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 's';
    }

Note that the return value of the handler is a signature character, not the actual return value, itself. Now, let's call it through a DCCallback object:

      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);

CONFORMING TO
The dyncallback library needs at least a c99 compiler with additional support for anonymous structs/unions (which were introduced officially in c11). Given that those are generally supported by pretty much all major c99 conforming compilers (as default extension), it should build fine with a c99 toolchain. Strictly speaking, dyncall conforms to c11, though.

SEE ALSO
dyncall(3), dynload(3) and the dyncall manual (available in HTML and PDF format) for more information.

AUTHORS
Daniel Adler <dadler@uni-goettingen.de> Tassilo Philipp <tphilipp@potion-studios.com>