function pointer usage in c

Question

#include<stdio.h>
struct file{
        int a;
        int b;
        int (*fp) (int ,int);
        };
static int sum(int a, int b)
{
        return(a+b);
}
void main()
{
        struct file var;


        int sum1=0;

        var.fp=&sum;
        sum1=fp(2,4);
        printf("\nsum is  %d ",sum1);
}

how to call the function..?? i am getting an error called as undefined reference to fp..???

I'd recommend removing the linux tag on this post since linux has nothing to do with this question. — David Weiser
– David Weiser, Commented Dec 24, 2010 at 21:49

Nikolai Fetissov · Accepted Answer · 2010-12-24 20:59:18Z

6

Since its a member of the structure you have to qualify it:

sum1 = var.fp( 2, 4 );

answered Dec 24, 2010 at 20:59

Nikolai Fetissov

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Comments

DigitalRoss · Accepted Answer · 2010-12-24 21:02:29Z

6

You meant to say sum1 = var.fp(...) or sum1 = (*var.fp)(...) but you typed fp(...). C implicitly defined an external fp() for you to call. The compiler has to do this in order to compile legacy C code.

Use cc -Wall ... to generate errors for missing forward declarations.

answered Dec 24, 2010 at 21:02

DigitalRoss

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Comments

David Weiser · Accepted Answer · 2010-12-24 20:59:40Z

2

Have you tried changing:

sum1=fp(2,4);

To:

sum1=var.fp(2,4);

?

answered Dec 24, 2010 at 20:59

David Weiser

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1 Comment

David Weiser Over a year ago

Good. Don't forget to mark the answer to the question as "accepted" (the green check mark). Doing so will make it more likely that others will answer the questions you ask in the future.

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