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passing a pointer to initialized structrue in a function

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  passing a pointer to initialized structrue in a function ArifulHossain tuhin <etothepowerpi@gmail.com> - 2012-03-02 07:39 -0800
    Re: passing a pointer to initialized structrue in a function ArifulHossain tuhin <etothepowerpi@gmail.com> - 2012-03-02 07:44 -0800
      Re: passing a pointer to initialized structrue in a function James Kuyper <jameskuyper@verizon.net> - 2012-03-02 11:26 -0500
    Re: passing a pointer to initialized structrue in a function James Kuyper <jameskuyper@verizon.net> - 2012-03-02 11:45 -0500
      Re: passing a pointer to initialized structrue in a function James Kuyper <jameskuyper@verizon.net> - 2012-03-02 11:47 -0500
      Re: passing a pointer to initialized structrue in a function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-03-02 17:02 +0000
        Re: passing a pointer to initialized structrue in a function James Kuyper <jameskuyper@verizon.net> - 2012-03-02 13:53 -0500
    Re: passing a pointer to initialized structrue in a function Ike Naar <ike@iceland.freeshell.org> - 2012-03-02 17:25 +0000
    Re: passing a pointer to initialized structrue in a function cri@tiac.net (Richard Harter) - 2012-03-02 19:40 +0000

#18110 — passing a pointer to initialized structrue in a function

I have this structure like following :

typedef struct{
char static_data[10];
int a;
int b;
} my_ds;

i have initialized it like following:

my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
memmove(my_ds->static_data, buf, len);
ds->a = c;
ds->b = d;

and passed it in a function:

int my_fun(void ** data);
my_fun((void *)&ds);

what happens inside my_fun, the first field, static[10] is initialized correctly but other two values are zero. What am i missing here?

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#18111

On Friday, March 2, 2012 9:39:25 PM UTC+6, ArifulHossain tuhin wrote:
> I have this structure like following :
> 
> typedef struct{
> char static_data[10];
> int a;
> int b;
> } my_ds;
> 
> i have initialized it like following:
> 
> my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
> memmove(my_ds->static_data, buf, len);
> ds->a = c;
> ds->b = d;
> 
> and passed it in a function:
> 
> int my_fun(void ** data);
> my_fun((void *)&ds);
> 
> what happens inside my_fun, the first field, static[10] is initialized correctly but other two values are zero. What am i missing here?

Sorry for dual posting. first one was not shown for a while. so i thought i did something wrong and posted again. group admin can delete one.

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#18112

On 03/02/2012 10:44 AM, ArifulHossain tuhin wrote:
...
> Sorry for dual posting. first one was not shown for a while. so i thought i did something wrong and posted again. group admin can delete one.

There is no group administrator. Since you used google groups to post
it, you can view the message in google groups, and then select the
appropriate option (I don't remember what it was) to delete the message.
That will remove it from Google's archives, and may cause a request to
be sent out to other news servers asking them to remove their copies of
the message as well. However, such requests are routinely ignored by
most news servers, because they are so easily faked and have so
frequently been abused.

Basically, once you post to usenet, copies of your message are extremely
unlikely to ever completely disappear (unless you don't want them to -
Murphy rules).

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#18113

On 03/02/2012 10:39 AM, ArifulHossain tuhin wrote:
> I have this structure like following :
> 
> typedef struct{
> char static_data[10];
> int a;
> int b;
> } my_ds;
> 
> i have initialized it like following:
> 
> my_ds * ds = (my_ds *)malloc(sizeof(my_ds));

It's better to do the following, it's less error prone and less likely
to cover up certain kinds of errors:

	my_ds *ds = malloc(sizeof *ds);

> memmove(my_ds->static_data, buf, len);
> ds->a = c;
> ds->b = d;
> 
> and passed it in a function:
> 
> int my_fun(void ** data);
> my_fun((void *)&ds);

ds object of type my_ds, and &ds is a pointer to that object. my_fun is
expecting to receive a pointer to an object of type void*, which is
completely different type of object. The result of your cast to void* is
to convert what would otherwise have been a constraint violation into
undefined behavior. It does not actually fix the problem. Here's one
approach that should work (assuming that this is the only problem in
your code):

	void *temp = &ds;
	my_fun(&temp);

If you still have problems, please put these code fragments together
into a single complete program, as simple as possible, that demonstrates
your problem. Make sure that it actually does demonstrate your problem.
Then post the entire program - that will make it a lot easier to figure
out. As it stands, your message left out a lot of details that might (or
might not) have been relevant:

1. The types and values of c, d, buf, and len.
2. The definition (NOT just the declaration!) of my_fun().
3. What you did to confirm that you were getting unexpected results.

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#18114

On 03/02/2012 11:45 AM, James Kuyper wrote:
...
> 	void *temp = &ds;

Correction:
	void *temp = ds;

> 	my_fun(&temp);

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#18115

James Kuyper <jameskuyper@verizon.net> writes:

> On 03/02/2012 10:39 AM, ArifulHossain tuhin wrote:
<snip>
>> my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
<snip>
>> memmove(my_ds->static_data, buf, len);
>> ds->a = c;
>> ds->b = d;
>> 
>> and passed it in a function:
>> 
>> int my_fun(void ** data);
>> my_fun((void *)&ds);
>
> ds object of type my_ds, and &ds is a pointer to that object.

You've missed a level of indirection (or maybe just several words got
lost from the sentence).  ds is an object of type pointer to my_ds.  &ds
is indeed a pointer to "that object", but it's type is pointer to
pointer to my_ds.

<snip>
-- 
Ben.

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#18118

On 03/02/2012 12:02 PM, Ben Bacarisse wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
> 
>> On 03/02/2012 10:39 AM, ArifulHossain tuhin wrote:
> <snip>
>>> my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
...
>> ds object of type my_ds, and &ds is a pointer to that object.
> 
> You've missed a level of indirection (or maybe just several words got
> lost from the sentence).

I did lose a level of indirection (without, oddly enough, invalidating
my conclusion). I posted a correction to my code, but I should have
corrected the text, too:

"ds is pointer to an object of type my_ds, and &ds is a pointer to that
pointer."

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#18116

On 2012-03-02, ArifulHossain tuhin <etothepowerpi@gmail.com> wrote:
> I have this structure like following :
>
> typedef struct{
> char static_data[10];
> int a;
> int b;
> } my_ds;
>
> i have initialized it like following:
>
> my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
> memmove(my_ds->static_data, buf, len);

Did you mean

  memmove(ds->static_data, buf, len);

here ?

> ds->a = c;
> ds->b = d;
>
> and passed it in a function:
>
> int my_fun(void ** data);
> my_fun((void *)&ds);
>
> what happens inside my_fun, the first field, static[10] is initialized
> correctly but other two values are zero. What am i missing here?

Apart from a few stylistic things, I don't see problems in the code that
could explain the behaviour (assuming c and d were nonzero, and that
len <= 10).
Can you show a *complete* small program (one that we can compile and run
without having to guess about the missing parts) that exhibits the faulty
behaviour?

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#18121

On Fri, 2 Mar 2012 07:39:25 -0800 (PST), ArifulHossain tuhin
<etothepowerpi@gmail.com> wrote:

>I have this structure like following :
>
>typedef struct{
>char static_data[10];
>int a;
>int b;
>} my_ds;
>
>i have initialized it like following:
>
>my_ds * ds = (my_ds *)malloc(sizeof(my_ds));
>memmove(my_ds->static_data, buf, len);
>ds->a = c;
>ds->b = d;
>
>and passed it in a function:
>
>int my_fun(void ** data);
>my_fun((void *)&ds);
>
>what happens inside my_fun, the first field, static[10] is initialized correctly but other two values are zero. What am i missing here?

Possibly that c and d are zero.  You need to show us the code that
gives them values.

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