Difference between revisions of "TWSocket.ReceiveFrom"
| Line 16: | Line 16: | ||
4) the size of the TSockAddr | 4) the size of the TSockAddr | ||
| − | The return value of ReceiveFrom specifies how many bytes | + | The return value of ReceiveFrom specifies how many bytes were actually read and placed in the buffer and the information about the sender is placed in the "from" variable. If the buffer is too small, OnDataAvailable will occur again until all data is received. |
| − | If the return value is 0 the connection the data should be read from has been closed and a value of less than 0 means an error occurred and the SOCKET_ERROR constant value will be returned. In order to retrieve the | + | If the return value is 0 the connection the data should be read from has been closed and a value of less than 0 means an error occurred and the SOCKET_ERROR constant value will be returned. In order to retrieve the actual error code you must check the LastError property immediately, before any other call might lead to another error. |
If the LastError error code is WSAE_WOULD_BLOCK, it means ReceiveFrom was called at a point in time when the system was not ready to process the call. Most likely you can ignore this error condition. | If the LastError error code is WSAE_WOULD_BLOCK, it means ReceiveFrom was called at a point in time when the system was not ready to process the call. Most likely you can ignore this error condition. | ||
| Line 38: | Line 38: | ||
== Best practices == | == Best practices == | ||
| − | * never do any message processing like Application.ProcessMessages or GetMessage inside | + | * never do any message processing like Application.ProcessMessages or GetMessage inside an event handler, since it will raise the same event again for the very same data. |
* if the data received needs lengthy processing, a separate thread for data processing might be a good idea since further data can only be received after the event has been processed completely. In most cases a separate thread is not necessary. | * if the data received needs lengthy processing, a separate thread for data processing might be a good idea since further data can only be received after the event has been processed completely. In most cases a separate thread is not necessary. | ||
== How to == | == How to == | ||
Latest revision as of 23:32, 26 October 2017
Main page -> ICS component reference -> TWSocket -> ReceiveFrom
Definition
function ReceiveFrom(Buffer:Pointer; BufferSize:integer; var from:TSockAddr;
var fromSize:integer):integer;
Description
ReceiveFrom is used in the OnDataAvailable-Event to fetch the received data and information about the sender.
The required parameters are: 1) buffer where the data will be placed 2) the size of the buffer 3) TSockAddr variable 4) the size of the TSockAddr
The return value of ReceiveFrom specifies how many bytes were actually read and placed in the buffer and the information about the sender is placed in the "from" variable. If the buffer is too small, OnDataAvailable will occur again until all data is received.
If the return value is 0 the connection the data should be read from has been closed and a value of less than 0 means an error occurred and the SOCKET_ERROR constant value will be returned. In order to retrieve the actual error code you must check the LastError property immediately, before any other call might lead to another error.
If the LastError error code is WSAE_WOULD_BLOCK, it means ReceiveFrom was called at a point in time when the system was not ready to process the call. Most likely you can ignore this error condition.
Example
procedure WSocket1OnDataAvailable(Sender: TObject; ErrCode: Word);
var
buf:array[1..64] of byte;
len, fromSize:integer;
from:TSockAddr;
begin
if ErrCode <> 0 then Exit;
fromSize:=sizeOf(from);
len:=ReceiveFrom(@buf, sizeof(buf), from, fromSize);
if (len > 0) then
// here your procesing of the data will take place
end;
Best practices
- never do any message processing like Application.ProcessMessages or GetMessage inside an event handler, since it will raise the same event again for the very same data.
- if the data received needs lengthy processing, a separate thread for data processing might be a good idea since further data can only be received after the event has been processed completely. In most cases a separate thread is not necessary.
