Converting from TClientSocket to Indy: What is equivalent to ReceiveLength? - sockets

Question

I am using XE7 and converting an app from using the ScktComp components to Indy, using TIdTCPClient in place of TClientSocket. At present I am just putting in likely equivalents to get it to compile. Most of it has been converted, except for this snippet:
if (Socket.ReceiveLength > 0) then
begin
s := Socket.ReceiveText;
which i have converted to
s := Socket.IOHandler.ReadLn
I have no equivalent for ReceiveLength.
Any ideas?

Answer 1

IOHandler.ReadLn() is not the correct equivalent of ReceiveText().
ReceiveLength() returns the number of unread bytes that are currently in the socket's buffer. ReceiveText() simply reads whatever raw bytes are currently in the socket's buffer and returns them in a string variable. It is a wrapper for a single call to ReceiveBuf() using ReceiveLength() as the buffer size.
IOHandler.ReadLn() reads from the IOHandler's own memory buffer, populating it with bytes from the socket's buffer as needed, until it encounters the specified terminator (which is a LF character by default), no matter how many reads it takes to accomplish that.
There is no direct translation of ReceiveLength() in Indy, however the closest equivalent to your snippet would be to call IOHandler.CheckForDataOnSource() followed by IOHandler.InputBufferAsString(), eg:
Socket.CheckForDataOnSource(0);
if not Socket.InputBufferIsEmpty then
begin
s := Socket.InputBufferAsString;
Alternatively, you can IOHandler.ReadBytes() with its AByteCount parameter set to -1, and then convert the returned byte array to a string:
buf := nil;
Socket.ReadBytes(buf, -1);
if buf <> nil then
begin
s := BytesToString(buf);
That being said, I have to ask why you are using ReceiveText() in the first place. It returns a string of arbitrary bytes, so it doesn't really lend itself well to most communication needs, can break up textual data in unpredictable ways, and is not suited for binary data at all. Network protocols usually have structure to them, and TClientSocket usage typically requires code to manually buffer bytes and parse structured data from that buffer - things Indy is designed to handle for you. You should focus more on the goal you want to achieve and less on the particulars of how to get it. If you need to read an integer, ask Indy to read an integer. If you need to read a string of a particular length or ending with a particular delimiter, ask Indy to read a string. If you need to read a block of X bytes, ask Indy to read X bytes. Indy has many read/write methods available to automate common tasks that you would normally have to do manually.