How to check out the first 4 bytes of a stock-market structure? - General

Igor Makanu 2019.07.07 16:32 #25001

fxsaber:

This is what happens.

tried it like this:

const uint FFFF=0xFFFFFFFF;
//+------------------------------------------------------------------+
struct A pack(4)
  {
   ushort            j1;
   ushort            j2;
  };
//+------------------------------------------------------------------+
struct B pack(8)
  {
   ushort            j1;
   ushort            j2;
  };
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
union UnionCheckByte
  {
   uint              byte_4x4[4];
   A                 a;
   B                 b;
  };
//+------------------------------------------------------------------+
void OnStart()
  {
   UnionCheckByte tst;
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   ArrayPrint(tst.byte_4x4);
// A   
   Print("A:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   A zeroA;
   ZeroMemory(zeroA);
   tst.a = zeroA;
   ArrayPrint(tst.byte_4x4);
   
// B
   Print("B:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   B zeroB;
   ZeroMemory(zeroB);
   tst.b = zeroB;
   ArrayPrint(tst.byte_4x4);
 
   Print("sizeof(A) = ",sizeof(A)," , sizeof(B) = ",sizeof(B));
  }
//+------------------------------------------------------------------+

2019.07.07 18:31:02.708 tst (EURUSD,H1) 4294967295 4294967295 4294967295 4294967295

2019.07.07 18:31:02.708 tst (EURUSD,H1) A

2019.07.07 18:31:02.708 tst (EURUSD,H1) 0 4294967295 4294967295 4294967295

2019.07.07 18:31:02.708 tst (EURUSD,H1) B

2019.07.07 18:31:02.708 tst (EURUSD,H1) 0 4294967295 4294967295 4294967295

2019.07.07 18:31:02.708 tst (EURUSD,H1) sizeof(A) = 4 , sizeof(B) = 4

we still only zero out the first 4 bytes

Igor Makanu 2019.07.07 16:42 #25002

tried another way:

const uint FFFF=0xFFFFFFFF;
//+------------------------------------------------------------------+
union UnionCheckByte
  {
   uint              byte_4x4[4];
   struct uA pack(4) {ushort j1; ushort j2; } a;
   struct uB pack(8) {ushort j1; ushort j2; } b;
  };
//+------------------------------------------------------------------+
void OnStart()
  {
   UnionCheckByte tst;
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   ArrayPrint(tst.byte_4x4);
// A   
   Print("A:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   
   ZeroMemory(tst.a);
   ArrayPrint(tst.byte_4x4);
   
// B
   Print("B:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   ZeroMemory(tst.b);
   ArrayPrint(tst.byte_4x4);
 
   Print("sizeof(A) = ",sizeof(tst.a)," , sizeof(B) = ",sizeof(tst.b));
  }
//+------------------------------------------------------------------+

still no result - only clears the first 4 bytes

fxsaber 2019.07.07 16:49 #25003

Igor Makanu:

still no result - only clears the first 4 bytes

Because physically the structure is of 4 bytes.

Igor Makanu 2019.07.07 17:02 #25004

fxsaber:

Because physically the structure is 4 bytes.

yes, we already figured out that pack() doesn't work inside MQL - I don't have any more options to checkpack()

I thought about checking in .dll in C# but there's a problem there too - MQL sees the signature of the called function (remember the const call in signatures didn't work for C# )

and when transferring data to C# will it be possible to do type conversion?

ZS: at most, write a .dll in C++ - there you can "clean" data to receive / send

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Alexey Viktorov 2019.07.07 17:09 #25005

fxsaber:

I don't remember if it's in the documentation.

Thank you, I found it. Everything works in my opinion. Since in your example there is only one type in the structure, there is no point in alignment. That's why it only gives the size of the type. And here you have

struct A pack(4)
{
  short j;
  int   z;
};

void OnStart()
{
  Print(sizeof(A)); // 8
}

Here is the structure

struct A pack(8)
{
  short   j;
  double  x;
  int     b;
};

void OnStart()
{
  Print(sizeof(A)); // 24
}

The documentation explains it very clearly in pictures.

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Igor Makanu 2019.07.07 17:23 #25006

Alexey Viktorov:

Thank you, I found it. Everything works in my opinion. Since in your example there is only one type in the structure, there is no point in alignment. That's why it gives only type size. And here we have

And this structure

The documentation explains it very clearly in pictures.

Yes, that's how it works:

const uint FFFF=0xFFFFFFFF;
//+------------------------------------------------------------------+
union UnionCheckByte
  {
   uint              byte_4x4[4];
   struct uA pack(4) {ushort j1; ulong j2; } a;
   struct uB pack(8) {ushort j1; ulong j2; } b;
  };
//+------------------------------------------------------------------+
void OnStart()
  {
   UnionCheckByte tst;
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   ArrayPrint(tst.byte_4x4);
// A   
   Print("A:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   
   ZeroMemory(tst.a);
   ArrayPrint(tst.byte_4x4);
   
// B
   Print("B:");
   tst.byte_4x4[0]=FFFF;   tst.byte_4x4[1]=FFFF;   tst.byte_4x4[2]=FFFF;   tst.byte_4x4[3]=FFFF;   // заполним память 0xFFFFFFFF FFFFFFFF
   ZeroMemory(tst.b);
   ArrayPrint(tst.byte_4x4);
 
   Print("sizeof(A) = ",sizeof(tst.a)," , sizeof(B) = ",sizeof(tst.b));
  }
//+------------------------------------------------------------------+

2019.07.07 19:16:27.100 tst (EURUSD,H1) 4294967295 4294967295 4294967295 4294967295

2019.07.07 19:16:27.100 tst (EURUSD,H1) A

2019.07.07 19:16:27.100 tst (EURUSD,H1) 0 0 0 0 4294967295

2019.07.07 19:16:27.100 tst (EURUSD,H1) B:

2019.07.07 19:16:27.100 tst (EURUSD,H1) 0 0 0 0

2019.07.07 19:16:27.100 tst (EURUSD,H1) sizeof(A) = 12 , sizeof(B) = 16

total: pack() performs data alignment in the structure by the size of the maximum sizeof() field of the structure

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Alexey Viktorov 2019.07.07 17:29 #25007

Igor Makanu:

summary: pack() aligns data in the structure by the size of the maximum sizeof() field of the structure

Not exactly.

struct A pack(4)
{
  short   j;
  double  x;
  int     b;
};

void OnStart()
{
  Print(sizeof(A)); // 16
}

The maximum field is 8 bytes; we set alignment at 4 and get 16. That is 2+2|4|2+2|2+addshort doubleint

Or 2+addition|4|4|4

Igor Makanu 2019.07.07 17:47 #25008

Alexey Viktorov:

Not exactly.

The maximum field is 8 bytes, set the alignment to 4 and get 16. So 2+2|4|2+2|2+add short doubleint

Or 2+addition|4|4|4

yes, that should probably sound right:

summary: pack() performs element by element alignment of data in the structure; each element will be appended with a value multiple of pack() taking into account the maximum sizeof() of the structure element (no more than the maximum value, less may be in appending)

pack(4) :

short = 2 + 2 appended = 4=pack(4)

double =8 - do not collapse

int =4 - not aligned

sizeof()=16 bytes

Packing structures in memory Get the number of My approach. The core

Alexey Viktorov 2019.07.07 18:06 #25009

Igor Makanu:

yes, that's probably how it should sound:

total: pack() performs data alignment in the structure element by element, each element will be a multiple of pack() taking into account the maximum sizeof() of the structure element (no more than the maximum value, less may be in addition)

pack(4) :

short = 2 + 2 appended = 4=pack(4)

double =8 - do not collapse

int =4 - not aligned

sizeof()=16 bytes

It sounds convoluted, but it makes sense to me.))

fxsaber 2019.07.07 18:51 #25010

Alexey Viktorov:

Sounds convoluted, but makes sense to me.)))

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fxsaber, 2019.07.07 15:03

Theorder of the fields affects memory consumption and apparently performance.

Haven't been able to fully understand it yet.

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