What is a "callback" in C and how are they implemented?

From the analysis that I have actually done, Core Audio counts greatly on callbacks (and also C+npls, yet that is an additional tale).

I recognize the principle (type of) of establishing a function that is called by an additional function repetitively to complete a job. I simply do not recognize just how they ready up and also just how they in fact function. Any kind of instances would certainly be valued.

174
2022-07-25 10:36:16
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Answers: 5

Here is an example of callbacks in C.

Let's say you want to write some code that allows registering callbacks to be called when some event occurs.

First define the type of function used for the callback:

typedef void (*event_cb_t)(const struct event *evt, void *userdata);

Now, define a function that is used to register a callback:

int event_cb_register(event_cb_t cb, void *userdata);

This is what code would look like that registers a callback:

static void my_event_cb(const struct event *evt, void *data)
{
    /* do stuff and things with the event */
}

...
   event_cb_register(my_event_cb, &my_custom_data);
...

In the internals of the event dispatcher, the callback may be stored in a struct that looks something like this:

struct event_cb {
    event_cb_t cb;
    void *data;
};

This is what the code looks like that executes a callback.

struct event_cb *callback;

...

/* Get the event_cb that you want to execute */

callback->cb(event, callback->data);
132
2022-07-25 21:39:00
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Callbacks in C are generally applied making use of function reminders and also a linked information reminder. You pass your function on_event() and also information reminders to a structure function watch_events() (as an example). When an occasion takes place, your function is called with your information and also some occasion - details information.

Callbacks are additionally made use of in GUI shows. The GTK+ tutorial has a wonderful area on the theory of signals and callbacks.

4
2022-07-25 13:10:40
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Usually this can be done by utilizing a function reminder, that is an unique variable that indicates the memory area of a function. You can after that utilize this to call the function with details debates. So there will possibly be a function that establishes the callback function. This will certainly approve a function reminder and afterwards store that resolve someplace where it can be made use of. Afterwards when the defined occasion is caused, it will certainly call that function.

0
2022-07-25 13:10:37
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This wikipedia article has an instance in C.

An example is that new components contacted boost the Apache Web web server register with the major apache procedure by passing them function reminders so those features are recalled to procedure website demands.

3
2022-07-25 13:10:28
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There is no "callback" in C - not greater than any kind of various other common shows principle.

They are applied making use of function reminders. Below is an example:

void populate_array(int *array, size_t arraySize, int (*getNextValue)(void))
{
    for (size_t i=0; i<arraySize; i++)
        array[i] = getNextValue();
}

int getNextRandomValue(void)
{
    return rand();
}

int main(void)
{
    int myarray[10];
    populate_array(myarray, 10, getNextRandomValue);
    ...
}

Here, the populate_array function takes a function reminder as its 3rd parameter, and also calls it to get the values to inhabit the array with. We've created the callback getNextRandomValue, which returns a random - ish value, and also passed a reminder to it to populate_array. populate_array will certainly call our callback function 10 times and also assign the returned values to the components in the offered array.

233
2022-07-25 13:07:50
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