How to prevent automatic padding by IDA?

Question

I defined a struct in a header file, similar to this one:

struct STRUCT
{
    char a;
    int b;
};

This is parsed successfully by IDA, however it adds padding bytes after the char:

00000000 STRUCT          struc ; (sizeof=0x4)
00000000 a               db ?
00000001                 db ? ; undefined
00000002 b               dw ?
00000004 STRUCT          ends

I can't remove the padding field using u, so the question is: How can one remove padding fields automatically inserted by IDA, or how can one prevent IDA from creating padding fields?

score 13 · Accepted Answer · answered May 17 '13 at 13:43

13

You can use #pragma pack(1) before the declaration.

answered May 17 '13 at 13:43

Igor Skochinsky

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How can I undo this if I define multiple structs per file? #pragma pack(0)? Does this syntax originate from MSVC? – heinrich5991 May 17 '13 at 13:44
7

#pragma pack(push, 1) / #pragma pack(pop) – Igor Skochinsky May 17 '13 at 15:11
1

Worth noting, that if you did wish a member of your struct to be padded, you can define it thus: __declspec(align(4)) char; – Orwellophile Aug 20 '16 at 09:49

score 9 · Answer 2 · edited Jun 17 '20 at 09:54

9

removing undefined byte

click on an undefined padding byte, then with Shrink struct (right-click menu, or Ctrl-S), choose how many bytes you want to remove - it automatically sets the right amount to the next defined offset.

preventing auto add

It depends on the parameter in the Options/Compiler menu: Change the default alignment to 1 to remove padding, then import your header

edited Jun 17 '20 at 09:54

Community

1

answered May 17 '13 at 13:33

Ange

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How to prevent automatic padding by IDA?

2 Answers2

removing undefined byte

preventing auto add