Thread: Pointers to function(function pointers)

dear freinds i have two problems.
1) Pointers to functions (Function Pointers)
i don't know what is the error in the following program....

#include<stdio.h>
#include<string.h>
void check(char *a, char *b, int(*cmp)());
main()
{

void check(char *a, char *b, int(*cmp)());
char s1[80],s2[80];
int (*p)();
p=strcmp;
gets(s1);
gets(s2);
check(s1,s2,p);
}

void check(char *a, char *b,int(*cmp)())
{
printf("testing for equality \n");
if (!(*cmp)(a,b))
printf("equal");
else
printf("not equal");
}

2) how to pass structures as an argument to a functiion e.g

intcal (struct {struct{ char name[10];
int num;
float amt;}bill)..
...
..
.....
etc..

> if (!(*cmp)(a,b))
Try

if ( cmp(a,b) == 0 )

> 2) how to pass structures as an argument to a functiion
You declare the structure outside the function prototype

Code:

struct foo { 
    char name[10];
};

void myfn ( struct foo var );

Hello,

For problem 1,

You declared a function pointer cmp like this

int (*cmp)()

which means that this function takes no arguments and just returns an int. What you should be doing infact is this,

int (*cmp)(char *, char *)

which says that cmp is a ptr-to-a-func that takes two char*'s as its arguments and returns an int value.


For problem 2,

C doesn't support passing structures like you specified in your code yet. You should create a variable which holds the required values and then pass 1. either the structure itself, or 2. a pointer to it.

for ex:

typedef struct {
int x,
y;
} point;

void print(point *p) {
printf("x is %d, y is %d", p->x, p->y);
}

int main() {
point p = {1, 2};
print( &p );

return 0;
}

> which means that this function takes no arguments
Actually, it means that the arguments are unspecified

func() is the equivalent of func(...) in C
func() is the equivalent of func(void) in C++

But it would be better written as you suggest anyway


> C doesn't support passing structures like you specified in your code yet
ANSI-C has allowed structures to be passed as parameters for over a decade now.

Hello all,


The big-wigs should excuse this ignorant soul, I thought one couldn't pass structures like this

Code:

void SomeFn( struct { int a;} b ) { //code }

I think I also made another mistake in my earlier post. The variable "cmp" was declared like this

Code:

int (*cmp)(char *, char *) = strcmp;

Instead it should be

Code:

int (*cmp)(const char *, const char *) = strcmp;

This I think should work just fine.