Help using memcpy to update structure members

**pk68** · 02-25-2011

I am trying to update the members of a structure using memcpy and running into issues determining the indexes of the elements. here is an example

Code:

struct AA{
  double a;
  double b;
  double c;
 }



struct BB{
  double d;
  int       e;
  double f;
  int       g;
}

double U[3]
double V[2]
int W[2]

(void) memcpy(&AA (double*)U,, sizeof(double*)U);

This works fine. However I am not sure how to update struct BB since it has mixed data types. I would like to update structure BB with the values of V and W. I want to use memcpy since the element declaration of these structures can change and I don't have access to them. Can someone help me how to access the location of each structure member.

**~~CommonTater~~** · 02-25-2011

Why wouldn't you just use the = sign?

AA.a = u[x];

**Bayint Naung** · 02-25-2011

I want to use memcpy since the element declaration of these structures can change and I don't have access to them. Can someone help me how to access the location of each structure member.

Can you elaborate more?
If it's opaque type, then better write setter/getter function?

**pk68** · 02-25-2011

Hello Bayint,
I am writing a C program to load a library(dll) and update certain values in the dll. I don’t have the source code of the dll. However the dll provides me the Structure, number of members and types of each member in the structure. I need to update these members with new values. Since I don’t know the member names AA.a=u[x] doesn’t work for me. The only way I can think of accomplishing this is by updating the addresses. So if all the member types are of the same type then “(void) memcpy(&AA (double*)U, sizeof(double*)U);” would work. However I am not sure how to accomplish this if the member types are mixed.

**quzah** · 02-25-2011

Be lazy:

Code:

union
{
    struct thingineedtoupdate foo;
    struct whateverithinkiamdoing bar;
} baz;

baz.bar = doingstuff;
dowhateverithinkiamupdating( baz.foo );

Quzah.

**pk68** · 02-26-2011

I am sorry, I don't understand this. Is there a way to access the address of structure members so that I can use memcpy to update them.

**quzah** · 02-26-2011

Sure, if you know what the members are.

Code:

&somestruct.somemember

I don't understand why you think you need to use memcpy if you know what every one of the members are. Just assign values directly, as you've already been told.

Quzah.

**pk68** · 02-27-2011

The problem is I don't know the members. All I know are the number of members and their data types and the datatypes are mixed within the structure.

**quzah** · 02-27-2011

Well I'm still not sure what you're stuck on. How can you be updating something if you don't know what it is?

Code:

struct foo
{
    type thingyouknow;
    type thingyoudonotknow;
    type otherthingyouknow;
};

How can you expect to accurately update 'thingyoudonotknow' if you don't know what it is or what its type is? What are you trying to put there ... and why, if you don't know anything about what it is? None of that makes sense.

Answer that first, and you should be able to do whatever you need with everything else here we've already given you.

Quzah.

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