Delphi: Alternative to using Assign/ReadLn for text file reading
- by Ian Boyd
i want to process a text file line by line. In the olden days i loaded the file into a StringList:
slFile := TStringList.Create();
slFile.LoadFromFile(filename);
for i := 0 to slFile.Count-1 do
begin
oneLine := slFile.Strings[i];
//process the line
end;
Problem with that is once the file gets to be a few hundred megabytes, i have to allocate a huge chunk of memory; when really i only need enough memory to hold one line at a time. (Plus, you can't really indicate progress when you the system is locked up loading the file in step 1).
The i tried using the native, and recommended, file I/O routines provided by Delphi:
var
f: TextFile;
begin
Assign(filename, f);
while ReadLn(f, oneLine) do
begin
//process the line
end;
Problem withAssign is that there is no option to read the file without locking (i.e. fmShareDenyNone). The former stringlist example doesn't support no-lock either, unless you change it to LoadFromStream:
slFile := TStringList.Create;
stream := TFileStream.Create(filename, fmOpenRead or fmShareDenyNone);
slFile.LoadFromStream(stream);
stream.Free;
for i := 0 to slFile.Count-1 do
begin
oneLine := slFile.Strings[i];
//process the line
end;
So now even though i've gained no locks being held, i'm back to loading the entire file into memory.
Is there some alternative to Assign/ReadLn, where i can read a file line-by-line, without taking a sharing lock?
i'd rather not get directly into Win32 CreateFile/ReadFile, and having to deal with allocating buffers and detecting CR, LF, CRLF's.
i thought about memory mapped files, but there's the difficulty if the entire file doesn't fit (map) into virtual memory, and having to maps views (pieces) of the file at a time. Starts to get ugly.
i just want Assign with fmShareDenyNone!
