Can I use a struct pointer inside a struct?

Question

System: Red Hat Linux Enterprise 4
BDB: 4.5.2.NC
Language: C
My data structure list below:
typedef struct a
int length;
float area;
}A;
typedef struct b
A * A1;
int others;
}B;
Because the length of array A1 is various, so I want to create at run time by malloc().
Codes list below:
B b1;
b1.A1 = (A*) malloc(sizeof(A)*n); /*n is set by parameter*/
for (int i = 0; i < n; i++)
b1.A1.length = x;
b1.A1.area = y;
memset(&Data, 0, sizeof(DBT));
Data.data = &b1;
Data.size = sizeof(B) + sizeof(A)*n;
ret = dbp->put(dbp, NULL, &Key, &Data, DB_NOOVERWRITE);
There was no error when putting data into BDB, but I can't get the data properly.
ret = dbp->get(dbp, NULL, &Key, &Data, 0);
B * b1;
b1 = (B*)(Data.data);
for (int j = 0; j < n; j++)
printf("%d, %dn", b1.A1[j].length, b1.A1[j].area);
Error appears when I access the item of array A1. : "Segementation fault"
I got the information from DB manual ''C Getting Started Guide: Chapter 3. Database Records". It's read from the very beginning:"They can therefore be used to store anything from simple primitive data to complex structures so long as the information you want to store resides in a single contiguous block of memory."
Does it mean that data.data can just be a block of memory? I mean that I have to specify the size of a char array (length of a string), etc.? Should not I use a dynamic size array? If could, how to do with it?

Answer 1

Hi Lesslie,
BDB stores data that is located at a specified address and of a particular size. Your data contains a field that points to dynamically allocated memory, meaning that data can be located in different, not necessarily contiguous, locations in the heap.
To resolve this the most efficient and easiest way is to pack your data into a single memory location (a buffer that can hold your data) and then store the data in that location (this is called marshalling).
So, you should pack the n structures of type A, and provide the Data DBT with the address of the buffer where they are marshaled and the size of the buffer holding the structures.
For more information consult the following section in the GSG guide for C on storing structures with pointers:
http://www.oracle.com/technology/documentation/berkeley-db/db/gsg/C/BerkeleyDB-Core-C-GSG.pdf
Regards,
Andrei