Select - a few questions - c++

Question

I have a few questions about select function, i wrote this code:
void TCPSerwer::sel()
{
struct timeval tv = {1, 0};
fd_set temp_list = m_RecvList;
//if(select(m_fdmax + 1, &temp_list, NULL, NULL, &tv) == SOCKET_ERROR)
if(select(0, &temp_list, NULL, NULL, &tv) == SOCKET_ERROR)
{
perror( "select" );
exit( 1 );
}
for(int i = 0; i <= m_fdmax; i++ )
{
if(FD_ISSET(i, &temp_list))
{
// New connection
if(i == m_ListenSocket)
{
acceptClient();
}
// Data from client
else
{
PacketHeader header;
int nbytes = recv(i, (char*)(&header), sizeof(PacketHeader),
// Error
if(nbytes < 0)
{
disconnectClient(i);
}
// success
else
{
std::cout << "type: " << header.type << " len: " << header.length << std::endl;
}
}
}
}
}
I can give first arg to select function and i can don't do that, but why ? Why a should give first arg to select ? m_fdmax is highest number of socket, but this code working without this arg.
Next question is, why select need timeout ? When i don't give this arg select marks all socket as socket that can be readable but select doing this when socket haven't any data to read. When i give this arg i don't have this problem. But why ?
if m_fdmax is highest number of socket, i have to find next highest number of socket when i close connection, Right ? And i should doing this that:
int size = m_ClientVector.size();
for(int i = 0; i < size; i++)
{
if(m_ClientVector[i] > m_fdmax)
m_fdmax = m_ClientVector[i];
}

Answer 1

I can give first arg to select function and i can don't do that, but why ? Why a should give first arg to select ? m_fdmax is highest number of socket, but this code working without this arg.
Read the documentation. The select() function on Windows ignores the first parameter, so it does not matter what you pass to it.
Next question is, why select need timeout ?
It does not NEED a timeout, but you can OPTIONALLY provide a timeout if desired. This way, if the requested socket state is not reached before the timeout elapses, select() can still exit and not deadlock the calling thread indefinitely, allowing it to do other things.
When i don't give this arg select marks all socket as socket that can be readable but select doing this when socket haven't any data to read.
If you do not provide a timeout, select() waits indefinitely until the requested socket state actually occurs. A socket can be marked as readable if it has data to read, but it can also be marked as readible if it has been gracefully disconnected by the other party. A subsequent call to recv() will tell you which is the case (recv() returns -1 on error, 0 on disconnect, an >0 on data). Again, read the documentation.
if m_fdmax is highest number of socket, i have to find next highest number of socket when i close connection, Right ?
If you want to calculate the highest socket number (which Windows does not care about, but other platforms do), then you would have to re-calculate the highest socket number every time you call select(), or at least whenever you re-prepare the fd_set structure (which you need to do every time you call select() anyway).
And i should doing this that
On Windows, no. On other platforms, yes.
With that said, try this code on Windows instead:
void TCPSerwer::sel()
{
struct timeval tv = {1, 0};
fd_set temp_list = m_RecvList;
int ret = select(0, &temp_list, NULL, NULL, &tv);
if (ret == SOCKET_ERROR)
{
perror( "select" );
exit( 1 );
}
if (ret == 0) // timeout
return;
for(u_int i = 0; i < temp_list.fd_count; ++i)
{
SOCKET s = temp_list.fd_array[i];
// New connection
if (s == m_ListenSocket)
{
acceptClient();
continue;
}
// Data from client
PacketHeader header;
char *pheader = (char*) &header;
int nbytes = 0;
do
{
ret = recv(s, pheader, sizeof(PacketHeader)-nbytes, 0);
// success
if (ret > 0)
nbytes += ret;
}
while ((ret > 0) && (nbytes < sizeof(PacktHeader)));
// Error or disconnect
if (nbytes < sizeof(PacktHeader))
{
disconnectClient(i);
continue;
}
// success
std::cout << "type: " << header.type << " len: " << header.length << std::endl;
}
}

Answer 2

About the timeout:
select can use a struct timeval to timeout. If you pass a NULL pointer, select will wait until an event comes. If you pass an address to a struct timeval, select will return even if there's not event (in your code, select will return every second).
About the fdmax: Yes you have to find the highest socket, and your snippet is correct.
Other: You don't have any FD_SET in your code. Usually, the sockets are setted (via FD_SET so) in the loop which find the highest socket.
EDIT: My bad I didn't seen the fd_set temp_list = m_RecvList; in your code. We will need more code in order to analyze your problem with select.

Answer 3

Thanks for help, I want use this code on Windows and Linux, and now I doing that:
When I have a new connection:
bool TCPSerwer::acceptClient()
{
SOCKET new_client = accept(m_ListenSocket, 0, 0);
if(new_client == INVALID_SOCKET)
return false;
m_ClientVector.push_back(new_client);
// Add to FD
FD_SET(new_client, &m_RecvList);
if(new_client > m_fdmax)
m_fdmax = new_client;
return true;
}
When i want to close connection:
void TCPSerwer::disconnectClient(const SOCKET& client)
{
int size = m_ClientVector.size();
for(int i = 0; i < size; i++)
{
if(m_ClientVector[i] == client)
{
closesocket(m_ClientVector[i]);
// Delete from FD
FD_CLR(m_ClientVector[i], &m_RecvList);
m_ClientVector.erase(m_ClientVector.begin() + i);
break;
}
}
// re-calculateing the highest socket number
size = m_ClientVector.size();
for(int i = 0; i < size; i++)
{
if(m_ClientVector[i] > m_fdmax)
m_fdmax = m_ClientVector[i];
}
}
I havr one question to you Remy Lebeau, your recv function look that:
recv(s, pheader, sizeof(PacketHeader)-nbytes, 0);
but recv save data at the being of bufor ? Meybe this should look that:
recv(s, pheader + nbytes, sizeof(PacketHeader)-nbytes, 0);