Question

I have the following struct:

struct Datastore_T
{
 Partition_Datastores_T  cmtDatastores; // bytes 0 to 499
 Partition_Datastores_T  cdhDatastores; // bytes 500 to 999
 Partition_Datastores_T  gncDatastores; // bytes 1000 to 1499
 Partition_Datastores_T  inpDatastores; // bytes 1500 1999
 Partition_Datastores_T  outDatastores; // bytes 2000 to 2499
 Partition_Datastores_T  tmlDatastores; // bytes 2500 to 2999
 Partition_Datastores_T  sm_Datastores; // bytes 3000 to 3499
};

I want to set a char* to point to a struct of this type like so:

struct Datastore_T datastores;

// Elided: datastores is initialized with data here

char* DatastoreStartAddr = (char*)&datastores;
memset(DatastoreStartAddr, 0, 3500);

The problem I have is that DatastoreStartAddr always has a value of zero when it should point to the struct that has been initialized with data.

What am I doing wrong?

Edit: What I mean by zero is that the “values” in the structure are all zeros even after I initialize the structure. The address is not zero, it is the values in the struct that are zero.

Edit: I think I am asking the question wrong. Let’s start over. If I have a struct that is initialized with data, and another object maintains a field member that is a pointer to that struct, if the struct is changed directly:

struct Datastore_T datastores;
char* DatastoreStartAddr = (char*)&datastores;

datastores.cmtDatastores.u16Region[0] = Scheduler.GetMinorFrameCount(); // byte 40,41
datastores.cmtDatastores.u16Region[1] = Scheduler.GetMajorFrameCount(); // byte 42,43

Shouldn’t I be able to access these changes using the DatastoreStartAddr pointer?

EDIT: The following code tries to read the data set in datastores, but using the pointer to the struct:

            CMT_UINT8_Tdef PayLoadBuffer[1500]= {NULL};
            int TDIS = 0;
            int DIS = 0;
            int DSA = 0;

            //copy DataStore info using address and size offsets
            if ((PayLoadBuffer + TDIS + DIS) < IndvDEMMax)
            {
               memcpy((PayLoadBuffer + TDIS), Datastores+DSA, DIS);
               TDIS += DIS;
            }

In the memcpy((PayLoadBuffer + TDIS), Datastores+DSA, DIS) line, Datastores should point to structure and attempts to access an offset in that structure. But since the value is always zero, it copies zero in the PayLoadBuffer.

Answer 1

I don’t know why you are getting an address of zero, but I would guess the code you don’t show has something to do with it. Some other points:

• Consider using an array of Partition_Datastores_T inside your struct
• Do not use magic numbers for struct sizes, you want sizeof(Datastore_T )
• There is no need for the intermediate char*

Edit: Bobby, to answer your supplementary question – yes you should be able to access it through a pointer, but not through a char * (without jumping through some hoops). You want:

struct Datastore_T datastores;
struct Datastore_T * DatastoreStartAddr = &datastores;

and when you use that pointer:

DatastoreStartAddr->cmtDatastores.u16Region[0] = Scheduler.GetMinorFrameCount(); 

Please note the use of the -> operator.