"Fixed" Structures exporting problem

IDA General Bugs
1

Hi, I feel there is a significant bug in exporting (to c) “fixed-size” structures:

  1. The size itself isn’t exported
  2. The alignment isn’t exported
  3. “padding bytes” inside aren’t exported

Experiment 1

created fixed-size struct “struct_test” in size 0x2a:

00000000 struct __fixed struct_test // sizeof=0x2A
00000000 {
00000000 __int16 field_0;
00000002 __int16 field_2;
00000004 char field_4;
00000005 char field_5;
00000006 char field_6;
00000007 int field_7;
0000000B __int16 field_B;
0000000D char field_D;
0000000E char field_E;
0000000F char field_F;
00000010 char field_10;
00000011 char field_11;
00000012 char field_12;
00000013 char field_13;
00000014 // padding byte
00000015 // padding byte
00000016 char field_16;
00000017 char field_17;
00000018 char field_18;
00000019 int field_19;
0000001D // padding byte
0000001E __int64 field_1E;
00000026 __int16 field_26;
00000028 char field_28;
00000029 // padding byte
0000002A };

When I export it to .h it looks:

struct __fixed struct_test
{ 
  __int16 field_0;
  __int16 field_2;
  char field_4;
  char field_5;
  char field_6;
  int field_7;
  __int16 field_B;
  char field_D;
  char field_E;
  char field_F;
  char field_10;
  char field_11;
  char field_12;
  char field_13;
  char field_16;
  char field_17;
  char field_18;
  int field_19;
  __int64 field_1E;
  __int16 field_26;
  char field_28;
};

And When I actually import it to IDA it looks:

00000000 struct struct_test // sizeof=0x30
00000000 {
00000000     __int16 field_0;
00000002     __int16 field_2;
00000004     char field_4;
00000005     char field_5;
00000006     char field_6;
00000007     // padding byte
00000008     int field_7;
0000000C     __int16 field_B;
0000000E     char field_D;
0000000F     char field_E;
00000010     char field_F;
00000011     char field_10;
00000012     char field_11;
00000013     char field_12;
00000014     char field_13;
00000015     char field_16;
00000016     char field_17;
00000017     char field_18;
00000018     int field_19;
0000001C     // padding byte
0000001D     // padding byte
0000001E     // padding byte
0000001F     // padding byte
00000020     __int64 field_1E;
00000028     __int16 field_26;
0000002A     char field_28;
0000002B     // padding byte
0000002C     // padding byte
0000002D     // padding byte
0000002E     // padding byte
0000002F     // padding byte
00000030 };

I work a lot with those fixed-size structure and now when I tried to share with colleagues some header files the IDBs getting messed up :frowning:

Would be glad if you could fix it soon, Please.

Regards, Uriel

2

Thank you for the report! I would suggest two workarounds:

  1. for important structures, turn off the “Fixed layout” flag and make sure the layout is still correct (e.g. add explicit padding members or alignment).
  2. use File > Produce file > Export typeinfo to IDC instead of header export. This uses low level API which should retain all struct attributes.
3

Thank you for the workarounds, I’ll use them in the meantime.

For the long term I’d really love to use the fixed-layout (great feature!), and I still prefer to export as (c) because I use the LabSync plugin which is much user-friendly this way. Do you have any estimation when it will be fixed?

4

We need to invent a way to represent fixed structures in C/C++, and more importantly, teach clang to support them. I’m not sure if it is possible to give you an estimate of the work needed for that.

5

perhaps LabSync can be modified to use the low-level APIs for fixed structs (or even all types)? We’ll be happy to help the maintainers in this task.

6

Well I’m not LabSync maintainer but I guess it’s a change that can be done, although I can see why it’s a great bonus to have your local types history backed up in a Git repo

Thank you.