Print a struct

[arnuld]

I am unable to add anything to this structI can't understand why I am
getting a Segfault:



#include <stdio.h>


struct macd
{
int price;
};


void print_combo( struct macd* );


int main(void)
{
struct macd* my_combo = NULL;

my_combo->price = 100;


return 0;
}


void print_combo( struct macd* mc )
{
if( mc )
{
printf("Price = %d\n", mc->price);

}
}





--

https://www.lispmachine.wordpress.com

my email is @ the above blog.

[LoYwEnG]

Re: Print a struct

On Nov 7, 9:28 pm, arnuld <sunr...@invali d.addresswrote:

I am unable to add anything to this structI can't understand why I am
getting a Segfault:
>
#include <stdio.h>
>
struct macd
{
  int price;
>
};
>
void print_combo( struct macd* );
>
int main(void)
{
  struct macd* my_combo = NULL;

Maybe because of this statement.

>
  my_combo->price = 100;
>
  return 0;
>
}
>
void print_combo( struct macd* mc )
{
  if( mc )
    {
      printf("Price = %d\n", mc->price);
>
    }
>
}
>
--www.lispmachine .wordpress.com
my email is @ the above blog.

[Martien Verbruggen]

Re: Print a struct

On Fri, 07 Nov 2008 18:28:50 +0500,
arnuld <sunrise@invali d.addresswrote:

I am unable to add anything to this structI can't understand why I am
getting a Segfault:

struct macd
{
int price;
};

int main(void)
{
struct macd* my_combo = NULL;

This is not a struct. It's a pointer to a struct. And it doesn't point
to anywhere where there could be a struct.

my_combo->price = 100;

You're trying to dereference the pointer that you just set to NULL
yourself. You can't do that. Either allocate memory with malloc, or use
a struct.


struct macd *my_combo ;
macd = malloc(sizeof *macd);
my_combo->price = 100;

OR

struct macd my_combo;
my_combo.price = 100;

Just like any other pointer, a struct pointer needs to point to
some memory.

Martien
--
|
Martien Verbruggen | Since light travels faster than sound, is
| that why some people appear bright until you
| hear them speak?

[Lew Pitcher]

Re: Print a struct

On November 7, 2008 08:28, in comp.lang.c, arnuld (sunrise@invali d.address)
wrote:

I am unable to add anything to this structI can't understand why I am
getting a Segfault:
>
>
>
#include <stdio.h>
>
>
struct macd
{
int price;
};
>
>
void print_combo( struct macd* );
>
>
int main(void)
{
struct macd* my_combo = NULL;

my_combo is a pointer to a structure. The pointer is set to NULL. There is
no space allocated to the structure.

my_combo->price = 100;

Here, you put a value into the structure. Where did this value wind up?
Remember, at this point, the structure only exists in potential, and /not/
as an object that can be modified. So, what did you modify?

Remember also that my_combo is a pointer, which (at the moment) points to
(the official) nowhere. So, again, where did your value of 100 go?

>
return 0;
}

[snip]

--
Lew Pitcher

Master Codewright & JOAT-in-training | Registered Linux User #112576
http://pitcher.digitalfreehold.ca/ | GPG public key available by request
---------- Slackware - Because I know what I'm doing. ------

[santoshsy]

Re: Print a struct

On Nov 7, 6:28 pm, arnuld <sunr...@invali d.addresswrote:

I am unable to add anything to this structI can't understand why I am
getting a Segfault:
>
#include <stdio.h>
>
struct macd
{
  int price;
>
};
>
void print_combo( struct macd* );
>
int main(void)
{
  struct macd* my_combo = NULL;
>

my_combo = malloc(sizeof(s truct macd)); // Allocating Memory

This should work fine.

  my_combo->price = 100;
>
  return 0;
>
}
>
void print_combo( struct macd* mc )
{
  if( mc )
    {
      printf("Price = %d\n", mc->price);
>
    }
>
}
>
--www.lispmachine .wordpress.com
my email is @ the above blog.

[William Pursell]

Re: Print a struct

On 11 Nov, 05:54, santoshsy <santos...@gmai l.comwrote:

    my_combo = malloc(sizeof(s truct macd));   // Allocating Memory

Better is:

my_combo = malloc( sizeof *my_combo );

Although in this case it is probably best to declare
an actual struct:

struct macd foo;
struct macd *my_combo;
my_combo = &foo;

Or even just do:

struct macd my_combo[ 1 ];

[William Pursell]

Re: Print a struct

On 7 Nov, 13:59, Lew Pitcher <lpitc...@teksa vvy.comwrote:

On November 7, 2008 08:28, in comp.lang.c, arnuld (sunr...@invali d.address)
wrote:

  struct macd* my_combo = NULL;
  my_combo->price = 100;

<snip>

>
Remember also that my_combo is a pointer, which (at the moment) points to
(the official) nowhere. So, again, where did your value of 100 go?
>

It went to nowhere plus offsetof( struct macd, price ), of course! :)