After locating the data chunk using mmioDescend, then how i suppose to read and display the sample data into for example into a memo in delphi 7?
I have follow the step like open the file, locating the riff, locating the fmt, locating data chunk.
if (mmioDescend(HMMIO, #ckiData, #ckiRIFF, MMIO_FINDCHUNK) = MMSYSERR_NOERROR) then
SetLength(buf, ckiData.cksize);
mmioRead(HMMIO, PAnsiChar(buf), ckiData.cksize);
I use mmioRead too but i don't know how to display the data.Can anyone help give an example how to use the mmioRead and then display the result?
Well, I'd probably read into a buffer that was declared using a more appropriate type.
For example, suppose your data are 16 bit integers, Smallint in Delphi. Then declare a dynamic array of Smallint.
var
buf: array of Smallint;
Then allocate enough space for the data:
Assert(ckiData.cksize mod SizeOf(buf[0])=0);
SetLength(buf, ckiData.cksize div SizeOf(buf[0]));
And then read the buffer:
mmioRead(HMMIO, PAnsiChar(buf), ckiData.cksize);
Now you can access the elements as Smallint values.
If you have different element types, then you can adjust your array declaration. If you don't know until runtime what the element type is you may be better off with array of Byte and then using pointer arithmetic and casting to access the actual content.
I'd say that the design of the interface to mmioRead is a little weak. The buffer isn't really a string. It's probably best considered as a byte array. But perhaps because C does not have separate byte and character types, the function is declared as taking a pointer to char array. Really the Delphi translation would be better exposing a pointer to byte or even better in my view, a plain untyped Pointer type.
I assumed that you were struggling with interpreting the output of mmioRead since that was the code that you included in the question. But, according to now deleted comments, your question is a GUI question.
You want to add content to a memo. Do it like this:
Memo1.Clear;
for i := low(buf) to high(buf) do
Memo1.Items.Add(IntToStr(buf[i]));
If you want to convert to floating point then, still assuming 16 bit signed data, do this:
Memo1.Clear;
for i := low(buf) to high(buf) do
Memo1.Items.Add(FormatFloat('0.00000', buf[i]/32768.0));//show 5dp
Related
So according to this (applies to the same page for XE4 to XE8):
When a pointer holds the address of another variable, we say that it points to the location of that variable in memory or to the data stored there. In the case of an array or other structured type, a pointer holds the address of the first element in the structure.
To me the above sounds exactly like this:
Remark: Free Pascal treats pointers much the same way as C does. This means that a pointer to some type can be treated as being an array of this type.
From this point of view, the pointer then points to the zeroeth element of this array.
FPC example:
program PointerArray;
var i : Longint;
p : ^Longint;
pp : array[0..100] of Longint;
begin
for i := 0 to 100 do pp[i] := i; { Fill array }
p := #pp[0]; { Let p point to pp }
for i := 0 to 100 do
if p[i]<>pp[i] then
WriteLn (’Ohoh, problem !’)
end.
The example obviously doesn't compile and complains about p - Array type required. I have never seen such shananigans in delphi, but I am very confused by the info from the embarcadero wiki. The wiki itself gives no examples of such use.
Could someone explain to me what is exactly meant by the wiki? And if it is in fact similar to the FPC/C in any way, could someone provide a working example?
In Delphi you have to enable a special compiler mode to be able to treat a pointer as if it were a pointer to an array. That mode is enabled with the POINTERMATH directive like this:
{$POINTERMATH ON}
From the documentation:
Pointer math is simply treating any given typed pointer in some narrow
instances as a scaled ordinal where you can perform simple arithmetic
operations directly on the pointer variable. It also allows you to
treat such a pointer variable as an unbounded array using the array []
operator. Notice in the example above that incrementing the index of
an array of a type is equivalent to incrementing a pointer to that
type. An increment of one bumps the pointer by the size an array
element in bytes, not by one byte.
The POINTERMATH directive has a local scope. That is, if you turn this
directive ON and do not turn it off in the module, it will remain on
only until the end of the module. Also, if you declare a typed pointer
with this directive ON, any variable of that type allows scaled
pointer arithmetic and array indexing, even after the directive has
been turned OFF Similarly, any block of code surrounded by this
directive allows arithmetic operations for ANY typed pointers within
the block regardless of whether the typed pointer was originally
declared with POINTERMATH ON.
This directive affects only typed pointers. Variables of type Pointer
do not allow the pointer math features, since type Pointer is
effectively pointing to a void element, which is 0 bytes in size.
Untyped var or const parameters are not affected because they are not
really pointers.
I'm curious about what happens with this piece of code in Delphi 2010:
function foo: WideString;
var
myUnicodeString: UnicodeString;
begin
for i:=1 to 1000 do
begin
myUnicodeString := ... something ...;
result := result + myUnicodeString; // This is where I'm interested
end;
end;
How many string conversions are involved, and are any particularly bad performance-wise?
I know the function should just return a UnicodeString instead, but I've seen this anti-pattern in the VCL streaming code, and want to understand the process.
To answer your question about what the code is actually doing, this statement:
result := result + myUnicodeString;
Does the following:
calls System._UStrFromWStr() to convert Result to a temp UnicodeString
calls System._UStrCat() to concatenate myUnicodeString onto the temp
calls System._WStrFromUStr() to convert the temp to a WideString and assign it back to Result.
There is a System._WStrCat() function for concatenating a WideString onto a WideString (and System._UStrCat() for UnicodeString). If CodeGear/Embarcadero had been smarter about it, they could have implemented a System._WStrCat() overload that takes a UnicodeString as input and a WideString as output (and vice versa for concatenating a WideString onto a UnicodeString). That way, no temp UnicodeString conversions would be needed anymore. Both WideString and UnicodeString are encoded as UTF-16 (well mostly, but I won't get into that here), so concatenating them together is just a matter of a single allocation and move, just like when concatenating two UnicodeStrings or two WideStrings together.
The performance is poor. There's no need for any encoding conversions since everything is UTF-16 encoded. However, WideString is a wrapper around the COM BSTR type which performs worse than native UnicodeString.
Naturally you should prefer to do all your work with the native types, either UnicodeString or TStringBuilder, and convert to WideString at the last possible moment.
That is generally a good policy. You don't want to use WideString internally since it's purely an interop type. So only convert to (and from) WideString at the interop boundary.
How is a set organized in memory in Delphi?
What I try to do is casting a simple type to a set type like
var
MyNumber : Word;
ShiftState : TShiftState;
begin
MyNumber:=42;
ShiftState:=TShiftState(MyNumber);
end;
Delphi (2009) won't allow this and I don't understand why. It would make my life a lot easier in cases where I get a number where single bits encode different enum values and I just could cast it like this. Can this be done?
One approach I was going to go for is:
var
ShiftState : TShiftState;
MyNumber : Word absolute ShiftState;
begin
MyNumber:=42;
end;
But before doing this I thought I'd ask for the memory layout. It's more a feeling than knowlege what I am having right now about this.
A Delphi set is a bit field who's bits correspond to the associated values of the elements in your set. For a set of normal enumerated types the bit layout is straight-forward:
bit 0 corresponds to set element with ordinal value 0
bit 1 corresponds to set element with ordinal value 1
and so on.
Things get a bit interesting when you're dealing with a non-contiguous set or with a set that doesn't start at 0. You can do that using Delphi's subrange type (example: set of 3..7) or using enumerated types that specify the actual ordinal value for the elements:
type enum=(seven=7, eight=8, eleven=11);
EnumSet = set of enum;
In such cases Delphi will allocate the minimum required amount of bytes that will include all required bits, but will not "shift" bit values to use less space. In the EnumSet example Delphi will use two bytes:
The first byte will have it's 7th bit associated with seven
The second byte will have bit 0 associated with eight
The second byte will have bit 3 associated with eleven
You can see some tests I did over here: Delphi 2009 - Bug? Adding supposedly invalid values to a set
Tests were done using Delphi 2010, didn't repeat them for Delphi XE.
Unfortunately I just now stumbled upon the following question: Delphi 2009 - Bug? Adding supposedly invalid values to a set
The accepted answer of Cosmin contains a very detailed description of what is going on with sets in Delphi. And why I better not used my approach with absolute. Apparently a set variable can take from 1 to 32 byte of memory, depending on the enum values.
You have to choose a correctly sized ordinal type. For me (D2007) your code compiles with MyNumber: Byte:
procedure Test;
var
MyNumber: Byte;
ShiftState: TShiftState;
begin
MyNumber := 42;
ShiftState := TShiftState(MyNumber);
end;
I have used this technique in some situations and didn't encounter problems.
UPDATE
The TShiftState type has been extended since Delphi 2010 to include two new states, ssTouch and ssPen, based on the corresponding doc page (current doc page). The Delphi 2009 doc still has TShiftState defined as a set of 7 states.
So, your attempt to convert Word to TShiftState would work in Delphi 2010+, but Byte is the right size for Delphi 2009.
I use this:
For <= 8 elements, PByte(#MyNumber)^, for <= 16 elements, PWord(#MyNumber)^, etc.
If the enum takes up more space (through the min enum size compiler option) this will still work.
I want a string type that is Unicode and that stores the string directly at the adress of the variable, as is the case of the (Ansi-only) ShortString type.
I mean, if I declare a S: ShortString and let S := 'My String', then, at #S, I will find the length of the string (as one byte, so the string cannot contain more than 255 characters) followed by the ANSI-encoded string itself.
What I would like is a Unicode variant of this. That is, I want a string type such that, at #S, I will find a unsigned 32-bit integer (or a single byte would be enough, actually) containing the length of the string in bytes (or in characters, which is half the number of bytes) followed by the Unicode representation of the string. I have tried WideString, UnicodeString, and RawByteString, but they all appear only to store an adress at #S, and the actual string somewhere else (I guess this has do do with reference counting and such). Update: The most important reason for this is probably that it would be very problematic if sizeof(string) were variable.
I suspect that there is no built-in type to use, and that I have to come up with my own way of storing text the way I want (which actually is fun). Am I right?
Update
I will, among other things, need to use these strings in packed records. I also need manually to read/write these strings to files/the heap. I could live with fixed-size strings, such as <= 128 characters, and I could redesign the problem so it will work with null-terminated strings. But PChar will not work, for sizeof(PChar) = 1 - it's merely an address.
The approach I eventually settled for was to use a static array of bytes. I will post my implementation as a solution later today.
You're right. There is no exact analogue to ShortString that holds Unicode characters. There are lots of things that come close, including WideString, UnicodeString, and arrays of WideChar, but if you're not willing to revisit the way you intend to use the data type (make byte-for-byte copies in memory and in files while still being using them in all the contexts a string could be allowed), then none of Delphi's built-in types will work for you.
WideString fails because you insist that the string's length must exist at the address of the string variable, but WideString is a reference type; the only thing at its address is another address. Its length happens to be at the address held by the variable, minus four. That's subject to change, though, because all operations on that type are supposed to go through the API.
UnicodeString fails for that same reason, as well as because it's a reference-counted type; making a byte-for-byte copy of one breaks the reference counting, so you'll get memory leaks, invalid-pointer-operation exceptions, or more subtle heap corruption.
An array of WideChar can be copied without problems, but it doesn't keep track of its effective length, and it also doesn't act like a string very often. You can assign string literals to it and it will act like you called StrLCopy, but you can't assign string variables to it.
You could define a record that has a field for the length and another field for a character array. That would resolve the length issue, but it would still have all the rest of the shortcomings of an undecorated array.
If I were you, I'd simply use a built-in string type. Then I'd write functions to help transfer it between files, blocks of memory, and native variables. It's not that hard; probably much easier than trying to get operator overloading to work just right with a custom record type. Consider how much code you will write to load and store your data versus how much code you're going to write that uses your data structure like an ordinary string. You're going to write the data-persistence code once, but for the rest of the project's lifetime, you're going to be using those strings, and you're going to want them to look and act just like real strings. So use real strings. "Suffer" the inconvenience of manually producing the on-disk format you want, and gain the advantage of being able to use all the existing string library functions.
PChar should work like this, right? AFAIK, it's an array of chars stored right where you put it. Zero terminated, not sure how that works with Unicode Chars.
You actually have this in some way with the new unicode strings.
s as a pointer points to s[1] and the 4 bytes on the left contains the length.
But why not simply use Length(s)?
And for direct reading of the length from memory:
procedure TForm9.Button1Click(Sender: TObject);
var
s: string;
begin
s := 'hlkk ljhk jhto';
{$POINTERMATH ON}
Assert(Length(s) = (PInteger(s)-1)^);
//if you don't want POINTERMATH, replace by PInteger(Cardinal(s)-SizeOf(Integer))^
showmessage(IntToStr(length(s)));
end;
There's no Unicode version of ShortString. If you want to store unicode data inline inside an object instead of as a reference type, you can allocate a buffer:
var
buffer = array[0..255] of WideChar;
This has two disadvantages. 1, the size is fixed, and 2, the compiler doesn't recognize it as a string type.
The main problem here is #1: The fixed size. If you're going to declare an array inside of a larger object or record, the compiler needs to know how large it is in order to calculate the size of the object or record itself. For ShortString this wasn't a big problem, since they could only go up to 256 bytes (1/4 of a K) total, which isn't all that much. But if you want to use long strings that are addressed by a 32-bit integer, that makes the max size 4 GB. You can't put that inside of an object!
This, not the reference counting, is why long strings are implemented as reference types, whose inline size is always a constant sizeof(pointer). Then the compiler can put the string data inside a dynamic array and resize it to fit the current needs.
Why do you need to put something like this into a packed array? If I were to guess, I'd say this probably has something to do with serialization. If so, you're better off using a TStream and a normal Unicode string, and writing an integer (size) to the stream, and then the contents of the string. That turns out to be a lot more flexible than trying to stuff everything into a packed array.
The solution I eventually settled for is this (real-world sample - the string is, of course, the third member called "Ident"):
TASStructMemHeader = packed record
TotalSize: cardinal;
MemType: TASStructMemType;
Ident: packed array[0..63] of WideChar;
DataSize: cardinal;
procedure SetIdent(const AIdent: string);
function ReadIdent: string;
end;
where
function TASStructMemHeader.ReadIdent: string;
begin
result := WideCharLenToString(PWideChar(#(Ident[0])), length(Ident));
end;
procedure TASStructMemHeader.SetIdent(const AIdent: string);
var
i: Integer;
begin
if length(AIdent) > 63 then
raise Exception.Create('Too long structure identifier.');
FillChar(Ident[0], length(Ident) * sizeof(WideChar), 0);
Move(AIdent[1], Ident[0], length(AIdent) * sizeof(WideChar));
end;
But then I realized that the compiler really can interpret array[0..63] of WideChar as a string, so I could simply write
var
MyStr: string;
Ident := 'This is a sample string.';
MyStr := Ident;
Hence, after all, the answer given by Mason Wheeler above is actually the answer.
Those 3 types are very similar...
TArray is the generic version of TBytes.
Both can be casted to PByteArray and used as buffer for calls to Windows API. (with the same restrictions as string to Pchar).
What I would like to know: Is this behavior "by design" or "By Implementation". Or more specifically, could it break in future release?
//Edit
As stated lower...
What I really want to know is: Is this as safe to typecast TBytes(or TArray) to PByteArray as it is to typecast String to PChar as far as forward compatibility is concerned. (Or maybe AnsiString to PAnsiChar is a better exemple ^_^)
Simply put, an array of bytes is an array of bytes, and as long as the definitions of a byte and an array don't change, this won't change either. You're safe to use it that way, as long as you make sure to respect the array bounds, since casting it out of Delphi's array types nullifies your bounds checking.
EDIT: I think I see what you're asking a bit better now.
No, you shouldn't cast a dynamic array reference to a C-style array pointer. You can get away with it with strings because the compiler helps you out a little.
What you can do, though, is cast a pointer to element 0 of the dynamic array to a C-style array pointer. That will work, and won't change.
Two of those types are similar (identical in fact). The third is not.
TArray is declared as "Array of Byte", as is TBytes. You missed a further very relevant type however, TByteArray (the type referenced by PByteArray).
Being a pointer to TByteArray, PByteArray is strictly speaking a pointer to a static byte array, not a dynamic array (which the other byte array types all are). It is typed in this way in order to allow reference to offsets from that base pointer using an integer index. And note that this indexing is limited to 2^15 elements (0..32767). For arbitrary byte offsets (> 32767) from some base pointer, a PByteArray is no good:
var
b: Byte;
ab: TArray<Byte>;
pba: PByteArray;
begin
SetLength(ab, 100000);
pba := #ab; // << No cast necessary - the compiler knows (magic!)
b := pba[62767]; // << COMPILE ERROR!
end;
i.e. casting an Array of Byte or a TArray to a PByteArray is potentially going to lead to problems where the array has > 32K elements (and the pointer is passed to some code which attempts to access all elements). Casting to an untyped pointer avoids this of course (as long as the "recipient" of the pointer then handles access to the memory reference by the pointer appropriately).
BUT, none of this is likely to change in the future, it is merely a consequence of the implementation details that have long since applied in this area. The introduction of a syntactically sugared generic type declaration is a kipper rouge.