CODEWITHSUNDEEP
delphiunicodefontsrange
Paul
Paul

Reputation: 26660

WinAPI: GetFontUnicodeRanges - I do not understand the result

I am trying to get Unicode font glyph ranges (Delphi 6):

var GS:PGlyphSet;
    GSSize:LongWord;
    rng:TWCRange;
begin
  GSSize := GetFontUnicodeRanges(Canvas.Handle, nil);
  GetMem(Pointer(GS), GSSize);
  try
    GS.cbThis:=GSSize;
    GS.flAccel:=0;
    GS.cGlyphsSupported:=0;
    GS.cRanges:=0;
    if GetFontUnicodeRanges(Canvas.Handle, GS)<>0 then begin
      for i:=0 to GS.cRanges-1 do begin
        rng := GS.ranges[i];

The strange thing is that Length(GS.ranges) is 1, but GS.cRanges is 309 and when I try to access the second range GS.ranges[1] I get, of course, a range check error. Before I turned range checking on it has worked in some magical way.

Types for reference (from Windows module):

PWCRange = ^TWCRange;
{$EXTERNALSYM tagWCRANGE}
tagWCRANGE = packed record
  wcLow: WCHAR;
  cGlyphs: SHORT;
end;
TWCRange = tagWCRANGE;

PGlyphSet = ^TGlyphSet;
{$EXTERNALSYM tagGLYPHSET}
tagGLYPHSET = packed record
  cbThis: DWORD;
  flAccel: DWORD;
  cGlyphsSupported: DWORD;
  cRanges: DWORD;
  ranges: array[0..0] of TWCRange;
end;
TGlyphSet = tagGLYPHSET;

Upvotes: 4

Views: 576

Answers (1)

David Heffernan
David Heffernan

Reputation: 612993

This struct makes use of the so-called struct hack:

The ranges member is a variable length array, placed inline in the struct. But you cannot actually encode that in a static C type. That's why you call the function to find out how much memory to allocate, and then heap allocate the struct. If you allocated it on the stack, or using SizeOf(...) then the struct would be too small.

The simplest thing to do is to disable range checking for the code that accesses ranges. Although the type declaration says that only 0 is a valid index for ranges, in fact 0..cRanges-1 are valid.

If you don't want to disable range checking for the relevant code, then take a pointer the element 0, and then use pointer arithmetic in your loop.

var
  rng: PWCRange; 
....
rng := @GS.ranges[0];
for i:=0 to GS.cRanges-1 do begin
  // use rng^
  inc(rng);
end;

This is, in my view, the cleanest way to write code for sequential access. For random access, and with range checking in force, you'd be compelled to declare some extra types to defeat range checking:

type 
  TWCRangeArray = array [0..(MaxInt div SizeOf(TWCRange))-1] of TWCRange; 
  PWCRangeArray = ^TWCRangeArray;

And then use type casting to access individual elements:

rng := PWCRangeArray(@GS.ranges)[i];

Upvotes: 6

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