Passing a parameter in a callback in C [duplicate]

Question

Possible Duplicate:
How do function pointers in C work?

Surfing on stackoverflow I found this example:

/* Validation functions start */
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);
    ...
}

I was wondering, imagine getNextRandomValue had a parameter getNextRandomValue(int i), how would I include this and making the function accepting inputs?

Many thanks

aragaer · Accepted Answer · 2013-01-11 06:22:55Z

Common practice is to pass a pointer to "data" together with the function. When function gets called, pass that "data" pointer into function and assume that the function itself knows what to do with that data. In fact the data is usually a pointer to a structure. So the code looks like this:

struct func1_data {
    int a;
    int b;
};

struct func2_data {
    char x[10];
};

int function1(void *data) {
    struct func1_data *my_data = (typeof(my_data)) data;
    /* do something with my_data->a and my_data->b */
    return result;
}

int function2(void *data) {
    struct func2_data *my_data = (typeof(my_data)) data;
    /* do something with my_data->x */
    return result;
}

and assume we have

int caller(int (*callback), void *data) {
    return callback(data);
}

Then you call all this like this:

struct func1_data data1 = { 5, 7 };
struct func2_data data2 = { "hello!" };
caller(function1, (void *) &data1);
caller(function2, (void *) &data2);

Oliver Charlesworth · Accepted Answer · 2013-01-11 02:45:58Z

2

It's probably a good idea to get familiar with function-pointer syntax. You need to change the argument to int (*getNextValue)(int).

answered Jan 11, 2013 at 2:45

Oliver Charlesworth

274k34 gold badges591 silver badges687 bronze badges

7 Comments

piggyback Over a year ago

What if I don't know how many parameters need? What happens if sometimes I want to pass callbacks that have one values and others that have 3 ?

Oliver Charlesworth Over a year ago

@haskellguy: You must know. Otherwise how would you call it?

piggyback Over a year ago

consider I have a callback doThisOne(int) and doThisTwo(int, int)

Oliver Charlesworth Over a year ago

@haskellguy: I understand what you're describing, but in practice it doesn't make sense. How could you call a function if you didn't know how many arguments it took?

s.bandara Over a year ago

Variadic function pointer for @haskellguy at lemoda.net/c/function-pointer-ellipsis/index.html, although it may be somewhat esoteric.

|

Adeel Ahmed · Accepted Answer · 2013-01-11 05:59:09Z

2

Then your code should be like this...

void populate_array(int *array, size_t arraySize, int (*getNextValue)(unsigned int))
{
    unsigned int seedvalue = 100;

    for (size_t i=0; i<arraySize; i++)
        array[i] = getNextValue(seedvalue);
}

int getNextRandomValue(unsigned int seed)
{
    srand(seed);
    return rand();
}

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

answered Jan 11, 2013 at 5:59

Adeel Ahmed

1,6019 silver badges10 bronze badges

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