All the functions in c are globally defined, then why we need to pass them to function as argument?

Question

In c, we have to define all the functions globally. while studying function pointer i got some program where programmer passes function name as parameter to other function. so why we need to pass function to other function if they all are globally defined?

here i am giving small sample program :

#include<stdio.h>

void bsort(int arr[],int n,int (*compare)(int,int))    //bubble sort
{
    int i,j,temp;
    for(i=0;i<n;i++){
        for(j=0;j<n-i-1;j++){
            if(compare(arr[j],arr[j+1]) > 0 ){
                temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
    }
}

int compare(int a,int b)
{
    if(a > b) return 1;
    return -1;
}

void main()
{
    int i;
    int arr[5]={6,5,1,9,2};
    bsort(arr,5,compare);

    for(i=0;i<5;i++)
    printf("%d ",arr[i]);   
}

in this code, if we remove 3rd argument in definition and calling part of bsort function then also our program will give us same output.so for function pointer this program doesn't make sense. can you please do some modification in this code and make it good example for function pointer.

Thanks.

Answer 1

Your code does not actual need passing functions as parameters. But your example is rather didactic to make you understand how function pointers work.

However it's important to understand them as they might become very useful especially when dealing with libraries loaded at runtime. There are tons of examples where function pointers are really a good tool.

Here's one:

#include <stdlib.h>
#include <stdio.h>
#include <dlfcn.h>

double doOperation(double arg, double (*func)(double))
{
    return (*func)(arg);
}

int main(int argc, char **argv) {
    void *handle;
    double (*cosine)(double);
    char *error;

    handle = dlopen ("/lib/libm.so.6", RTLD_LAZY);
    if (!handle) {
        fputs (dlerror(), stderr);
        exit(1);
    }

    cosine = dlsym(handle, "cos");
    if ((error = dlerror()) != NULL)  {
        fputs(error, stderr);
        exit(1);
    }

    printf ("%f\n", doOperation(2.0, cosine);
    dlclose(handle);
}

You open a library, search for the cosine function, and then pass it as an argument to doOperation.

You can also look here for more info.

Answer 2

Function pointers are used when you might need to change the behavior of your function call depending on some dynamic requirement.

This page sums it up short and sweet:

A function pointer is a variable that stores the address of a function that can later be called through that function pointer. This is useful because functions encapsulate behavior. For instance, every time you need a particular behavior such as drawing a line, instead of writing out a bunch of code, all you need to do is call the function. But sometimes you would like to choose different behaviors at different times in essentially the same piece of code.

A few excellent examples are available there are well.

Answer 3

You pass a function [pointer] to another function because you want to dynamically direct which function the receiver calls. Or more precisely, because the receiver function is built to allow you to do so, thereby requiring you to do so. The pointed-to function does not need to be available at the time the receiver is compiled.

The comparison function required as a qsort() argument is the prototypical example.

Answer 4

Q: Why pass a function to another function?

Without re-writing bsort(), code can sort more than 1 way by only changing the compare function.

#include <stdio.h>
#include <stdarg.h>
int compare_up(int a,int b) {
    if(a > b) return 1;
    return -1;
}

int compare_down(int a,int b) {
    if(a < b) return 1; // reverse the compare
    return -1;
}

int compare_random(int a,int b) {
   return rand() & 1;  // mix them up
}

int main(void) {
    int i;
    int arr[5]={6,5,1,9,2};

    bsort(arr,5,compare_up);
    for(i=0;i<5;i++) printf("%d ",arr[i]); puts("");

    bsort(arr,5,compare_down);
    for(i=0;i<5;i++) printf("%d ",arr[i]); puts("");

    bsort(arr,5,compare_random);
    for(i=0;i<5;i++) printf("%d ",arr[i]); puts("");

    return 0;
}