I am trying to read data using the following code from a socket:
n = read(fd, buffer, 50000);
The question is: when the data from the web server is larger than the tcp package size, these data will be splited into multi packages. In this case, will read function just read one data package from fd, or it will read all the packages from fd?
Note that read function is called only once.
Because you are using TCP, your socket is of type SOCK_STREAM. A SOCK_STREAM socket is a byte stream and does not maintain packet boundaries, so the call to read() or recv() will read data that came from multiple packets if multiple packets of data have been received and there is sufficient space in your buffer. It may also return data from a portion of a packet if your buffer if not large enough to hold all of the data. The next read() will continue reading from the next byte.
The function read receives at maximum the specified count of bytes, in your example 50000.
When the function read returns, you need to check the return value. The actual number of bytes written to buffer is in your variable n.
Related
I've been reading through Beej's Guide to Network Programming to get a handle on TCP connections. In one of the samples the client code for a simple TCP stream client looks like:
if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}
buf[numbytes] = '\0';
printf("Client: received '%s'\n", buf);
close(sockfd);
I've set the buffer to be smaller than the total number of bytes that I'm sending. I'm not quite sure how I can get the other bytes. Do I have to loop over recv() until I receive '\0'?
*Note on the server side I'm also implementing his sendall() function, so it should actually be sending everything to the client.
See also 6.1. A Simple Stream Server in the guide.
Yes, you will need multiple recv() calls, until you have all data.
To know when that is, using the return status from recv() is no good - it only tells you how many bytes you have received, not how many bytes are available, as some may still be in transit.
It is better if the data you receive somehow encodes the length of the total data. Read as many data until you know what the length is, then read until you have received length data. To do that, various approaches are possible; the common one is to make a buffer large enough to hold all data once you know what the length is.
Another approach is to use fixed-size buffers, and always try to receive min(missing, bufsize), decreasing missing after each recv().
The first thing you need to learn when doing TCP/IP programming: 1 write/send call might take
several recv calls to receive, and several write/send calls might need just 1 recv call to receive. And anything in-between.
You'll need to loop until you have all data. The return value of recv() tells you how much data you received. If you simply want to receive all data on the TCP connection, you can loop until recv() returns 0 - provided that the other end closes the TCP connection when it is done sending.
If you're sending records/lines/packets/commands or something similar, you need to make your own protocol over TCP, which might be as simple as "commands are delimited with \n".
The simple way to read/parse such a command would be to read 1 byte at a time, building up a buffer with the received bytes and check for a \n byte every time. Reading 1 byte is extremely inefficient, so you should read larger chunks at a time.
Since TCP is stream oriented and does not provide record/message boundaries it becomes a bit more tricky - you'd
have to recv a piece of bytes, check in the received buffer for a \n byte, if it's there - append the bytes to previously received bytes and output that message. Then check the remainder of the buffer after the \n - which might contain another whole message or just the start of another message.
Yes, you have to loop over recv() until you receive '\0' or an
error happen (negative value from recv) or 0 from recv().
For the first option: only if this zero is part of your
protocol (the server sends it). However from your code it seems that
the zero is just to be able to use the buffer content as a
C-string (on the client side).
The check for a return value of 0 from recv:
this means that the connection was closed (it could be part
of your protocol that this happens.)
I wrote a simple client-server program. Network.h is a header file which uses Winsock2.h (TCP/IP mode) to create socket, accept/connect in blocking mode, send/recv in non-blocking mode. I made it so that the function string TNetwork::Recv(int size) will return the string "Nothing" if it gets WSAWOULDBLOCK error (no data is received yet)
Here is my main function:
int main(){
string Ans;
TNetwork::StartUp(); //WSA start up, etc
cin >> Ans;
if (Ans == "0"){ // 0 --> server
TNetwork::SetupAsServer(); //accept connection (in blocking mode!)
while (true){
TNetwork::Send("\nAss" + '\0'); //without null terminator, the client may read extra bytes, causing undefined behavior (?)
TNetwork::Send("embly" + '\0');
cin >> Ans;
}
}
else{ // others --> regard Ans as IP address. e.g. I can type "127.0.0.1"
TNetwork::SetupAsClient(Ans);
string Rec;
while (true){
Rec = TNetwork::Recv(1000);
if (Rec != "Nothing"){
cout << Rec;
}
}
}
system("PAUSE");
}
Supposedly, the client would print "Assembly" when connected, and when the server enters anything to its console window. Sometimes, though, the client would only print out "\nAss" in the console without the "embly.
To my understanding, TCP/IP ensures all data to be sent and in the correct order, so I guess what happens is that both packets arrive at the same time, which happen quite often over the unstable internet. And due to this null terminator, the client would ignore the "embly", since the Recv() function stopped reading when it hits a null terminator.
So, how can I ensure that the client will always read all data packets correctly?
Yes, the network stack will send the data in the correct order and doesn't care what termination type you use. This has to do with how you're receiving and processing the data stream (note: not packets, stream). If you receive all 11 bytes and print it to the screen, the print function will stop when it reaches the zero, but the rest of the data is still there.
Note: since it's a stream, what happens if you received only 10 bytes of data from the stream? You need to scan what you receive for the zero to know if you've received a full "zero-terminated string" if that's how you want to communicate your data.
EDIT: Also, I don't think "\nAss" + '\0' is doing what you think it is. Instead of adding a 0 character to the end of the string (which already has one, by the way), it's adding 0 to your string pointer.
As #mark points out, TCP is all about streams, not packets. TCP takes care of ensuring that data is reliably transmitted from A to B and that the data is delivered to the consumer in the order in which it was transmitted. Yes, the data is packetized on the wire, but the TCP stack on the system takes those packets and builds the stream which it makes available to you through the recv() function. The TCP stack handles out-of-order data, missing data, and duplicated data such that by the time your application sees it, the stream is a mirror-copy of when the sender sent.
To properly receive TCP data, you will typically need some kind of loop that reads data from the socket when it becomes available. The way I normally do this is to have a thread that is dedicated to servicing the socket. In the thread function is a loop that reads data from the socket when it becomes available and is idle otherwise. This loop reads data into a buffer of, say, 1 KB. Once the data is received from the socket into this buffer, the buffer is copied to another thread for processing. In the thread function for the processing thread is a loop that receives the 1 KB buffers from the socket thread and adds them to the back end of a master buffer of, say, 1 MB. The processing thread then processes the messages out of this master buffer and makes them available to the application.
For a simple demo application, two threads may be overkill. The two threads I've described could be certainly be combined into one, but for my application, it is more efficient to have two threads and take advantage of the multiple cores on my system. The point is, if you're going to have a front-end UI, there's not going to be a way around using at least one thread and still have the UI be responsive.
One other thing. There are two commonly-used mechanisms for protocol design. You're using one, namely, a marker (e.g., a null terminator, etc.) to signal the begin/end of a message. I don't prefer this mechanism mainly because the marker may actually need to be part of the message at some point. The other mechanism is to have a header on each message that tells, at a minimum, how long the message is. I prefer this mechanism and include in my headers a sync word and the message type as well. For example,
struct Header
{
__int16 _sync; // a hex pattern, e.g., 0xABCD
__int16 _type;
__int32 _length;
}
That's a total of 8 bytes. So when processing from the master buffer, I read the first 8 bytes, verify the sync word, and get the length. I determine if there are 'length' bytes available in the master buffer. If not, I have to wait until the socket thread provides me more data before checking again. If so, I extract 'length' bytes from the master buffer and pass that to an object created according to the specified type, which knows how to interpret that particular message. Then repeat.
As I mentioned, I use a master buffer of 1 MB or so. As messages are processed, it is important to remove them from the master buffer so there is additional space available for new data on the back end. This involves simply copying the unprocessed data, if any, to the beginning of the buffer. In cases where data comes in faster than you can process it, the master buffer may need the ability to resize itself to accommodate the additional data.
I hope that's not overwhelming. Start simple and add as you go.
I'm using boost 1.53 and serialization to transfer an array of 520 floats over TCP/IP. I put a debug code printout to see the amount of data to be send : it's about 5 K. No problem for me here, but this value somehow depends on the actual data to be serialized. It could be 5400, 5500 and so on.
The question is : what is the right way to receive such data block? For the moment I use read_some() call. But as I've figured out it doesn't guarantee that the whole serialized block of data will be read out. Am I wrong?
How to ensure that there will be a complete archive at RX side? Is there any exception to be thrown when it is not possible to deserialize a chunk of data?
as far as tcpip packet can be received to a number of smaller packets so I'd recommend to add some additional data to tcpip
something like this:
serialize you data to stream
get size of stream
send to tcpip buffer starting with size of stream and then data from the stream
receiver reads size and then reads the rest of the packet.
after you received the full packet - call deserialization
Yes. read_some is potentially a no-op on conforming implementations[1].
Instead do a loop using read() and gcount(), like:
std::istream& is = gotten_from_somewhere_or_a_parameter();
std::vector<byte> v(256);
std::streamsize bytes_read;
do
{
is.read(v.data(),v.size());
bytes_read = stream.gcount ();
// do something with the bytes read
} while(bytes_read);
[1] Notably, gcc's standard library implementation seems to always return something for std::filebuf but on MSVC, the first call will simply always return 0 bytes read :)
I have a TCP client connecting to my server which is sending raw data packets. How, using Boost.Asio, can I get the "whole" packet every time (asynchronously, of course)? Assume these packets can be any size up to the full size of my memory.
Basically, I want to avoid creating a statically sized buffer.
Typically when you build a custom protocol on the top of TCP/IP you use a simple message format where first 4 bytes is an unsigned integer containing the message length and the rest is the message data. If you have such a protocol then the reception loop is as simple as below (not sure what is ASIO notation, so it's just an idea)
for(;;) {
uint_32_t len = 0u;
read(socket, &len, 4); // may need multiple reads in non-blocking mode
len = ntohl(len);
assert (len < my_max_len);
char* buf = new char[len];
read(socket, buf, len); // may need multiple reads in non-blocking mode
...
}
typically, when you do async IO, your protocol should support it.
one easy way is to prefix a byte array with it's length at the logical level, and have the reading code buffer up until it has a full buffer ready for parsing.
if you don't do it, you will end up with this logic scattered all over the place (think about reading a null terminated string, and what it means if you just get a part of it every time select/poll returns).
TCP doesn't operate with packets. It provides you one contiguous stream. You can ask for the next N bytes, or for all the data received so far, but there is no "packet" boundary, no way to distinguish what is or is not a packet.
I've been reading through Beej's Guide to Network Programming to get a handle on TCP connections. In one of the samples the client code for a simple TCP stream client looks like:
if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}
buf[numbytes] = '\0';
printf("Client: received '%s'\n", buf);
close(sockfd);
I've set the buffer to be smaller than the total number of bytes that I'm sending. I'm not quite sure how I can get the other bytes. Do I have to loop over recv() until I receive '\0'?
*Note on the server side I'm also implementing his sendall() function, so it should actually be sending everything to the client.
See also 6.1. A Simple Stream Server in the guide.
Yes, you will need multiple recv() calls, until you have all data.
To know when that is, using the return status from recv() is no good - it only tells you how many bytes you have received, not how many bytes are available, as some may still be in transit.
It is better if the data you receive somehow encodes the length of the total data. Read as many data until you know what the length is, then read until you have received length data. To do that, various approaches are possible; the common one is to make a buffer large enough to hold all data once you know what the length is.
Another approach is to use fixed-size buffers, and always try to receive min(missing, bufsize), decreasing missing after each recv().
The first thing you need to learn when doing TCP/IP programming: 1 write/send call might take
several recv calls to receive, and several write/send calls might need just 1 recv call to receive. And anything in-between.
You'll need to loop until you have all data. The return value of recv() tells you how much data you received. If you simply want to receive all data on the TCP connection, you can loop until recv() returns 0 - provided that the other end closes the TCP connection when it is done sending.
If you're sending records/lines/packets/commands or something similar, you need to make your own protocol over TCP, which might be as simple as "commands are delimited with \n".
The simple way to read/parse such a command would be to read 1 byte at a time, building up a buffer with the received bytes and check for a \n byte every time. Reading 1 byte is extremely inefficient, so you should read larger chunks at a time.
Since TCP is stream oriented and does not provide record/message boundaries it becomes a bit more tricky - you'd
have to recv a piece of bytes, check in the received buffer for a \n byte, if it's there - append the bytes to previously received bytes and output that message. Then check the remainder of the buffer after the \n - which might contain another whole message or just the start of another message.
Yes, you have to loop over recv() until you receive '\0' or an
error happen (negative value from recv) or 0 from recv().
For the first option: only if this zero is part of your
protocol (the server sends it). However from your code it seems that
the zero is just to be able to use the buffer content as a
C-string (on the client side).
The check for a return value of 0 from recv:
this means that the connection was closed (it could be part
of your protocol that this happens.)