CODEWITHSUNDEEP
cfunction-pointersansi-c
nick_name
nick_name

Reputation: 1373

ANSI C - Functions as Arguments to Other Functions

I've got these prototypes:

int *my_func(int x, void (*other_func)(int a, int b));

int func2(int val1, int val2);

Assume there is a function written that matches it.

If I want to actually call my_func, how would I do it? I've tried the following with no luck:

my_func(1,func2);

Here's the error:

warning: passing argument 2 of ‘my_func’ from incompatible pointer type

Upvotes: 0

Views: 2494

Answers (3)

paxdiablo
paxdiablo

Reputation: 881503

Your original code had void* for the first argument of the passed function and a return type of int, and that was causing your issue:

int *my_func(int x, void (*other_func)(void * a, int b)) { return 0;}
int func2(int val1, int val2) {}
int main (void) {
    my_func (42, func2);
    return 0;
}

With the following edits, changing it to an int and returning void, there is no issue:

int my_func(int x, void (*other_func)(int a, int b)) { return 0;}
void func2(int val1, int val2) {}
int main (void) {
    my_func (42, func2);
    return 0;
}

Compiling the former gives you:

warning: passing argument 2 of 'my_func' from incompatible pointer type

Compiling the latter gives you no errors or warnings and the object file is created fine.

Upvotes: 2

David Cary
David Cary

Reputation: 5490

Often(1)(2) we use "typedef" to make sure that the kind of function we have matches the kind of function expected as an argument to the other function.

typedef void (*pt2Func)(void *, int *);

int *my_func(int x, pt2Func other_func);

pt2Func func3;

int main(void){
    my_func(1,func3);
}

which means the same thing as

int *my_func(int x, void (*other_func)(void *a, int *b));

void func3(void *val1, int *val2);

int main(void){
    my_func(1,func3);
}

Upvotes: 0

Mysticial
Mysticial

Reputation: 471229

That's because the function prototypes don't match:

void (*other_func)(void *a, int *b)

is not the same as:

int func2(int val1, int val2);

One takes a void* and int*, while the other takes two ints.

EDIT:

Also, the return type doesn't match.

EDIT 2:

Since you've fixed both errors, this answer is out of context. I just tested it with these two fixes, and it compiles:

int *my_func(int x, void (*other_func)(int a, int b)){
    return 0;
}
void func2(int val1, int val2){
    printf("blah");
}


int main(){
    my_func(1,func2);

    return 0;
}

Upvotes: 5

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