SOLVED: Detect if macOS file is readable

[ChrisR]

For GUI applications, a file may be readable based on file permissions, but not readable as it is outside the applications sand box. This is described hear with detection code:
  https://wiki.freepascal.org/macOS_Programming_Tips#Determining_if_a_file_is_readable

However, this detection fails if the file path is longer than 255 bytes
  https://github.com/neurolabusc/surf-ice/issues/35

Are there any comments regarding my suggested code, which handles long file names:

Code: Pascal  [Select][+][-]

1. function IsReadable (fnm: string): boolean;
2. //https://wiki.freepascal.org/macOS_Programming_Tips#Determining_if_a_file_is_readable
3. var
4.   fs : TFileStream;  
5.   b: byte;
6. begin
7.   result := false;
8.   if not fileexists(fnm) then begin
9.     //writeln('File does not exist: '+fnm);
10.     exit;
11.   end;
12.   if FSize(fnm) < 2 then begin
13.     //writeln('Empty file: '+fnm);
14.     exit;
15.   end;
16.   {$IFDEF Darwin}
17.   {$I-}
18.   try
19.   fs := TFileStream.Create (fnm, fmOpenRead or fmShareDenyWrite);
20.   except
21.     //writeln('Unable to open (permissions): '+fnm);
22.     exit(false);
23.   end;
24.   try
25.     fs.Seek(0, soFromBeginning);
26.     fs.Read(b, 1);
27.     //writeln(inttostr(b));
28.   except
29.     //writeln('Unable to read: '+fnm);
30.     exit(false);
31.   end;
32.   {$ENDIF}
33.   result := true;
34. end;
35.  

[KodeZwerg]

You forgot to free the created object.

[KodeZwerg]

tested :

Code: Pascal  [Select][+][-]

1. function IsReadable(const AFilename: string): Boolean;
2. var
3.   fs: TFileStream;
4.   b : Byte;
5. begin
6.   // whatever happen, it is wrong :-)
7.   Result := False;
8.   try
9.     // try connect to target
10.     fs := TFileStream.Create(AFilename, fmOpenRead or fmShareDenyWrite);
11.     try
12.       // ensure that what we want to read is available
13.       if (fs.Size >= SizeOf(Byte)) then
14.         begin
15.           try
16.             // turn compiler warning off
17.             b := 0;
18.             // jump to offset
19.             fs.Position := 0;
20.             // compare if the size we want to test is given back
21.             Result := (SizeOf(Byte) = fs.Read(b, SizeOf(Byte)));
22.           except
23.             // something went wrong at reading
24.           end;
25.         end;
26.     finally
27.       // free the object
28.       fs.Free;
29.     end;
30.   except
31.     // something went wrong on accessing or in general
32.     // file not found
33.     // access error like forbid, no permission etc...
34.   end;
35. end;
36.  

added explanation.

[ChrisR]

Thanks for the elegant code!

[KodeZwerg]

Thanks for the elegant code!

You are very welcome!