Using the adler32 checksum in Delphi 2007

Delphi 2 Minutes

Delphi 2007 comes with the RTL unit zlib.pas which links to adler32.obj. adler32.obj is a precompiled object file implementing the Adler-32 checksum (most likely implemented in c).

Delphi apparently doesn’t actually use that function since it is simply declared as …

procedure adler32; external;

… which is plain wrong but gets the job done of forcing the compiler / linker to actually include that object file so it is available for the zlib implementation which itself is another .obj file.

Later Delphi versions declare this function as …

function adler32(adler: LongWord; buf: PByte; len: Cardinal): LongWord; cdecl;

… which matches the declaration in the official zlib documentation.

Trying to simply use that object file in Delphi 2007 like this …

Unit u_Adler32;

function adler32(adler: LongWord; buf: PByte; len: Cardinal): LongWord; cdecl;

implementation

{$L adler32.obj} // you need to copy the obj file to the same directory as this unit

function adler32; external;

… results in a compiler error

[DCC Error] u_Adler32.pas(72): E2065 Unsatisfied forward or external declaration: '_llmod'

Fortunately I am not the first one to stumble upon this problem. Arnaud Bouchez with this answer on StackOverflow helped me, to declare the missing functions:

procedure _llmod;
asm
  jmp System.@_llmod
end;

procedure free(P: Pointer); cdecl; { always cdecl }
begin
  FreeMem(P);
end;

function malloc(size: cardinal): Pointer; cdecl; { always cdecl }
begin
  GetMem(Result, size);
end;

So, now it compiles. But does it actually work?

Let’s try the Wikipedia example:

var
  s: AnsiString;
  adler: LongWord;
begin
  s := 'Wikipedia';
  adler := adler32(0, nil, 0);
  adler := adler32(adler, PByte(@s[1]), Length(s));
  WriteLn(adler);

This prints 300286872 which is exactly what the example states, so it can’t be too wrong.

Now, we need some adler-32 checksums for known files to do some more tests or an implementation that is known to work.

I found Adler 32 Hash at conversion-tool.com and let it create checksums for a few texts and files and the result always matched, so now I am fairly certain the above works.

Finding a Windows tool for calculating Adler-32 turned out to be a challenge. I downloaded several, let them virus check on VirusTotal and they came up as containting viruses or trojans. Even though only one virus scanner out of 44 claimed that, I didn’t want to take any risk. I ended up with rehash by Dominik Reichl, which passed the virus scan. The Adler-32 checksums calculated by that tool also matched the above results.

But don’t just take my word for it, do your own tests!

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