format ELF64

struc Struc_DA {
    .items dq 0
    .count dq 0
    .capacity dq 0
    align 16
}

section '.text' executable align 16

extrn _exit

public _start

;; expects rdi - ptr to dynamic array
;; expects rsi - expected capacity
;
;; returns garbage
da_reserve:
    push rbp
    mov rbp, rsp
    sub rsp, 16

    lea rax, [rdi]

    ;; fasm error: undefined symbol 'Struc_DA.capacity'.
    cmp qword [rax+Struc_DA.capacity], rsi

    ;; . . .

    leave
    ret

_start:
    lea rdi, [my_da]

    call da_reserve

    mov rdi, [my_da.items]
    call plt _exit


section '.data' writeable align 16
my_da Struc_DA
    

struc Struc_DA {
    .items dq 0
    .count dq 0
    .capacity dq 0
    align 16
}
virtual at 0
        Struc_DA Struc_DA   ; <--- label 'Struc_DA' has the same name as the struc 'Struc_DA'
end virtual    

There needs to be a label defined that instantiates the struc:

Code:

struc Struc_DA {
    .items dq 0
    .count dq 0
    .capacity dq 0
    align 16
}
virtual at 0
        Struc_DA Struc_DA   ; <--- label 'Struc_DA' has the same name as the struc 'Struc_DA'
end virtual