About structs

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jorido

Joined: 23 Jan 2017
Posts: 53

jorido

I've read the documentation but I can't figure out this. In Nasm I'd first declare a struct in ".bss" and then define it in ".data":

Code:

                section ".bss"

                struc my_struct
                    .a resw 1
                    .b resw 1
                    .c resb 1
                    .d resb 1
                endstruc


                section ".data"

                my_struct_var1 istruc my_struct
                    at my_struct.a, dw 123
                    at my_struct.b dw, 0x123
                    at my_struct.c db, "fdsfds"
                    at my_struct.d db 2222
                endstruc

How can I do this in FASM exactly?

Code:

        ; declaring

        struct my_struct
            .a rw 1
            .b rw 1
            .c rb 1
            .d rb 1
        ends

        ; or
        ; what's the difference between these 2?
        
        struct my_struct
            .a dw ?
            .b dw ?
            .c db ?
            .d db ?
        ends

1) Is that correct? Or should I use the macros "sturc { ... }" If so, how exactly?

2) How can I initialize it in ".data"?

3) also there's a question in my code

It's for Linux x64

30 Jan 2017, 08:35

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16931
Location: In your JS exploiting you and your system

revolution

struc is the fasm native version, struct is the fasm high-level macro.

In fasm you can declare struc's anywhere before you use them no matter the current section, it makes no difference.

Code:

;declare
struc my_struc a,b,c,d {
        .a dw a
        .b dw b
        .c db c
        .d db d
}

;instantiate
my_label my_struc 0x3456,0x9876,'X','r'

;access
mov ax,[my_label.a]

31 Jan 2017, 03:10

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Can "struc" be used interchangeably with "struct" everywhere then?

31 Jan 2017, 06:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16931
Location: In your JS exploiting you and your system

revolution

No. They are different things.

For example:
struc uses curly brackets, struct doesn't.
struc uses dots, struct doesn't.
struc allows embedded instructions, struct doesn't
etc.

31 Jan 2017, 06:08

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

I mean, when I want to is merely define a simple structure with a few fields:

Code:

MyStruct:
  var1 'aaa'
  var2 ?
  var3 ?
  var4 123

can "struc" be used interchangeably with "struct"?

31 Jan 2017, 08:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16931
Location: In your JS exploiting you and your system

revolution

No. Both the syntax and the operation are different.

31 Jan 2017, 09:12

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

What those differences are? And when should be 1st and when 2nd?

31 Jan 2017, 13:09

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16931
Location: In your JS exploiting you and your system

revolution

struc defines a new macro with all the normal features of a macro, except that it requires a label first, but otherwise there is no difference. You can make it do almost anything you need. If you have special or unusual requirements then this might be the one you want.

struct is already a macro with a whole lot of extra processing to define things like the size of each element, and it allows union and various other things. But the usage is mostly fixed. You can't make it do special processing for example, but it can already do many things so it might be just what you need for a particular purpose.

To decide which one to use it depends upon what you need from it. It would be too complex to try to enumerate all the various situations that could come up. Your question is very broad. If you have a particular use case then please ask about that and we can try to help you.

31 Jan 2017, 13:36

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Thanks.

31 Jan 2017, 14:33

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