Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
printf("ID = %d",(*myBuff->id));
5  1902 
<mi*****@gmail. comwrote in message
news:11******** **************@ 22g2000hsm.goog legroups.com...
Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
printf("ID = %d",(*myBuff->id));
Are you sure you get a pointer to mem, and not the value of mem itself?
Ie a union Member **, not a union Member * ?
--
Free games and programming goodies. http://www.personal.leeds.ac.uk/~bgy1mm
On Aug 31, 11:08 am, "Malcolm McLean" <regniz...@btin ternet.com>
wrote:
<mihi...@gmail. comwrote in message
news:11******** **************@ 22g2000hsm.goog legroups.com...
Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
printf("ID = %d",(*myBuff->id));
Are you sure you get a pointer to mem, and not the value of mem itself?
Ie a union Member **, not a union Member * ?
--
Free games and programming goodies.http://www.personal.leeds.ac.uk/~bgy1mm- Hide quoted text -
- Show quoted text -
I am getting following:
Buffer received_buff;
void * my_ptr;
my_ptr = (void *)received_buff .mem;
So I have my_ptr, from here I need to get "id".
Hope I am clear. mi*****@gmail.c om wrote:
On Aug 31, 11:08 am, "Malcolm McLean" <regniz...@btin ternet.com>
wrote:
><mihi...@gmail .comwrote in message
news:11******* *************** @22g2000hsm.goo glegroups.com.. .
>>Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
printf("ID = %d",(*myBuff->id));
Are you sure you get a pointer to mem, and not the value of mem itself?
Ie a union Member **, not a union Member * ?
--
Free games and programming goodies.http://www.personal.leeds.ac.uk/~bgy1mm- Hide quoted text -
- Show quoted text -
I am getting following:
Buffer received_buff;
void * my_ptr;
my_ptr = (void *)received_buff .mem;
So I have my_ptr, from here I need to get "id".
Hope I am clear.
It is clear from the declarations you just posted. receiver.mem is a
pointer. You code, shown above, has not initialized it, so my_ptr is
set to an uninitialized value. However, you want to use a pointer to
received_buff.m em, so
my_ptr = &received_buff. mem;
then
Buffer *myBuff;
GetMemFromBuf (myBuff, (my_ptr);
printf("ID = %d",(myBuff->id));
Note the two changes made to your code. This assumes that
received_buff.i d has been initialized.
--
Thad
On Fri, 31 Aug 2007 16:24:04 -0000, mi*****@gmail.c om wrote:
>On Aug 31, 11:08 am, "Malcolm McLean" <regniz...@btin ternet.com>
wrote:
><mihi...@gmail .comwrote in message
news:11******* *************** @22g2000hsm.goo glegroups.com.. .
Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
No you don't. See the comments below.
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
The apparent purpose of this macro is to find the start of a
particular Buffer structure given the address of the member mem in
that particular structure.
>>
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
What is myMem? What do you think the cast is doing?
At this point in time, you do not have any object of type Buffer. You
do have a pointer but it doesn't point anywhere until the macro
expansion assigns it a value. If you insert the following before the
macro call
Buffer b;
then the macro call
GetMemFromBuf (myBuff, &b.mem);
will work and myBuff will point to b.
printf("ID = %d",(*myBuff->id));
The code invokes undefined behavior because you never initialized id
to contain the address of an enum object (possibly because you never
had an actual struct).
>>
Are you sure you get a pointer to mem, and not the value of mem itself?
Ie a union Member **, not a union Member * ?
I am getting following:
Buffer received_buff;
void * my_ptr;
my_ptr = (void *)received_buff .mem;
The cast is unnecessary and the code invokes undefined behavior
because mem has never been initialized with or assigned a value.
>
So I have my_ptr, from here I need to get "id".
You have my_ptr but it does not contain the address of mem. Malcolm's
question hits the nail on the head.
If you want the address of mem, then use
my_ptr = &received_buff. mem;
Remove del for email
On Fri, 31 Aug 2007 21:54:11 -0600, Thad Smith <Th*******@acm. org>
wrote:
>mi*****@gmail. com wrote:
>On Aug 31, 11:08 am, "Malcolm McLean" <regniz...@btin ternet.com>
wrote:
>><mihi...@gmai l.comwrote in message
news:11****** *************** *@22g2000hsm.go oglegroups.com. ..
Hi,
I have following structures and union
typedef struct
{
int a[100];
int b;
char *c;
}TEST_1;
typedef struct
{
int a[100];
char c[100];
}TEST_2;
union Member
{
TEST_1 test_1;
TEST_2 test_2;
};
typedef enum{
START_ID_0 = 0x2000,
START_ID_1,
START_ID_2,
START_ID_3,
START_ID_4
}SomeIdd;
typedef struct BufferTag
{
SomeId *id;
union Member *mem;
}Buffer;
In my program somehow I get pointer to "mem" in struct "Buffer". Now I
want to get value stored in "id". Is there a way. I read at multiple
places that I can use offsetof but when I try it gives by Segmentation
fault. Can you experts check this and let me know.
FOllowing is what I am doing.
#define GetMemFromBuf (out,source) \
(out) = (Buffer *) \
((char *) (source) - (offsetof(Buffe r, mem)))
Following is that way I am accessing it.
Buffer *myBuff;
GetMemFromBuf (myBuff, (void *)myMem);
printf("ID = %d",(*myBuff->id));
Are you sure you get a pointer to mem, and not the value of mem itself?
Ie a union Member **, not a union Member * ?
--
Free games and programming goodies.http://www.personal.leeds.ac.uk/~bgy1mm- Hide quoted text -
- Show quoted text -
I am getting following:
Buffer received_buff;
void * my_ptr;
my_ptr = (void *)received_buff .mem;
So I have my_ptr, from here I need to get "id".
Hope I am clear.
It is clear from the declarations you just posted. receiver.mem is a
pointer. You code, shown above, has not initialized it, so my_ptr is
set to an uninitialized value. However, you want to use a pointer to
received_buff. mem, so
my_ptr = &received_buff. mem;
then
Buffer *myBuff;
GetMemFromBuf (myBuff, (my_ptr);
printf("ID = %d",(myBuff->id));
Note the two changes made to your code. This assumes that
received_buff. id has been initialized.
The member id is a pointer. To print the value it points to with %d,
you need to dereference it as his original code did. If you want to
print the address it points to, you need to use %p and cast it to
void*.
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