CODEWITHSUNDEEP
assemblyx86nasm
Alison Costa
Alison Costa

Reputation: 83

Pointer for the first struct member list in nasm assembly

How can i declare a pointer for the first element in a list of a struct like this:

section .bss
struc agenda
     name   resb 10
     number resd 10
     type   resw 10
endstruc

Upvotes: 5

Views: 7226

Answers (2)

Frank Kotler
Frank Kotler

Reputation: 3119

Simply declaring the structure does not reserve memory for it. You need an instance of it. Either: 

section .bss
    my_agenda resb agenda_size
; or perhaps...
    agenda_array resb agenda_size * MAX_COUNT
; or...
section .data
    an_agenda istruc agenda
    at name db "fred"
    at number db "42"
    at type db "regular"
    iend
section .text
    mov esi, an_agenda
    mov al, [esi + name]

Something like that?

Heh! Jester has just posted essentially the same thing. He introduces the '.' notation for local labels. Probably a good idea. Without it, name is a global identifier and can't be reused. It takes slightly more typing - agenda.name, agenda.number, agenda.type. Probably worth it for increased clarity.

Upvotes: 6

Jester
Jester

Reputation: 58822

As usual, you should have consulted the fine nasm manual before asking. It's not too late to do that now, but I'll quickly give you the important points from there.

struc itself doesn't allocate a structure, it defines a type. As recommended practice, you should name your fields starting with a dot (.). Each field label will be equal to its offset in the structure, though a base address can also be added. Having declared your struct, you can allocate initialized instances with the istruc directive. In the .bss section, you'd want to use resb instead, using the struc_size symbol that the assembler defines for you.

A complete example which declares the type, defines a zero-initialized instance in the bss section and loads the address of the first member may look like this:

struc agenda
     .name   resb 10
     .number resb 10
     .type   resb 10
endstruc

section .bss

myagenda: resb agenda_size

section .text
    mov eax, myagenda + agenda.name

Note: agenda.name is of course 0, I have written it out just as an illustration how you'd do it for other members.

Upvotes: 6

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