sizeof returns different values for a struct when compiling different programs

I have 2 programs that share a header file. This header file defines a structure with a few members. There is then a #define: #define STRUCTURE_SIZE sizeof(OUR_STRUCTURE).

This structure is then used in shared memory with STRUCTURE_SIZE being used for the size argument to shmget().

Unfortunately, for the one program, STRUCTURE_SIZE ends up being 20758, while in the other one, it ends up being 20764. So when the second program tries to get the shared mem, shmget() returns EINVAL.

uname 开发者_运维技巧-a:

Linux machine 2.6.30.10-105.2.23.fc11.i686.PAE #1 SMP Thu Feb 11 07:05:37 UTC 2010 i686 i686 i386 GNU/Linux

g++ --version:

g++ (GCC) 4.4.1 20090725 (Red Hat 4.4.1-2)

A few possibilities:

• you're compiling the two programs with different compilers and/or compiler switches
• you have another header which appears before the header in question in one of the programs and this messes with #pragma pack or similar and doesn't restore the setting
• a type that is used in the shared struct is defined differently in the two programs

(Note: the last two points can be applied recursively to any other structs which are used within the problem struct.)

The problem could be a packing or alignment issue. Find out how to tell your compiler how it should align structures.

Other possibilities:

• The set of defines used in the two compilations differs, and the structure is defined conditionally on these defines
• Some types can have different sizes in kernel and user space (this is rare, but it can happen)

You might be able to gain some understanding by writing some code to check the offsets of field in OUR_STRUCTURE and print them out, using the two compilations in turn. Break down the total struct size to determine the size caused by each of its fields.

struct OUR_STRUCTURE
{
  double d;
  other_structure other;
  bool flag;
};

OUR_STRUCTURE ours;
cout << &ours.d - &ours << endl;
cout << &ours.other - &ours << endl;
cout << &ours.flag - &ours << endl;
cout << &ours + sizeof(OUR_STRUCTURE) - &ours.flag << endl;