dyncallback(3)
NAME
dyncallback – callback interface of dyncall
SYNOPSIS
#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
Note: 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.
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
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⟩
December 6, 2022
