Hi,

I encountered unpredictable behavior with the following code:

var
  fs: TFileStream;
  s: string;
begin
  ... ...
  fs.Write(s, Length(s));
  ... ...
end;

But if I do this, then problem is gone:

var
  fs: TFileStream;
  s: string;
  buf: PChar;
begin
  ... ...
  buf := GetMem(Length(s) + 1);
  StrPCopy(buf, s);
  fs.Write(buf^, Length(s));
  ... ...
end;

My question is, why the above code with string works sometime, but not alwasy fail? What is the internal structure of string and variable array (i.e. array of something)? What is the difference between @string and @string[1] or @array and @array[0]?

Thanks
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My question is, why the above code with string works sometime, but not

alwasy fail?

TStream.Write expects a buffer, typically the first element of an item (NOT a pointer to it) and the length (number of elements after first one).

fs.Write(s, Length(s));

Here you try to give the WHOLE string, which is variable depending on the string type. It could be plain array for ShortString, but also pointer to a dynamically allocated structure for other strings. For ShortStrings, however, the above code will have the string length (if I'm not mistaken) before the first character in the written file.

To do it correctly without converting to PChar is to give the first element:

fs.Write(s[1], Length(s));

What is the internal structure of string and variable array (i.e. array of something)?

Explained in docs:
- static array:
http://www.freepascal.org/docs-html/prog/progsu161.html#x204-2170008.2.9
- dynamic array:
http://www.freepascal.org/docs-html/prog/progsu162.html#x205-2180008.2.10
- strings:
http://www.freepascal.org/docs-html/prog/progsu159.html#x202-2120008.2.7

What is the difference between @string and @string[1] or @array and @array[0]?

@string means pointer to to the string STRUCTURE, for ShortString, this is the same as @string[0] (the length)
@string[1] means pointer to the first character in the string @array means pointer to the array STRUCTURE, for static array, this is the same as @array[<lowest index>]
@array[0] means pointer to the element at index 0

If a buffer expect the first "element", and count is the number of elements to write, then what is the unit size of that element? is it always "byte"?

2013/3/3 leledumbo

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Xiangrong Fang wrote

If a buffer expect the first "element", and count is the number of elements to write, then what is the unit size of that element? is it always "byte"?

Yes - count is the number of bytes, not the number of elements.

If a buffer expect the first "element", and count is the number of elements

to write, then what is the unit size of that element? is it always "byte"?

From
http://www.freepascal.org/docs-html/rtl/classes/thandlestream.write.html: "Write overrides the Write method of TStream. It uses the Handle property to write the Count BYTES from Buffer."