Structure sizes differ in C and C++?

[cme0848]

I am attempting to write a program which passes an array struct from an ANSI C environment into a C++ DLL. The struct is defined as this:

struct device
{
   int ID;
   char name[25];
};
 
struct device myDevices[100];
 
 

I was attempting to update the structure in the C++ DLL and somehow the pointer gets off and so i have incomplete data. To my surprise this struct is sizeof 29 in C++ and 32 in C. Could this be my problem? Why are they different sizes if they are the same struct type? If this is my issue, how do i resolve it?

Confused,
Craig

[weaknessforcats]

I just did a sizeof your struct and compiled as C and then as C++ and got 32 both times. Are you using different compilers for C and C++??

I would expect 32 since that keeps everything aligned on a word boundary. There would be three slack bytes after the array. However, different compilers may align differently.

I hope you didnt do sizeof(var.Id) + sizeof(var.name) because that will give you 29.

[cme0848]

I fixed my issue. .

I declared my int as a short. This made both struct sizes equal to 27 and allowed me to get the desired output.

FYI, I was using National Instruments CVI (ANSI C) and Visual Studio 2005 for C++. Still not sure why my C++ did not conform to a 32 bit word boundary but maybe its because C works on word boundaries and C++ does not? There seems to be some dissent on this topic among these forums. (see http://www.thescripts.com/forum/thread163278.html )

Thank you,
Craig

[weaknessforcats]

Word alignment is compiler design and not part of the C ot C++ language.

Alignment on workd boundaries is more efficient and all major compilers do this. However, it is compiler depenedent.