DYNCALLBACK(3)

okay bestie, welcome to dyncallback(3) — the callback interface of dyncall, and honestly? iconic. 💖 This is where you learn how to make callback objects you can pass to functions as callback function pointers, then iterate dynamically over the arguments once the callback gets called back. lowkey magical. ✨

🌸 NAME 🌸

dyncallback — callback interface of dyncall.

🦋 SYNOPSIS 🦋

here's the whole cast, exactly as declared, no edits bestie:

#include <dyncall_callback.h>

typedef DCsigchar
(DCCallbackHandler)(DCCallback* pcb, DCArgs* args, DCValue* result,
    void* userdata);

DCCallback *
dcbNewCallback(const DCsigchar * signature, DCCallbackHandler * funcptr,
    void * userdata);

DCCallback *
dcbNewCallback2(const DCsigchar * signature,
    DCCallbackHandler * funcptr, void * userdata,
    DCaggr *const * aggrs);

void
dcbInitCallback(DCCallback * pcb, const DCsigchar * signature,
    DCCallbackHandler * funcptr, void * userdata);

void
dcbInitCallback2(DCCallback * pcb, const DCsigchar * signature,
    DCCallbackHandler * funcptr, void * userdata,
    DCaggr *const * aggrs);

void
dcbFreeCallback(DCCallback * pcb);

void
dcbGetUserData(DCCallback * pcb);

DCbool
dcbArgBool(DCArgs * p);

DCchar
dcbArgChar(DCArgs * p);

DCshort
dcbArgShort(DCArgs * p);

DCint
dcbArgInt(DCArgs * p);

DClong
dcbArgLong(DCArgs * p);

DClonglong
dcbArgLongLong(DCArgs * p);

DCuchar
dcbArgUChar(DCArgs * p);

DCushort
dcbArgUShort(DCArgs * p);

DCuint
dcbArgUInt(DCArgs * p);

DCulong
dcbArgULong(DCArgs * p);

DCulonglong
dcbArgULongLong(DCArgs * p);

DCfloat
dcbArgFloat(DCArgs * p);

DCdouble
dcbArgDouble(DCArgs * p);

DCpointer
dcbArgPointer(DCArgs * p);

DCpointer
dcbArgAggr(DCArgs * p, DCpointer target);

void
dcbReturnAggr(DCArgs * args, DCValue * result, DCpointer ret);

🎀 DESCRIPTION 🎀

The dyncallback dyncall library has an interface to create callback objects that can be passed to functions as callback function pointers. In other words, a pointer to the callback object can be “called”, directly. A generic callback handler invoked by this object then allows iterating dynamically over the arguments once called back. 💅

dcbNewCallback2() 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), and funcptr is a pointer to a generic callback handler (see below). The signature is needed in the generic callback handler to correctly retrieve the arguments provided by the caller of the callback. Note that the generic handler's function type/declaration is always the same for any callback. userdata is a pointer to arbitrary user data to be available in the generic callback handler. If the callback expects aggregates (struct, union) to be passed or returned by value, a pointer to an array of DCaggr* descriptions must be provided (exactly one per aggregate, in the same order as in the signature) via the aggrs parameter, otherwise pass NULL. This pointer must point to valid data during callback. 🫶

dcbNewCallback() is the same as dcbNewCallback2(), with an implicit NULL passed via the aggrs parameter, meaning it can only be used for callbacks that do not use any aggregate by value.

⚠️ NOTE: C++ non-trivial aggregates (check with the std::is_trivial type trait) do not use aggregate descriptions, so the respective pointers in the provided array must be NULL. See dyncall(3) for more information on C++ non-trivial aggregates. 💀

Use the pointer returned by dcbNewCallback*() as argument in functions requiring a callback function pointer.

dcbInitCallback() and dcbInitCallback2() (re)initialize the callback object. For a description of their parameters, see dcbNewCallback*().

dcbFreeCallback() destroys and frees the callback handler.

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

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

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

cb is a pointer to the DCCallback object in use, args is to be used with the dcbArg*() functions to iterate over the arguments passed to the callback, and result is a pointer to an object used to store the callback's return value (output, to be set by the handler). Finally, userdata is the user defined data pointer set when creating or (re)initializing the callback object. The handler itself must return a signature character (see manual or dyncall_signature.h for format) specifying the data type of result. ✨

Retrieving aggregates by value from the generic handler's args argument can be done via dcbArgAggr(), where target must point to memory large enough for the aggregate to be copied to, iff the aggregate is trivial (see below for non-trivial C++ aggregates), in which case target is returned.

To return a trivial aggregate by value, a helper function dcbReturnAggr() needs to be used in order to correctly place the aggregate pointed to by ret into result, then let the generic handler return DC_SIGCHAR_AGGREGATE.

Retrieving or returning C++ non-trivial aggregates (check with the std::is_trivial type trait) is done differently, as dyncall cannot know how to do this copy and the C++ ABI handles those differently:

• When retrieving a C++ non-trivial aggregate via dcbArgAggr(), target is ignored, and a pointer to the non-trivial aggregate is returned (the user should then do a local copy).
• To return a C++ non-trivial aggregate by value via dcbReturnAggr(), pass NULL for ret, which will make result->p point to (implicit, caller-provided) memory where the aggregate should be copied to. 🫶

✨ EXAMPLES ✨

okay bestie, examples time 💖 quick heads-up before we dive in: for simplicity, none of the examples below do any error checking. Also, none of them pass the callback object pointer as an argument to a function doing the respective callback (e.g. compar in qsort(3), etc.), but demonstrate calling it, directly, for clarity. no cap, that's just the page's whole vibe ✨

Let's say we want to create a callback object and call it. First, we need to define our callback handler — the following handler illustrates how to access the passed-in arguments, optional userdata, and how to return values:

             DCsigchar 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, and 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);

             /* call the callback object */
             result = ((short(*)(int, float, short, double, long long))cb)
               (123, 23.f, 3, 1.82, 9909ll);

             dcbFreeCallback(cb);

🧸 C/trivial aggregates by-value 🧸

Onto an example calling back a function which takes an aggregate by value (note that this is only available on platforms where macro DC__Feature_AggrByVal is defined). E.g. with the following function f() and struct S:

             struct S { char x[3]; double y; };
             int f(struct S, float);

the callback handler would look like:

             DCsigchar cbHandler(DCCallback* cb,
                                 DCArgs*     args,
                                 DCValue*    result,
                                 void*       userdata)
             {
               struct S arg1;
               float arg2;
               dcbArgAggr(args, (DCpointer)&arg1);
               arg2 = dcbArgFloat(args);

               /* ... */

               result->i = 1;
               return 'i';
             }

and the callback object as well as the aggregate field/layout description are set up (and the former called back) as follows:

             struct S s = { { 56, -23, 0 }, -6.28 };
             int result;

             DCCallback* cb;

             DCaggr *a = dcNewAggr(2, sizeof(struct S));
             dcAggrField(a, DC_SIGCHAR_CHAR,   offsetof(struct S, x), 3);
             dcAggrField(a, DC_SIGCHAR_DOUBLE, offsetof(struct S, y), 1);
             dcCloseAggr(a);

             /* an array of DCaggr* must be passed as last arg, with one
              * entry per 'A' signature character; we got only one, here
              */
             cb = dcbNewCallback2("Af)v", &cbHandler, NULL, &a);

             /* call the callback object */
             result = ((int(*)(struct S, float))cb)(s, 42.f);

             dcbFreeCallback(cb);
             dcFreeAggr(a);

Let's extend the last example, so that the callback function also returns struct S by value. The struct definition, function declaration and handler definition would be:

             /* callback function decl */
             struct S f(struct S, float);

             struct S { char x[3]; double y; };

             DCsigchar cbHandler(DCCallback* cb,
                                 DCArgs*     args,
                                 DCValue*    result,
                                 void*       userdata)
             {
               struct S arg1, r;
               float arg2;
               dcbArgAggr(args, (DCpointer)&arg1);
               arg2 = dcbArgFloat(args);

               /* ... */

               /* use helper to write aggregate return value to result */
               dcbReturnAggr(args, result, (DCpointer)&r);
               return 'A';
             }

and the callback object as well as the aggregate field/layout descriptions are set up (and the former called back) as follows:

             struct S s = { { 33, 29, -1 }, 6.8 };
             struct S result;

             DCCallback* cb;

             DCaggr *a = { dcNewAggr(2, sizeof(struct S)) };
             dcAggrField(a, DC_SIGCHAR_CHAR,   offsetof(struct S, x), 3);
             dcAggrField(a, DC_SIGCHAR_DOUBLE, offsetof(struct S, y), 1);
             dcCloseAggr(a);

             /* an array of DCaggr* must be passed as last arg, with one
              * entry per 'A' signature character
              */
             cb = dcbNewCallback2("Af)A", &cbHandler, NULL, (DCaggr*[2]){a,a});

             /* call the callback object */
             result = ((struct S(*)(struct S, float))cb)(s, 42.f);

             dcbFreeCallback(cb);
             dcFreeAggr(a);

💅 C++ 💅

In our next example, let's look at setting up a DCCallback object to call back a simple C++ method (illustrating the need to specify the thiscall calling convention). If the class and method is declared as:

             class Klass {
             public:
                     virtual void Method(float, int);
             };

the respective callback handler would be something along the lines of:

             DCsigchar cbHandler(DCCallback* cb,
                                 DCArgs*     args,
                                 DCValue*    result,
                                 void*       userdata)
             {
               Klass*    thisptr = (Klass*)dcbArgPointer(args);
               float     arg1 = dcbArgFloat(args);
               int       arg2 = dcbArgInt(args);

               /* ... */

               return 'v';
             }

and the callback object would be used as follows:

             DCCallback* cb;
             cb = dcbNewCallback("_*pfi)v", &cbHandler, NULL);

             /* HACK: this is a hack just for this example to force the compiler
              * generating a thiscall, below (creates a fake vtable mimicking
              * Klass, setting all of its possible entries to our callback handler;
              */
             DCpointer fakeClass[sizeof(Klass)/sizeof(DCpointer)];
             for(int j=0; j<sizeof(Klass)/sizeof(DCpointer); ++j)
                     fakeClass[j] = &cb;

             /* (this)call the callback object */
             ((Klass*)&fakeClass)->Method(8, 23.f);

             dcbFreeCallback(cb);

NOTE: In a real world scenario one would figure out the precise location of the vtable entry of Klass::Method(), of course; the above example omits this for simplicity.

🌈 CONFORMING TO 🌈

okay so 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. periodt ✨

🦋 SEE ALSO 🦋

dyncall(3), dynload(3) and the dyncall manual (available in HTML and PDF format) for more information. that's the whole see-also vibe 🫶

👑 AUTHORS 👑

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

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mwah 💋 that's a wrap on dyncallback(3), bestie. The footer's source/version + date energy is $Mdocdate$ and dyncallback(3) — go be iconic 🎀