| re: Pointer to Structure Casts
Alexei A. Frounze wrote:[color=blue]
> <caleb.vandyke@gmail.com> wrote in message
> news:1127261462.826812.90780@g49g2000cwa.googlegro ups.com...[color=green]
> > I am working with some code that is doing some pointer to structure
> > casts and I can't figure out how the cast is being done. Here is
> > basically the code.[/color]
> ...[color=green]
> > typedef struct diffRecord
> > {
> > struct record *next[1];
> > } DiffRecord;
> >
> > typedef struct record
> > {
> > struct record *next;
> > int value;
> > } Record;
> >
> > int main(int argc, char* argv[])
> > {
> > Record *rec1 = malloc(sizeof(Record));
> > rec1->value = 5;
> >
> > Record *rec2 = malloc(sizeof(Record));
> > rec2->value = 6;
> >
> > rec1->next = rec2;
> > rec2->next = NULL;
> >
> > DiffRecord *diffRec = ((DiffRecord*)rec1)->next[0];
> > Record *testRec = (Record*)diffRec;
> >
> > printf("The value is: %i\n", testRec->value); //prints 6[/color]
> ...[color=green]
> > How is the pointer contained in record being coerced into the array in
> > the diffRecord structure?[/color]
>
> Here:
> struct record *next[1];
> struct record *next;
> One struct contains array of 1 pointer.
> The other contains 1 pointer, w/o being part of any array.
> But, they both contain a pointer anyway, so there's no problem here. It's
> not much different from the case when you have an array of 1 int and just 1
> int, simply the type is different.
> So, both structs contain in their beginning a pointer (or array of one
> pointer -- doesn't matter how you think of it, in this particular case it's
> essentially the same thing and from the memory's standpoint the two things
> are equivalent)...
> I can only say that this is a bit odd code... I'd expect this way of casting
> Record *testRec = (Record*)diffRec;
> rather than this
> DiffRecord *diffRec = ((DiffRecord*)rec1)->next[0];
> But you have both. What for?
>
> Alex[/color]
This code actually is from another author. Their code looks something
like this:
#include <stdio.h>
#include <stdlib.h>
typedef struct diffRecord
{
struct diffRecord *next[2];
} DiffRecord;
typedef struct record
{
struct record *next;
struct record *previous;
int value;
} Record;
static void printValue(int index, Record *rec)
{
DiffRecord *diffRec = (DiffRecord*)rec;
Record *testRec = (Record*)diffRec->next[index];
printf("Value: %i\n", testRec->value);
}
int main(int argc, char* argv[])
{
Record *rec1 = malloc(sizeof(Record));
Record *rec2 = malloc(sizeof(Record));
Record *rec3 = malloc(sizeof(Record));
rec1->value = 5;
rec2->value = 6;
rec3->value = 7;
rec1->next = rec2;
rec1->previous = NULL;
rec2->previous = rec1;
rec2->next = rec3;
rec3->previous = rec2;
rec3->next = NULL;
printValue(0, rec2);
printValue(1, rec2);
free(rec3);
free(rec2);
free(rec1);
return 0;
}
Output:
Value: 7
Value: 5
I think their intention was for the parameter index in printValue() to
be some sort of selector for the pointers in the rec parameter. The
haven't seen a cast like this before but I am assuming that this works
becuase the first two parameters of Record are pointers?
If say Record looks like this:
typedef struct record
{
struct record *next;
int value;
struct record *previous;
} Record;
and I cast struct record to a struct diffRecord would I get some
undefined behaviour trying to access next[1] field of diffRecord?
Thanks for the help,
Caleb Van Dyke
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