const in struct

[mabdelouahab]

Hello forum members
How do I define the STRUCT, be one of the variables constant. like:
s   Struct
   A_   dd   25  ; < ---- Const and read only
   B_   dd   13  ; < ---- Const and read only
   C_   dd   ?
   D_   dd   ?
s    ends

, Or a pointer to a constant, where I can use directly, Like:
.const
   s0   Struct
      A_   dd   ?
      B_   dd   ?
   s0    ends
   t  s0    <25,13>
.data?
   s   Struct
      t0   s0   <t>   ; <--------- const
      C_   dd   ?
      D_   dd   ?
   s    ends
Or a change (dynamic ) Attribute of "A_ and B_" variable in the structure "s" to become read-only

[jj2007]

This works, but you cannot set selected elements to read only:

s   Struct
   A_   dd   25  ; < ---- Const and read only
   B_   dd   13  ; < ---- Const and read only
   C_   dd   ?
   D_   dd   ?
s    ends

.data
MyS   s <>

Well, you can put it in the .code section to make it read-only, but then also C_ and D_ are read-only...

[mabdelouahab]

Hi jj2007
What I want to do is to make A_ and B_(only)  a read-only, and leave the rest as is

[jj2007]

I've understood what you want, but it's not possible. Why do you want it that way?

[mabdelouahab]

I want to create  (object)
where :variables, constants, and functions.
In the form of STRUCT
For example:
Obj struc
     A_                    DWORD      10   ;< -- Const
     B_            DWORD      255   ;< -- Const
     C_            Byte      ?   ;< -- Variable
     D_            Word      ?   ;< -- Variable
     F01_            Lp1TDF      ?   ;< -- Function
     F02_            Lp2TDF      ?   ;< -- Function
Obj ends

[jj2007]

You can do that, no problem. Just don't write to A_ and B_ ...

A structure is just an area in memory. You cannot write-protect 8 bytes and leave the following n bytes unprotected. Check VirtualProtect for more info.

[mabdelouahab]

I am using the function "VirtualProtect" (dynamic), but has disables all access to A_ and B_
I want to also use the method works with all platforms windows

[dedndave]

changing memory attributes is on a "per-page" basis
on all current versions of windows, a page is 4 Kb

in assembly language, it's a little bit unnecessary to write-protect variables
that's because the programmer has complete control over what is and isn't written to

[qWord]

Actual such attribute (const) does not require the memory to be read only - it is the compiler (in our case this would be the assembler) who check the "access direction" while compiling and throws errors when writing to read-only members. Unless not replacing all instructions with macros who does the check, I can't see a chance  to implement such feature with MASM.

[MichaelW]

In my test, using the VC++ Toolkit 2003 compiler, my attempt to modify a const structure member in C code was flagged as an error, but I could modify the member in inline assembly, so clearly it was not stored as read-only.

Code Select Expand


#include <windows.h>
#include <conio.h>
#include <stdio.h>

struct
{
    int x;
    int const y;
} s;

int main( void )
{
    s.x = 1;
    //s.y = 2; // test.c(14) : error C2166: l-value specifies const object
    __asm mov s.y, 3
    printf("%d\n", s.y);
    getch();
}


And same result for this version:

Code Select Expand


#include <windows.h>
#include <conio.h>
#include <stdio.h>

struct S
{
    int x;
    int const y;
};

int main( void )
{
    struct S s = {1,2};
    s.x = 1;
    //s.y = 2; // test.c(15) : error C2166: l-value specifies const object
    __asm mov s.y, 3
    printf("%d\n", s.y);
    getch();
}