Put data back in socket buffer - windows

Short question, didn't seem to find anything useful here or on Google: in the Winsock2 API, is it possible to put data back in the sockets internal buffer when you have retrieved it using recv() for example, so that is seems it was never actually read from the buffer?

No, it is not possible to inject data back into the socket's internal buffer. Either use the MSG_PEEK flag to read data without removing it from the socket's buffer, or else read the socket data into your own buffer, and then do whatever you want with your buffer. You could have your reading I/O logic always look for data in your buffer first, and then read more data from the socket only when your buffer does not have enough data to satisfy the read operation. Any data you inject back into your buffer will be seen by subsequent read operations.

You can use the MSG_PEEK flag in your recv() call

Related

How to save, and then serve again data of type io.Reader?

I would like to parse several times with gocal data I retrieve through a HTTP call. Since I would like to avoid making the call for each of the parsing, I would like to save this data and reuse it.
The Body I get from http.Get is of type io.ReadCloser. The gocal parser requires io.Reader so it works.
Since I can retrieve Body only once, I can save it with body, _ := io.ReadAll(get.Body) but then I do not know how to serve []byte as io.Reader back (to the gocal parser, several times to account for different parsing conditions)
As you have figured, the http.Response.Body is exposed as an io.Reader, this reader is not re usable because it is connected straight to the underlying connection* (might be tcp/utp/or any other stream like reader under the net package).
Once you read the bytes out of the connection, new bytes are sitting their waiting for another read.
In order to save the response, indeed, you need to drain it first, and save that result within a variable.
body, _ := io.ReadAll(get.Body)
To re use that slice of bytes many time using the Go programming language, the standard API provides a buffered reader bytes.NewReader.
This buffer adequately offers the Reset([]byte) method to reset the state of the buffer.
The bytes.Reader.Reset is very useful to read multiple times the same bytes buffer with no allocations. In comparison, bytes.NewReader allocates every time it is called.
Finally, between two consecutive calls to c.Parser, you should reset the buffer with bytes buffer you have collected previously.
such as :
buf := bytes.NewReader(body)
// initialize the parser
c.Parse()
// process the result
// reset the buf, parse again
buf.Reset(body)
c.Parse()
You can try this version https://play.golang.org/p/YaVtCTZHZEP It uses the strings.NewReader buffer, but the interface and behavior are similar.
not super obvious, that is the general principle, the transport reads the headers, and leave the body untouched unless you consume it. see also that.

file descriptor in kernel space

I'm developing a charachter device driver for Linux.
I want to implement file-descriptor-targeted read() operation which will be a bit specific every time you open a device.
It is possible to identify the process where read() called from (using kernel current macro), but there can be several file descriptor associated with my device in this process.
I know that file descriptors got mapped to struct file objects just before making system call but can I get it back?
welcome to stackoverflow!
To achieve the goal you have specified in comment there are two methods:
ioctl and read :
Here you will have multiple buffers for each consumer to read from, and write buffer is different from read buffer. Each consumer immediatly after opening the device will fire an ioctl which will result in new buffer being allocated and a new token being generated for that buffer (something like this token numeber means this buffer). this token number should be passed back to the concernted consumer.
Now each consumer before making a read call will fire the ioctl giving the token number that will switch the current read buffer to that associated with that token number.
Now this method adds over head and you need to add locks too. Also no more than one consumer at a time can read from the device.
ioctl and mmap:
you can mmap the read buffer for each consumer and let it read from it at its own pace, using ioctl to request new data etc.
This will allow multiple consumers to read at the same time.
Or, you can malloc a new data buffer to read from on each open call and store the pointer to buffer in the private field of the file structure.
when ever a read is called this way you can just read the private data field of the file structure passed with the call and see which buffer is being talked about.
Also you can embed the whole structure containing the buffer pointer and size etc in the private field.

Is it possible to determine when a message has been entirely read when reading from a Named Pipe in Overlapped I/O?

It is very easy to determine if a message has been entirely read when reading form a pipe with read mode set to PIPE_READMODE_MESSAGE in synchronous I/O. (If the ReadFile function returns FALSE and GetLastError() returns ERROR_MORE_DATA, it means that the message is incomplete and that subsequent reads are necessary to retrieve the full message.)
Now, if the Named Pipe operates in Overlapped I/O instead and a read operation is pending (ReadFile function returns FALSE and GetLastError() returns ERROR_IO_PENDING), how do I know if I retrieved the full message when the operation completes? All I can determine is the number of bytes that were actually transferred by calling the GetOverlappedResult function, but it does not tell me whether or not the full message has been read…
Am I missing something here?
I think the easiest way ist to know, how long the messages is that you are expecting. Your protocol may give you the information.
For example the protocol always delivers a WORD as the first 2 bytes that tells you the length of the complete message.
So I use overlapped I/O with ReadFile to get the first 2 Bytes of the WORD. When I receive them I use ReadFile without overlapped I/O using the known message length and so I get all data.

libusb on Windows: Checking for readable bytes

Using libusb under Windows (where poll/select support is not quite there), is there a way to check for pending bytes on some USB endpoint, i.e. to check whether a following read will succeed (and read one or more bytes)?
I worked around the problem by implementing a ring buffer with already read bytes; checking for pending bytes looks in the ring buffer and (if it is empty) performs a non-blocking read on the endpoint (writing into the ring buffer if there's anything), and reading must use the ring buffer indirection instead of reading directly from the endpoint. This works for me, but is not very elegant.

Handling streamed data via pipes

A Win32 application (the "server") is sending a continuous stream of data over a named pipe. GetNamedPipeInfo() tells me that input and output buffer sizes are automatically allocated as needed. The pipe is operating in byte mode (although it is sending data units that are bigger than 1 byte (doubles, to be precise)).
Now, my question is this: Can I somehow verify that my application (the "client") is not missing any data when reading from the pipe? I know that those read/write operations are buffered, but I suppose the buffers will not grow indefinitely if the client doesn't fetch the data quickly enough. How do I know if I missed something? Does the server (or the pipe?) silently discard data that is not read in time by the client?
BTW, can I rely on proper alignment of the data the client reads using ReadFile()? As far as I understood, ReadFile() may return with less bytes read than specified, i.e. NumberOfBytesRead <= NumberOfBytesToRead. Do I have to check every time that NumberOfBytesRead is a multiple of sizeof(double)?
The write operation will block if there is no more room in the pipe's buffers. This is from my (old) copy of the SDK manual:
When an application uses the WriteFile
function to write to a pipe, the write
operation may not finish if the pipe
buffer is full. The write operation is
completed when a read operation (using
the ReadFile function) makes more
buffer space available.
Sorry, didn't find out how to comment on your post, Neil.
The write operation will block if there is no more room in the pipe's buffers.
I just discovered that Sysinternals' FileMon can also monitor pipe operations. For testing purposes I connected the client to the named pipe and did no read operations, just waiting. The server writes a few hundred kB to the pipe every 4--5 seconds, even though nobody is fetching the data from the pipe on the client side. No blocking write operation ... And so far no limits in buffer-size seem to have been reached.
This is either a very big buffer ... or the server does some magic additional to just using WriteFile() and waiting for the client to read.

Resources