I have a fd_set "write_set" which contains sockets that I want to use in a send(...) call. When I call select(maxsockfd+1, NULL, &write_set, NULL, &tv) there it always returns 0 (timeout) although I haven't sent anything over the sockets in the write_set yet and it should be possible to send data.
Why is this? Shouldn't select return instantly when it's possible to send data over the sockets in write_set?
Thanks!
Edit: My code..
// _read_set and _write_set are the master sets
fd_set read_set = _read_set;
fd_set write_set = _write_set;
// added this for testing, the socket is a member of RemoteChannelConnector.
std::list<RemoteChannelConnector*>::iterator iter;
for (iter = _acceptingConnectorList->begin(); iter != _acceptingConnectorList->end(); iter++) {
if(FD_ISSET((*iter)->getSocket(), &write_set)) {
char* buf = "a";
int ret;
if ((ret = send((*iter)->getSocket(), buf, 1, NULL)) == -1) {
std::cout << "error." << std::endl;
} else {
std::cout << "success." << std::endl;
}
}
}
struct timeval tv;
tv.tv_sec = 10;
tv.tv_usec = 0;
int status;
if ((status = select(_maxsockfd, &read_set, &write_set, NULL, &tv)) == -1) {
// Terminate process on error.
exit(1);
} else if (status == 0) {
// Terminate process on timeout.
exit(1);
} else {
// call send/receive
}
When I run it with the code for testing if my socket is actually in the write_set and if it is possible to send data over the socket, I get a "success"...
I don't believe that you're allowed to copy-construct fd_set objects. The only guaranteed way is to completely rebuild the set using FD_SET before each call to select. Also, you're writing to the list of sockets to be selected on, before ever calling select. That doesn't make sense.
Can you use poll instead? It's a much friendlier API.
Your code is very confused. First, you don't seem to be setting any of the bits in the fd_set. Secondly, you test the bits before you even call select.
Here is how the flow generally works...
Use FD_ZERO to zero out your set.
Go through, and for each file descriptor you're interested in the writeable state of, use FD_SET to set it.
Call select, passing it the address of the fd_set you've been calling the FD_SET function on for the write set and observe the return value.
If the return value is > 0, then go through the write set and use FD_ISSET to figure out which ones are still set. Those are the ones that are writeable.
Your code does not at all appear to be following this pattern. Also, the important task of setting up the master set isn't being shown.
Related
I am working on a project where I have to use zmq_poll. But I did not completely understand what it does.
So I also tried to implement it:
zmq_pollitem_t timer_open(void){
zmq_pollitem_t items[1];
if( items[0].socket == nullptr ){
printf("error socket %s: %s\n", zmq_strerror(zmq_errno()));
return;
}
else{
items[0].socket = gsock;
}
items[0].fd = -1;
items[0].events = ZMQ_POLLIN;
// get a timer
items[0].fd = timerfd_create( CLOCK_REALTIME, 0 );
if( items[0].fd == -1 )
{
printf("timerfd_create() failed: errno=%d\n", errno);
items[0].socket = nullptr;
return;
}
int rc = zmq_poll(items,1,-1);
if(rc == -1){
printf("error poll %s: %s\n", zmq_strerror(zmq_errno()));
return;
}
else
return items[0];
}
I am very new to this topic and I have to modify an old existing project and replace the functions with the one of zmq. On other websites I saw examples where they used two items and the zmq_poll function in an endless loop. I have read the documentation but still could not properly understand how this works. And these are the other two functions I have implemented. I do not know if it is the correct way to implement it like this:
void timer_set(zmq_pollitem_t items[] , long msec, ipc_timer_mode_t mode ) {
struct itimerspec t;
...
timerfd_settime( items[0].fd , 0, &t, NULL );
}
void timer_close(zmq_pollitem_t items[]){
if( items[0].fd != -1 )
close(items[0].fd);
items[0].socket = nullptr;
}
I am not sure if I need the zmq_poll function because I am using a timer.
EDIT:
void some_function_timer_example() {
// We want to wait on two timers
zmq_pollitem_t items[2] ;
// Setup first timer
ipc_timer_open_(&items[0]);
ipc_timer_set_(&items[0], 1000, IPC_TIMER_ONE_SHOT);
// Setup second timer
ipc_timer_open_(&items[1]);
ipc_timer_set_(&items[1], 1000, IPC_TIMER_ONE_SHOT);
// Now wait for the timers in a loop
while (1) {
//ipc_timer_set_(&items[0], 1000, IPC_TIMER_REPEAT);
//ipc_timer_set_(&items[1], 5000, IPC_TIMER_REPEAT);
int rc = zmq_poll (items, 2, -1);
assert (rc >= 0); /* Returned events will be stored in items[].revents */
if (items [0].revents & ZMQ_POLLIN) {
// Process task
std::cout << "revents: 1" << std::endl;
}
if (items [1].revents & ZMQ_POLLIN) {
// Process weather update
std::cout << "revents: 2" << std::endl;
}
}
}
Now it still prins very fast and is not waiting. It is still waiting only in the beginning. And when the timer_set is inside the loop it waits properly, only if the waiting time is the same like: ipc_timer_set(&items[1], 1000,...) and ipctimer_set(&items[0], 1000,...)
So how do I have to change this? Or is this the correct behavior?
zmq_poll works like select, but it allows some additional stuff. For instance you can select between regular synchronous file descriptors, and also special async sockets.
In your case you can use the timer fd as you have tried to do, but you need to make a few small changes.
First you have to consider how you will invoke these timers. I think the use case is if you want to create multiple timers and wait for them. This would be typically the function in yuor current code that might be using a loop for the timer (either using select() or whatever else they might be doing).
It would be something like this:
void some_function() {
// We want to wait on two timers
zmq_pollitem items[2];
// Setup first timer
ipc_timer_open(&item[0]);
ipc_timer_set(&item[0], 1000, IPC_TIMER_ONE_REPEAT);
// Setup second timer
ipc_timer_open(&item[1]);
ipc_timer_set(&item[1], 5000, IPC_TIMER_ONE_SHOT);
// Now wait for the timers in a loop
while (1) {
int rc = zmq_poll (items, 2, -1);
assert (rc >= 0); /* Returned events will be stored in items[].revents */
}
}
Now, you need to fix the ipc_timer_open. It will be very simple - just create the timer fd.
// Takes a pointer to pre-allocated zmq_pollitem_t and returns 0 for success, -1 for error
int ipc_timer_open(zmq_pollitem_t *items){
items[0].socket = NULL;
items[0].events = ZMQ_POLLIN;
// get a timer
items[0].fd = timerfd_create( CLOCK_REALTIME, 0 );
if( items[0].fd == -1 )
{
printf("timerfd_create() failed: errno=%d\n", errno);
return -1; // error
}
return 0;
}
Edit: Added as reply to comment, since this is long:
From the documentation:
If both socket and fd are set in a single zmq_pollitem_t, the ØMQ socket referenced by socket shall take precedence and the value of fd shall be ignored.
So if you are passing the fd, you have to set socket to NULL. I am not even clear where gsock is coming from. Is this in the documentation? I couldn't find it.
And when will it break out of the while(1) loop?
This is application logic, and you have to code according to what you require. zmq_poll just keeps returning everytime one of the timer hits. In this example, every second the zmq_poll returns because the first timer (which is a repeat) keeps triggering. But at 5 seconds, it will also return because of the second timer (which is a one shot). Its up to you to decide when you exit the loop. Do you want this to go infinitely? Do you need to check for a different condition to exit the loop? Do you want to do this for say 100 times and then return? You can code whatever logic you want on top of this code.
And what kind of events are returned back
ZMQ_POLLIN since timer fds behave like readable file descriptors.
I have the given code:
#include <winsock2.h>
#include <sys/time.h>
#include <iostream>
int main()
{
WSADATA wsaData;
if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
{
std::cout << "WSA Initialization failed!" << std::endl;
WSACleanup();
}
timeval time;
time.tv_sec = 1;
time.tv_usec = 0;
int retval = select(0, NULL, NULL, NULL, &time);
if (retval == SOCKET_ERROR)
{
std::cout << WSAGetLastError() << std::endl;
}
return 0;
}
It prints 10022, which means error WSAEINVAL. According to this page, I can get this error only if:
WSAEINVAL: The time-out value is not valid, or all three descriptor parameters were null.
However, I have seen a few examples calling select() without any FD_SETs. Is it possible somehow? I need to do it in a client-side code to let the program sleep for short periods while it is not connected to the server.
However, I have seen a few examples calling select() without any
FD_SETs.
It will work in most OS's (that aren't Windows).
Is it possible somehow [under Windows]?
Not directly, but it's easy enough to roll your own wrapper around select() that gives you the behavior you want even under Windows:
int proper_select(int largestFileDescriptorValuePlusOne, struct fd_set * readFS, struct fd_set * writeFS, struct fd_set * exceptFS, struct timeVal * timeout)
{
#ifdef _WIN32
// Note that you *do* need to pass in the correct value
// for (largestFileDescriptorValuePlusOne) for this wrapper
// to work; Windows programmers sometimes just pass in a dummy value,
// because the current Windows implementation of select() ignores the
// parameter, but that's a portability-killing hack and wrong,
// so don't do it!
if ((largestFileDescriptorValuePlusOne <= 0)&&(timeout != NULL))
{
// Windows select() will error out on a timeout-only call, so call Sleep() instead.
Sleep(((timeout->tv_sec*1000000)+timeout->tv_usec)/1000);
return 0;
}
#endif
// in all other cases we just pass through to the normal select() call
return select(maxFD, readFS, writeFS, exceptFS, timeout);
}
... then just call proper_select() instead of select() and you're golden.
From the notorious and offensive Winsock 'lame list':
Calling select() with three empty FD_SETs and a valid TIMEOUT structure as a sleezy delay function.
Inexcusably lame.
Note the mis-spelling. The document is worth reading, if you can stand it, just to see the incredible depths hubris can attain. In case they've recanted, or discovered that they didn't invent the Sockets API, you could try it with empty FD sets instead of null parameters, but I don't hold out much hope.
I am using select to handle connections on a udp server. If I do not get a packet for some period, I would like to time out. The probelm is, it seems I can either timeout correctly and only read from one client, or read from all clients and not time out.
The difference in this functionality has to do with the first argument to select, the int nfds
Here is my code:
int TIMEOUT = 5;
for (;;) {
FD_ZERO(&read_handles);
FD_SET(udpFD, &read_handles);
timeout.tv_sec = TIMEOUT;
timeout.tv_usec = 0;
if (select(udpFD+1, &read_handles, NULL, NULL, &timeout) == 0) {
printf("Select has timed out...\n");
return 1;
} else {
int length = 1;
if (FD_ISSET(udpFD, &read_handles)) {
//process read.
}
}
}
This version does not time out. If I change the select line to:
if(select(udpFD, &read_handles, NULL, NULL, &timeout) == 0)
It does timeout, but it only receives data from one of my clients.
udpFD is the only handle I am looking at, but it has a value of 4 because it is not the first descriptor I have made. I do not know if that makes a difference because it is the max value.
How can I both timeout and get data from both of my clients?
Using if(select(udpFD+1, &read_handles, NULL, NULL, &timeout) == 0) is the correct way to go.
This will work.
My error was later in the code I was not resetting a length field I read, and was getting stuck in the recvfrom loop, and only calling select once.
I'm using select() in a thread to monitor a datagram socket, but unless data is being sent to the socket before the thread starts, select() will continue to return 0.
I'm mixing a little C and C++; here's the method that starts the thread:
bool RelayStart() {
sock_recv = socket(AF_INET, SOCK_DGRAM, 0);
memset(&addr_recv, 0, sizeof(addr_recv));
addr_recv.sin_family = AF_INET;
addr_recv.sin_port = htons(18902);
addr_recv.sin_addr.s_addr = htonl(INADDR_ANY);
bind(sock_recv, (struct sockaddr*) &addr_recv, sizeof(addr_recv));
isRelayingPackets = true;
NSS::Thread::start(VIDEO_SEND_THREAD_ID);
return true;
}
The method that stops the thread:
bool RelayStop() {
isSendingVideo = false;
NSS::Thread::stop();
close(sock_recv);
return true;
}
And the method run in the thread:
void Run() {
fd_set read_fds;
int select_return;
struct timeval select_timeout;
FD_ZERO(&read_fds);
FD_SET(sock_recv, &read_fds);
while (isRelayingPackets) {
select_timeout.tv_sec = 1;
select_timeout.tv_usec = 0;
select_return = select(sock_recv + 1, &read_fds, NULL, NULL, &select_timeout);
if (select_return > 0 && FD_ISSET(sock_recv, &read_fds)) {
// ...
}
}
}
The problem is that if there isn't a process already sending UDP packets to port 18902 before RelayStart() is called, select() will always return 0. So, for example, I can't restart the sender without restarting the thread (in the correct order.)
Everything seems to work fine as long as the sender is started first.
The Run thread only constructs read_fds once.
The select call updates read_fds to have all its bits cleared for all descriptors that did not have data ready, and all its bits set for those that were set before and do have data ready.
Hence, if no descriptor has any data ready and the select call times out (and returns 0), all the bits in read_fds are now cleared. Further calls passing the same all-zero bit-mask will scan no file descriptors.
You can either re-construct the read-set on each trip inside the loop:
while (isRelayingPackets) {
FD_ZERO(&read_fds);
FD_SET(sock_recv, &read_fds);
...
}
or use an auxiliary variable with a copy of the bit-set:
while (isRelayingPackets) {
fd_set select_arg = read_fds;
... same as before but use &select_arg ...
}
(Or, of course, there are non-select interfaces that are easier to use in some ways.)
How were you expecting it to behave? The point of select() is to sleep to a timeout until data are available to be read; in this case, it will time out after 1 second and return 0. Perhaps you don't actually want a timeout before the start of a stream?
I'm trying to make a select-server in order to receive connection from several clients (all clients will connect to the same port).
The server accepts the first 2 clients, but unless one of them disconnects, it will not accept a new one.
I'm starting to listen the the server port like this:
listen(m_socketId, SOMAXCONN);
and using the select command like this:
int selected = select(m_maxSocketId + 1, &m_socketReadSet, NULL, NULL, 0);
I've added some code.
bool TcpServer::Start(char* ipAddress, int port)
{
m_active = true;
FD_ZERO(&m_socketMasterSet);
bool listening = m_socket->Listen(ipAddress, port);
// Start listening.
m_maxSocketId = m_socket->GetId();
FD_SET(m_maxSocketId, &m_socketMasterSet);
if (listening == true)
{
StartThread(&InvokeListening);
StartReceiving();
return true;
}
else
{
return false;
}
}
void TcpServer::Listen()
{
while (m_active == true)
{
m_socketReadSet = m_socketMasterSet;
int selected = select(m_maxSocketId + 1, &m_socketReadSet, NULL, NULL, 0);
if (selected <= 0)
continue;
bool accepted = Accept();
if (accepted == false)
{
ReceiveFromSockets();
}
}
}
bool TcpServer::Accept()
{
int listenerId = m_socket->GetId();
if (FD_ISSET(listenerId, &m_socketReadSet) == true)
{
struct sockaddr_in remoteAddr;
int addrSize = sizeof(remoteAddr);
unsigned int newSockId = accept(listenerId, (struct sockaddr *)&remoteAddr, &addrSize);
if (newSockId == -1) // Invalid socket...
{
return false;
}
if (newSockId > m_maxSocketId)
{
m_maxSocketId = newSockId;
}
m_clientUniqueId++;
// Remembering the new socket, so we'll be able to check its state
// the next time.
FD_SET(newSockId, &m_socketMasterSet);
CommEndPoint remote(remoteAddr);
CommEndPoint local = m_socket->GetLocalPoint();
ClientId* client = new ClientId(m_clientUniqueId, newSockId, local, remote);
m_clients.Add(client);
StoreNewlyAcceptedClient(client);
char acceptedMsg = CommInternalServerMsg::ConnectionAccepted;
Server::Send(CommMessageType::Internal, client, &acceptedMsg, sizeof(acceptedMsg));
return true;
}
return false;
}
I hope it's enough :)
what's wrong with it?
The by far most common error with select() is not re-initializing the fd sets on every iteration. The second, third, and forth arguments are updated by the call, so you have to populate them again every time.
Post more code, so people can actually help you.
Edit 0:
fd_set on Windows is a mess :)
It's not allowed to copy construct fd_set objects:
m_socketReadSet = m_socketMasterSet;
This combined with Nikolai's correct statement that select changes the set passed in probably accounts for your error.
poll (On Windows, WSAPoll) is a much friendlier API.
Windows also provides WSAEventSelect and (Msg)WaitForMultipleObjects(Ex), which doesn't have a direct equivalent on Unix, but allows you to wait on sockets, files, thread synchronization events, timers, and UI messages at the same time.