why this program is crashing

[James]

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void))
{
t1.i=i;
t1.f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

Thanks

[Daniel Etzold]

This works:

[...]

int construct(struct type* t1,int i,void (*f)(void))
{
t1->i=i;
t1->f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(&t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

You have to call "construct" with the address of the struct.

Regards,
Daniel

James wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void))
{
t1.i=i;
t1.f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

Thanks

[Artie Gold]

James wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void)) You realize, of course, that `t1' is being passed by value? Any change
you make to `t1' only affects the local copy. {
t1.i=i;
t1.f=f;
return 0;
}
Make that:

int construct(struct type * t1, int i, void (*f)(void))
/* why are you bothering to return anything here? */
{
t1->i = 1;
t1->f = f;
return 0;
}
int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct); You should have looked at the value of t1 *here*. ;-(
construct(&t1, 2, funct); t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

HTH,
--ag
--
Artie Gold -- Austin, Texas
http://it-matters.blogspot.com (new post 12/5)
http://www.cafepress.com/goldsays

[Martin Ambuhl]

James wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

[OP's code replaced]

#include <stdio.h>

struct type
{
int i;
void (*f) (void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int OPs_construct(struct type t1, int i, void (*f) (void))
{
t1.i = i;
t1.f = f;
return 0;
}
int corrected_construct(struct type *t1, int i, void (*f) (void))
{
t1->i = i;
t1->f = f;
return 0;
}

int main(void)
{
struct type t1 = { 0, 0 };
printf("t1.i starts as %d.\n"
"It should be 2 after \"construct\"\n", t1.i);
OPs_construct(t1, 2, funct);
printf("After calling OP's construct function, \n"
" t1.i = %d\n", t1.i);
printf("Since the OP's construct function doesn't work,\n"
" we won't bother with 't1.f();'\n\n");

corrected_construct(&t1, 2, funct);
printf("After calling the corrected construct function, \n"
" t1.i = %d\n", t1.i);
printf("Calling t1.f:\n");
t1.f();
return 0;
}

[output]
t1.i starts as 0.
It should be 2 after "construct"
After calling OP's construct function,
t1.i = 0
Since the OP's construct function doesn't work,
we won't bother with 't1.f();'

After calling the corrected construct function,
t1.i = 2
Calling t1.f:
this is funct

[Robert Gamble]

On Sun, 17 Apr 2005 07:20:18 -0700, James wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void))
{
t1.i=i;
t1.f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

Thanks

When you call your "construct" function you are passing a copy of t1 on
which the function operates, the original struct is unchanged, this is
because arguments are passed by-value in C as opposed to by-reference.
When your program calls t1.f(), it is trying to execute whatever happens
to be at the (uninitialized) address stored in t1.f causing your
segmentation fault. You have a couple of options, you can return a struct
from your "construct" function assigning the new value to your old struct,
or you can pass your function a pointer to the struct and have the
function operate on the original struct.

Example 1 (Return struct):

/* Change return type */
struct type construct(struct type t1, int i, void (*f)(void))
{
t1.i=i;
t1.f=f;
return t1; /* Return the modified copy */
}

int main(void)
{
struct type t1;
printf("This is main\n");
t1 = construct(t1, 2, &funct); /* Store the return value in t1 */
t1.f();
return 0;
}

Example 2 (Pass pointer to struct):

/* Change first parameter to be pointer to struct */
int construct(struct type * t1, int i, void (*f)(void))
{
t1->i=i; /* Operate on original struct */
t1->f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(&t1, 2, &funct); /* Pass address of t1 */
t1.f();
return 0;
}
Rob Gamble

[Barry Schwarz]

On Sun, 17 Apr 2005 16:34:54 +0200, Daniel Etzold <de*****@gmx.de>
wrote:

This works:

[...]

int construct(struct type* t1,int i,void (*f)(void))
{
t1->i=i;
t1->f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(&t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

You have to call "construct" with the address of the struct.
Or return the updated copy of the struct back to the calling function.

Regards,
Daniel

James wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void))
{
t1.i=i;
t1.f=f;
return 0;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct);
t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

Thanks

<<Remove the del for email>>

[Barry Schwarz]

On 17 Apr 2005 07:20:18 -0700, "James" <ja********@yahoo.com> wrote:

hello,
The program given below is giving segmentation fault. By debugging I
inferred that the problem is in the statement t1.f(), but I am not sure
that what is the problem.

#include <stdio.h>

struct type {
int i;
void (*f)(void);
};

void funct(void)
{
printf("this is funct\n");
return;
}

int construct(struct type t1,int i,void (*f)(void))
struct type construct(...
{
t1.i=i;
t1.f=f;
return 0;
return t1;
}

int main(void)
{
struct type t1;
printf("This is main\n");
construct(t1,2,&funct);
t1 = construct(...
t1.f(); // Most probably the problem is in this line.
return 0;
}

Can anybody please tell me that what is the real problem?

Thanks

<<Remove the del for email>>