void pointers and scanf/printf

P

pereges

have i written this program correctly ?

it is giving me correct output but i am little suspicious in the
following two statements -

scanf("%d", &ptr->data) and printf("%d\n", ptr->data).

/********** LINK LIST **********/

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

typedef struct node_s
{
void *data;
struct node_s *next;

}node;


int add_to_link_list(node **head)
{
node *ptr;
int temp;

ptr = malloc(sizeof(node));

if (ptr == NULL)
{
fprintf(stderr, "Memory allocation failed\n");
return (1);
}

ptr->data = malloc(sizeof(int));
if (ptr->data == NULL)
{
fprintf(stderr, "Memory allocation failed\n");
return (1);
}

printf("Enter data\n");
if (scanf("%d", &ptr->data) != 1)
{
fprintf(stderr, "Error while entering data\n");
return (1);
}

ptr->next = *head;
*head = ptr;

return (0);
}

int main(void)
{
node *head = NULL;
node *ptr;
int n, i;

printf("How many numbers\n");
if (scanf("%d", &n) != 1)
{
fprintf(stderr, "Error while enterning list size\n");
return (EXIT_FAILURE);
}

for (i = 0; i < n; i++)
{
if (add_to_link_list(&head))
{
fprintf(stderr, "add_to_link_list failed\n");
return (EXIT_FAILURE);
}
}

ptr = head;

while (ptr != NULL)
{
printf("%d\n", ptr->data);
ptr = ptr->next;
}

return (EXIT_SUCCESS);
}
 
V

voidpointer

have i written this program correctly ?

it is giving me correct output but i am little suspicious in the
following two statements -

scanf("%d", &ptr->data) and printf("%d\n", ptr->data).

/********** LINK LIST **********/

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

typedef struct node_s
{
void *data;
struct node_s *next;

}node;

int add_to_link_list(node **head)
{
node *ptr;
int temp;

ptr = malloc(sizeof(node));

if (ptr == NULL)
{
fprintf(stderr, "Memory allocation failed\n");
return (1);
}

ptr->data = malloc(sizeof(int));
if (ptr->data == NULL)
{
fprintf(stderr, "Memory allocation failed\n");
return (1);
}

printf("Enter data\n");
if (scanf("%d", &ptr->data) != 1)
{
fprintf(stderr, "Error while entering data\n");
return (1);
}

ptr->next = *head;
*head = ptr;

return (0);

}

int main(void)
{
node *head = NULL;
node *ptr;
int n, i;

printf("How many numbers\n");
if (scanf("%d", &n) != 1)
{
fprintf(stderr, "Error while enterning list size\n");
return (EXIT_FAILURE);
}

for (i = 0; i < n; i++)
{
if (add_to_link_list(&head))
{
fprintf(stderr, "add_to_link_list failed\n");
return (EXIT_FAILURE);
}
}

ptr = head;

while (ptr != NULL)
{
printf("%d\n", ptr->data);
ptr = ptr->next;
}

return (EXIT_SUCCESS);

}
Click to expand...

your program don't work, have a segfault, compile with -Wall (if
you're using gcc) and try solve the errors, any doubt post here
 
P

pereges

your program don't work, have a segfault, compile with -Wall (if
you're using gcc) and try solve the errors, any doubt post here
Click to expand...

Hello, Can you please check the first post again ? I have changed the
code. I got no warnings from my dmc/pellesC compilers and the outpu is
correct.
 
V

voidpointer

Hello, Can you please check the first post again ? I have changed the
code. I got no warnings from my dmc/pellesC compilers and the outpu is
correct.
Click to expand...

Hello, now work but have one warning message,

In function 'add_to_link_list':
test.c:30: warning: format '%d' expects type 'int *', but argument 2
has type 'void *'

you can solve this with a cast in your scanf, like this scanf("%d",
(int*)ptr->data)
and you have a lot of memory leak in your code, you doesn't free()d
the allocated space by malloc,
other thing, if you expect work with int, so use int, because void*
can be hard to work,
and need a lot of cast in your code
 
V

voidpointer

Hello, now work but have one warning message,

In function 'add_to_link_list':
test.c:30: warning: format '%d' expects type 'int *', but argument 2
has type 'void *'

you can solve this with a cast in your scanf, like this scanf("%d",
(int*)ptr->data)
and you have a lot of memory leak in your code, you doesn't free()d
the allocated space by malloc,
other thing, if you expect work with int, so use int, because void*
can be hard to work,
and need a lot of cast in your code
Click to expand...

only a correction, use scanf("%d", (int*)&ptr->data), sorry
 
B

Ben Bacarisse

voidpointer said:
Hello, now work but have one warning message,

In function 'add_to_link_list':
test.c:30: warning: format '%d' expects type 'int *', but argument 2
has type 'void *'
Click to expand...

Check the code, your compiler or the message you really get. The
argument is of type void **.
you can solve this with a cast in your scanf, like this scanf("%d",
(int*)ptr->data)
Click to expand...

This won't fix the problem.
 
B

Ben Bacarisse

pereges said:
have i written this program correctly ?

it is giving me correct output but i am little suspicious in the
following two statements -

scanf("%d", &ptr->data) and printf("%d\n", ptr->data).
Context:
typedef struct node_s
{
void *data;
struct node_s *next;

}node;
Click to expand...

You are right to be. You can't use %d to scanf into a void * and the
reverse is also wrong.

If you want a linked list of ints, then 'data' should be an int. If
you want something more generic, you could use union. More generalt
still comes from keeping the data field as void *, but allocating
storage for it to point to whatever you need to store.

In the Bad Old Days, it was a common trick to smuggle small data types
into a generic list by putting them into the void * (actually I've not
seen it done since void * arrived, but the same applies). It is not a
good idea.
 
N

neobakuer

but ptr->data is already address then why &ptr->data ?
Click to expand...

this is only to avoid the warning message.

But as said voidpointer and Ben, if you want work with int, use int.
 
B

Ben Bacarisse

pereges said:
but ptr->data is already address then why &ptr->data ?
Click to expand...

You some problems if you are asking this. I thought you'd written

scanf("%d", &ptr->data)

because you were knowingly breaking the type rules to put an int into
value into a void * object.

Both

scanf("%d", ptr->data)

and the type-correct version:

scanf("%d", (int *)ptr->data)

are wrong because they try to use an indeterminate pointer. The data
field has not been set to point anywhere. If you want data to point
to an int and then to read into it you must write:

ptr->data = malloc(sizeof(int));
if (ptr->data && scanf("%d", (int *)ptr->data) == 1) ...

Both what you had

scanf("%d", &ptr->data)

and the slightly better

scanf("%d", (int *)&ptr->data)

are wrong because they try to put an into a void * object. It often
works (as you have found out) but it is much better to do this:

struct node {
union {
int i;
void *vp;
} data;
struct node *next;
};

if you want to put ints into the actual nodes themselves. You then

scanf("%d", &ptr->data.i)

in a type-safe manner.
 
H

Harald van Dijk

this is only to avoid the warning message.
Click to expand...

(int*)&ptr->data avoids the warning message and is incorrect in this case.
(int*)ptr->data avoids the warning message and is correct. This is not
directed at you specifically, but please try to understand warnings before
modifying code to work around them.
 
H

Harald van Dijk

You some problems if you are asking this. I thought you'd written

scanf("%d", &ptr->data)

because you were knowingly breaking the type rules to put an int into
value into a void * object.

Both

scanf("%d", ptr->data)

and the type-correct version:

scanf("%d", (int *)ptr->data)

are wrong because they try to use an indeterminate pointer. The data
field has not been set to point anywhere.
Click to expand...

To quote the original message:

ptr->data = malloc(sizeof(int));
if (ptr->data == NULL)
{
fprintf(stderr, "Memory allocation failed\n");
return (1);
}

printf("Enter data\n");
if (scanf("%d", &ptr->data) != 1)
{
fprintf(stderr, "Error while entering data\n");
return (1);
}

ptr->data has been properly initialised, so

scanf("%d", (int *)ptr->data)

is correct.
 
P

pereges

I had delete the previous code and posted a new code. I don't know how
others can still see the previous code but it happens always with
google and its quite irritating.

Anyway, thanks for the help Harald and Ben.
 
K

Keith Thompson

Richard Heathfield said:
pereges said:


Google has nothing to do with it. Usenet is not Google, any more than the
World Wide Web is Internet Explorer, email is Outlook Express, or
spreadsheets are Lotus 1-2-3.

When you post a Usenet article, it is sent to a great many servers all over
the world. It used to be pretty easy to cancel messages until that service
got abused. Now, few servers honour cancellation requests - it's a shame,
but it's easy to understand why. Even if Google offers a cancellation
option, the rest of the world is under no obligation to follow suit.
Click to expand...

And the lesson is, if you want to correct something you've posted,
post the correction *as a followup* and clearly indicate in the text
of the followup that it's a correction. It won't hurt to try to
cancel the original article, but it's not likely to do much good.
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Similar Threads

void pointers and scanf/printf 0
Adding adressing of IPv6 to program 1
Please help with C programming to save GPS reception data in Raspberry Pi. 0
Linux: using "clone3" and "waitid" 0
about scanf() 2
Could you fix my code please? (i get no output after inputing the number) 1
Pointers 16
Fibonacci 0

Members online

No members online now.
Total: 265 (members: 1, guests: 264)
Robots: 248

Forum statistics

Threads
473,954
Messages
2,570,116
Members
46,704
Latest member
BernadineF

Latest Threads

Top