void pointers and scanf/printf

Discussion in 'C Programming' started by pereges, Jul 20, 2008.

  1. pereges

    pereges Guest

    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);
    }
    pereges, Jul 20, 2008
    #1
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  2. pereges

    voidpointer Guest

    On Jul 20, 5:53 pm, pereges <> wrote:
    > 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);
    >
    > }


    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
    voidpointer, Jul 20, 2008
    #2
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  3. pereges

    pereges Guest

    On Jul 21, 2:23 am, voidpointer <> wrote:

    > 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


    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.
    pereges, Jul 20, 2008
    #3
  4. pereges

    voidpointer Guest

    On Jul 20, 6:28 pm, pereges <> wrote:
    > On Jul 21, 2:23 am, voidpointer <> wrote:
    >
    > > 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

    >
    > 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.


    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
    voidpointer, Jul 20, 2008
    #4
  5. pereges

    voidpointer Guest

    On Jul 20, 6:57 pm, voidpointer <> wrote:
    > On Jul 20, 6:28 pm, pereges <> wrote:
    >
    > > On Jul 21, 2:23 am, voidpointer <> wrote:

    >
    > > > 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

    >
    > > 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.

    >
    > 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


    only a correction, use scanf("%d", (int*)&ptr->data), sorry
    voidpointer, Jul 20, 2008
    #5
  6. pereges

    pereges Guest

    On Jul 21, 3:12 am, voidpointer <> wrote:

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


    but ptr->data is already address then why &ptr->data ?
    pereges, Jul 20, 2008
    #6
  7. voidpointer <> writes:

    > On Jul 20, 6:28 pm, pereges <> wrote:
    >> On Jul 21, 2:23 am, voidpointer <> wrote:
    >>
    >> > 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

    >>
    >> 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.

    >
    > 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 *'


    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)


    This won't fix the problem.

    --
    Ben.
    Ben Bacarisse, Jul 20, 2008
    #7
  8. pereges <> writes:

    > 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;


    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.

    --
    Ben.
    Ben Bacarisse, Jul 20, 2008
    #8
  9. pereges

    neobakuer Guest

    On Jul 20, 7:15 pm, pereges <> wrote:
    > On Jul 21, 3:12 am, voidpointer <> wrote:
    >
    > > only a correction, use scanf("%d", (int*)&ptr->data), sorry

    >
    > but ptr->data is already address then why &ptr->data ?


    this is only to avoid the warning message.

    But as said voidpointer and Ben, if you want work with int, use int.
    neobakuer, Jul 20, 2008
    #9
  10. pereges <> writes:

    > On Jul 21, 3:12 am, voidpointer <> wrote:
    >
    >> only a correction, use scanf("%d", (int*)&ptr->data), sorry

    >
    > but ptr->data is already address then why &ptr->data ?


    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.

    --
    Ben.
    Ben Bacarisse, Jul 20, 2008
    #10
  11. On Sun, 20 Jul 2008 15:51:13 -0700, neobakuer wrote:
    > On Jul 20, 7:15 pm, pereges <> wrote:
    >> On Jul 21, 3:12 am, voidpointer <> wrote:
    >> > only a correction, use scanf("%d", (int*)&ptr->data), sorry

    >>
    >> but ptr->data is already address then why &ptr->data ?

    >
    > this is only to avoid the warning message.


    (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.
    Harald van Dijk, Jul 21, 2008
    #11
  12. On Sun, 20 Jul 2008 23:58:16 +0100, Ben Bacarisse wrote:
    > pereges <> writes:
    >> On Jul 21, 3:12 am, voidpointer <> wrote:
    >>
    >>> only a correction, use scanf("%d", (int*)&ptr->data), sorry

    >>
    >> but ptr->data is already address then why &ptr->data ?

    >
    > 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.


    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.
    Harald van Dijk, Jul 21, 2008
    #12
  13. Harald van Dijk <> writes:

    > On Sun, 20 Jul 2008 23:58:16 +0100, Ben Bacarisse wrote:
    >> 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.

    >
    > To quote the original message:
    >
    > ptr->data = malloc(sizeof(int));

    <snip>
    > ptr->data has been properly initialised, so
    >
    > scanf("%d", (int *)ptr->data)
    >
    > is correct.


    Blimey! Must go to sleep. Totally missed that!

    --
    Ben.
    Ben Bacarisse, Jul 21, 2008
    #13
  14. pereges

    pereges Guest

    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.
    pereges, Jul 21, 2008
    #14
  15. Richard Heathfield <> writes:
    > pereges said:
    >
    >> 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.

    >
    > 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.


    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.

    --
    Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
    Nokia
    "We must do something. This is something. Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"
    Keith Thompson, Jul 21, 2008
    #15
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