I'm trying to send data to the connected client, even when the client did not send me a message first.
This is my current code:
while (true) {
// open a new socket to transmit data per connection
int sock;
if ((sock = accept(listen_sock, (sockaddr *) &client_address, &client_address_len)) < 0) {
logger.log(TYPE::ERROR, "server::could not open a socket to accept data");
exit(0);
}
int n = 0, total_received_bytes = 0, max_len = 4096;
std::vector<char> buffer(max_len);
logger.log(TYPE::SUCCESS,
"server::client connected with ip address: " + std::string(inet_ntoa(client_address.sin_addr)));
// keep running as long as the client keeps the connection open
while (true) {
n = recv(sock, &buffer[0], buffer.size(), 0);
if (n > 0) {
total_received_bytes += n;
std::string str(buffer.begin(), buffer.end());
KV key_value = kv_from(vector_from(str));
messaging.set_command(key_value);
}
std::string message = "hmc::" + messaging.get_value("hmc") + "---" + "sonar::" + messaging.get_value("sonar") + "\n";
send(sock, message.c_str(), message.length(), 0);
}
logger.log(TYPE::INFO, "server::connection closed");
close(sock);
}
I thought by moving the n = recv(sock, &buffer[0], buffer.size(), 0); outside the while condition that it would send the data indefinitely, but that is not what happened.
Thanks in advance.
Solution
Adding MSG_DONTWAIT to the recv function enabled non-blocking operations which I was looking for.
First I will explain, why it does not work, then I will make a proposal for solutions. Basically you will find the answer in the man7.org > Linux > man-pages and for recv specifially here.
When the function "recv" is called, then it will not return, until data is available and can be read. This behavior of functions is called "blocking". Means, the current execution thread is blocked until data has been read.
So, calling the function
n = recv(sock, &buffer[0], buffer.size(), 0);
as you did, causes the trouble. You need also to check the return code. 0 means, connection closed, -1 means error and you must check errno for further information.
You can modify the socket to work in non-blocking mode with the function fnctl and the O_NONBLOCK flag, for the lifetime of the socket. You can also use the the flag MSG_DONTWAIT as 4th parameter (flags), to unblock the function on a per-function-call base.
In both cases, if no data is available, the functions returns a -1 and you need to check errno for EAGAIN or EWOULDBLOCK.
return value 0 indicates that the connection has been closed.
But from the architecture point of view, I would not recommend to use this approach. You could use multiple threads for receiving and sending data, or, using Linux, one of select, poll or similar functions. There is even a common design pattern for this. It is called "reactor", There are also related patterns like "Acceptor/Connector" and "Proactor"/"ACT" available. If you plan to write a more robust application, then you may consider those.
You will find an implementation of Acceptor, Connector, Reactor, Proactor, ACT here
Hope this helps
Related
I'm sending and receiving info with a unix socket, but I do not completely understand how it works. Basically, I send a message like this:
int wr_bytes = write(sock, msg.c_str(), msg.length());
And receive message like this:
int rd_bytes = read(msgsock, buf, SOCKET_BUFFER_SIZE);
This code works perfectly with thousands of bytes, what I don't understand is, how does the read function knows when the other part is done sending the message? I tried to read the read documentation and, on my understanding read will return once it reaches EOF or the SOCKET_BUFFER_SIZE, is that correct?
So I'm guessing that when I give my string to the write function, it adds an EOF at the end of my content so the read function knows when to stop.
I'm asking this question because, I did not add any code that checks whether the other part finished sending the message, however, I'm receiving big messages (thousands of bytes) without any problem, why is that happening, why am I not getting only parts of the message?
Here is the full function I'm using to send a message to a unix socket server:
string sendSocketMessage(string msg) {
int sock;
struct sockaddr_un server;
char buf[1024];
sock = socket(AF_UNIX, SOCK_STREAM, 0);
if (sock < 0) {
throw runtime_error("opening stream socket");
}
server.sun_family = AF_UNIX;
strcpy(server.sun_path, "socket");
if (connect(sock, (struct sockaddr *) &server, sizeof(struct sockaddr_un)) < 0) {
close(sock);
throw runtime_error("connecting stream socket");
}
if (write(sock, msg.c_str(), msg.length()) < 0){
throw runtime_error("writing on stream socket");
close(sock);
}
bzero(buf, sizeof(buf));
int rval = read(sock, buf, 1024);
return string( reinterpret_cast< char const* >(buf), rval );
}
And here is my server function (a little bit more complicated, the type vSocketHandler represents a function that I call to handle requests):
void UnixSocketServer::listenRequests(vSocketHandler requestHandler){
int sock, msgsock, rval;
struct sockaddr_un server;
char buf[SOCKET_BUFFER_SIZE];
sock = socket(AF_UNIX, SOCK_STREAM, 0);
if (sock < 0) {
throw runtime_error("opening stream socket");
}
server.sun_family = AF_UNIX;
strcpy(server.sun_path, SOCKET_FILE_PATH);
if (bind(sock, (struct sockaddr *) &server, sizeof(struct sockaddr_un))) {
throw runtime_error("binding stream socket");
}
listen(sock, SOCKET_MAX_CONNECTIONS);
while(true) {
msgsock = accept(sock, 0, 0);
if (msgsock == -1){
throw runtime_error("accept socket");
} else {
bzero(buf, sizeof(buf));
if((rval = read(msgsock, buf, SOCKET_BUFFER_SIZE)) < 0)
throw runtime_error("reading stream message");
else if (rval == 0){
//do nothing, client closed socket
break;
} else {
string msg = requestHandler(string( reinterpret_cast< char const* >(buf), rval ));
if(write(msgsock, msg.c_str(), msg.length()) < 0)
throw runtime_error("sending stream message");
}
close(msgsock);
}
}
close(sock);
unlink(SOCKET_FILE_PATH);
}
what I don't understand is, how does the read function knows when the other part is done sending the message?
For a stream-type socket, such as you're using, it doesn't. For a datagram-type socket, communication is broken into distinct chunks, but if a message spans multiple datagrams then the answer is again "it doesn't". This is indeed one of the key things to understand about the read() and write() (and send() and recv()) functions in general, and about sockets more specifically.
For the rest of this answer I'll focus on stream oriented sockets, since that's what you're using. I'll also suppose that the socket is not in non-blocking mode. If you intend for your data transmitted over such a socket to be broken into distinct messages, then it is up to you to implement an application-level protocol by which the other end can recognize message boundaries.
I tried to read the read documentation and, on my understanding read will return once it reaches EOF or the SOCKET_BUFFER_SIZE, is that correct?
Not exactly. read() will return if it reaches the end of the file, which happens when the peer closes its socket (or at least shuts down the write side of it) so that it is certain that no more data will be sent. read() will also return in the event of any of a variety of error conditions. And read() may return under other unspecified circumstances, provided that it has transferred at least one byte. In practice, this last case is generally invoked if the socket buffer fills, but it may also be invoked under other circumstances, such as when the buffer empties.
So I'm guessing that when I give my string to the write function, it adds an EOF at the end of my content so the read function knows when to stop.
No, it does no such thing. On success, the write() function sends some or all of the bytes you asked it to send, and nothing else. Note that it is not guaranteed even to send all the requested bytes; its return value tells you how many of them it actually did send. If that's fewer than "all", then ordinarily you should simply perform another write() to transfer the rest. You may need to do this multiple times to send the whole message. In any event, only the bytes you specify are sent.
I'm asking this question because, I did not add any code that checks whether the other part finished sending the message, however, I'm receiving big messages (thousands of bytes) without any problem, why is that happening, why am I not getting only parts of the message?
More or less because you're getting lucky, but the fact that you're using UNIX-domain sockets (as opposed to network sockets) helps. Your data are transferred very efficiently from sending process to receiving process through the kernel, and it is not particularly surprising that large writes() are received by single read()s. You cannot safely rely on that always to happen, however.
I'm trying to write a web program, using poll(). I'm creating a UDP in a struct pollfd array, and then polling it. However, the poll() returns 0 every time, no matter how many times I send it a message. When I just call recvfrom, it works just fine. So here's my code:
Creating and binding the socket:
struct pollfd[2] fds;
// ...
fds[0].fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fds[0].fd < 0)
syserr("socket");
listen_address = { 0 };
listen_address.sin_family = AF_INET;
listen_address.sin_addr.s_addr = htonl(INADDR_ANY);
listen_address.sin_port = htons(m_port);
if (bind(
fds[0].fd,
(struct sockaddr*) &listen_address,
(socklen_t) sizeof(listen_address)
) < 0)
syserr("bind");
Now this works:
for (;;) {
memset(buffer, 0, BUF_SIZE + 1);
rval = recvfrom(
fds[0].fd,
buffer,
BUF_SIZE,
0,
(struct sockaddr *) &respond_address,
&rcva_len
);
std::cout << buffer << std::endl;
}
But this doesn't
finished = false;
do {
fds[0].revents = fds[1].revents = 0;
ret = poll(fds, 2, 0);
if (ret < 0) {
perror("poll");
} else if (ret == 0) {
// the loop always enters here
} else {
// the loop never enters here,
// even though I send messages to the socket
}
} while (!finished);
For testing, I use a command like this
echo -n “foo” | nc -4u -w1 <host> <udp port>
I would appreciate some insight
There are two critical problems with the way that you use poll():
You must set the revents field, for each file descriptor, to indicate which events you are interested in, such as POLLIN and/or POLLOUT. Your code fails to set revents to anything.
The third parameter to poll() is the timeout setting. Which you are setting to 0. This means "check the file descriptors for whether or not any of the requested events have occured, but always return immediately in any case, and if no file descriptors have the requested events then return 0".
Which is the behavior you are seeing.
Instead of telling you what you need to set the third parameter to, in order to wait until any of the file descriptors's events have occured, I'll just refer you to poll()'s manual page. Reading manual pages is good. They explain everything.
In conclusion:
Initialize each file descriptor's revents parameter properly.
Pass the correct parameters to poll(), as described in its manual page. If you want poll() to wait indefinitely, until any of the passed file descriptors' requested events have occured, there's a specific value to do this. Look it up.
After poll() returns, check each file descriptor's events parameter to determine the file descriptor's status.
I am writing the client side of the Socket. When there is something to read my code works fine but when there is nothing to read, the recv never returns. Help please.
Code:
m_socket = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in dest;
if ( m_socket )
{
memset(&dest, 0, sizeof(dest)); /* zero the struct */
dest.sin_family = AF_INET;
dest.sin_addr.s_addr = inet_addr(address); /* set destination IP number */
dest.sin_port = htons(port);
if (connect(m_socket, (struct sockaddr *)&dest, sizeof(struct sockaddr)) == SOCKET_ERROR)
{
return false;
}
else
{
std::vector<char> inStartup1(2);
int recvReturn = recv(Socket, &inStartup1.at(0), inStartup1.size(), 0);
}
recv is a blocking call. This would help you:-
The recv() call is normally used only on a connected socket.It returns the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded DEPENDING on the type of socket the message is received from.
If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking, in which case the value -1 is returned and the external variable errno is set to EAGAIN or EWOULDBLOCK. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.
Taking this one step further, on a server this is how you would correctly handle a connection (socket or serial port does not matter):
make the socket/port non-blocking: this is the first important step; it means that recv() will read what is available (if anything) and return the number of read bytes or -1 in case of an error.
use select(), with a timeout, to find out when data becomes available. So now you wait for a certain amount of time for data to become available and than read it.
The next problem to handle is making sure you read the full message. Since there is no guarantee that the whole message will be available when you call recv(), you need to save whatever is available and go back to select() and wait for the next data to become available.
Put everything in a while(cond) construct to make sure you read all the data.
The condition in the while is the only thing left to figure out - you either know the length of the expected message or you use some delimiters to mark the end of the message.
Hope this helps!
In got the following problem:
I made a server which is able to handle multiple connection by using select(). But select returns a client(index of FD_SET) also if the socket just got an error like "client disconnect" or whatever.
Is it possible to check a socket without calling recv(). Because to receive I need to get a buffer out of my "BufferPool"
Sample code:
int ret = recv(client, buffer_pool->get(), BUFFER_SIZE, 0);
if(ret == -1) ... // something went wrong
Well then I have to release the buffer again, and it was pretty much a waste of one buffer in my pool. (for a short time)
So isn't it possible to check the socket without calling recv()
I am not sure about the Windows, but using getsockopt() works like a charm on POSIX-compliant systems. Though before you use it - make sure that getting your buffer from the pool is more expensive than making an extra system call. Here is a code snippet:
int my_get_socket_error(int fd)
{
int err_code;
socklen_t len = sizeof(err_code);
if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &err_code, &len) != 0)
err_code = errno;
else
errno = err_code;
return err_code;
}
UPDATE:
According to this document, it seems like Windows supports it too.
No, there is no way to avoid the recv() call. If select() reports that the socket is readable, then you have to read from the socket to determine its new state. If the client disconnected gracefully, recv() will return 0, not -1. If you do not want to waste a pooled buffer, then you will have to read into a temporary local buffer first, and then if recv() returns any data, you can retrieve a pooled buffer and copy the read data into it.
Calling recv and similar function does not work directly with networking devices or something similar.
When you send or receive data, all you do is questioning OS for available data, or to put data in queue for sending. Then OS will do the other job when your code is already went further.
That is why you receive errors after next call of socket function that will "contact" OS networking layers.
It is normal to get errors on that point, and you have to deal with them.
But to prevent blocking sockets and wasting buffers, check out online techniques of implementing or ready libraries that gives you asynchronous way of working with sockets, that way you don't need to define anything before socket will trigger receive callback function where you have to do actual receiving.
As well, it is not good technique to receive big amount of data in one go, because you will face problems with merged or broken apart data through TCP layer, because it is stream based layer. It is recommended to have header in you packets (few bytes) and receive them, that way you don't need pull for header, but only after header you want to read rest of message based on length provided in header. This is just possible example.
After some minutes of work and your help I just receive 1byte before receiving the full amount:
SOCKET client = ...;
char temp = 0x00;
int len = recv(client, &temp, 1, 0);
if(len == 0)
{
// .. client error handling
return;
}
char* buffer = m_memory_pool->Get();
len = recv(client, buffer + 1, m_memory_pool->buffer_size() - 1, 0);
buffer[0] = temp;
// data handling
I tried also to set a timeout for recv() but seems that under Windows it does not work, this is my code:
...
long timeout_ms = 10;
struct timeval interval = {timeout_ms / 1000, (timeout_ms % 1000) * 1000};
if (interval.tv_sec < 0 || (interval.tv_sec == 0 && interval.tv_usec <= 0))
{
interval.tv_sec = 0;
interval.tv_usec = 10000;
}
setsockopt(s_sktIx, SOL_SOCKET, SO_RCVTIMEO, (char *)&interval, sizeof(struct timeval));
...
I'm having a strange problem while attempting to transform a blocking socket server into a nonblocking one. Though the message was only received once when being sent with blocking sockets, using nonblocking sockets the message seems to be received an infinite number of times.
Here is the code that was changed:
return ::write(client, message, size);
to
// Nonblocking socket code
int total_sent = 0, result = -1;
while( total_sent < size ) {
// Create a temporary set of flags for use with the select function
fd_set working_set;
memcpy(&working_set, &master_set, sizeof(master_set));
// Check if data is available for the socket - wait 1 second for timeout
timeout.tv_sec = 1;
timeout.tv_usec = 0;
result = select(client + 1, NULL, &working_set, NULL, &timeout);
// We are able to write - do so
result = ::write(client, &message[total_sent], (size - total_sent));
if (result == -1) {
std::cerr << "An error has occured while writing to the server."
<< std::endl;
return result;
}
total_sent += result;
}
return 0;
EDIT: The initialization of the master set looks like this:
// Private member variables in header file
fd_set master_set;
int sock;
...
// Creation of socket in class constructor
sock = ::socket(PF_INET, socket_type, 0);
// Makes the socket nonblocking
fcntl(sock,F_GETFL,0);
FD_ZERO(&master_set);
FD_SET(sock, &master_set);
...
// And then when accept is called on the socket
result = ::accept(sock, NULL, NULL);
if (result > 0) {
// A connection was made with a client - change the master file
// descriptor to note that
FD_SET(result, &master_set);
}
I have confirmed that in both cases, the code is only being called once for the offending message. Also, the client side code hasn't changed at all - does anyone have any recommendations?
fcntl(sock,F_GETFL,0);
How does that make the socket non-blocking?
fcntl(sock, F_SETFL, O_NONBLOCK);
Also, you are not checking if you can actually write to the socket non-blocking style with
FD_ISSET(client, &working_set);
I do not believe that this code is really called only once in the "non blocking" version (quotes because it is not really non-blocking yet as Maister pointed out, look here), check again. If the blocking and non blocking versions are consistent, the non blocking version should return total_sent (or size). With return 0 instead caller is likely to believe nothing was sent. Which would cause infinite sending... is it not what's happening ?
Also your "non blocking" code is quite strange. You seem to use select to make it blocking anyway... Ok, with a timeout of 1s, but why don't you make it really non blocking ? ie: remove all the select stuff and test for error case in write() with errno being EWOULDBLOCK. select or poll are for multiplexing.
Also you should check errors for select and use FD_ISSET to check if socket is really ready. What if the 1 s timeout really happen ? Or if select is stopped by some interruption ? And if an error occurs in write, you should also write which error, that is much more useful than your generic message. But I guess this part of code is still far from finished.
As far as I understand your code it should probably look somewhat like that (if the code is running in an unique thread or threaded, or forking when accepting a connection would change details):
// Creation of socket in class constructor
sock = ::socket(PF_INET, socket_type, 0);
fcntl(sock, F_SETFL, O_NONBLOCK);
// And then when accept is called on the socket
result = ::accept(sock, NULL, NULL);
if (result > 0) {
// A connection was made with a client
client = result;
fcntl(client, F_SETFL, O_NONBLOCK);
}
// Nonblocking socket code
result = ::write(client, &message[total_sent], (size - total_sent));
if (result == -1) {
if (errno == EWOULDBLOCK){
return 0;
}
std::cerr << "An error has occured while writing to the server."
<< std::endl;
return result;
}
return size;