03-30-2011
You need just one. Your socket_fd is usually not the socket with which the client and server communicate. It is rather just the point where the client connects to first and then waits for the server to open another socket on which the real data exchange takes place.
So if you have
listen(socket_fd, 5);
This means that 5 connetions are allowed to be in the queue and wait to be served. If all of these slots are taken, they will be blocked.
You can then accept the connections in this queue with the accept() call and this function will return a new file descripter which marks the direct connetion to the client.
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LEARN ABOUT REDHAT
listen
LISTEN(2) Linux Programmer's Manual LISTEN(2)
NAME
listen - listen for connections on a socket
SYNOPSIS
#include <sys/socket.h>
int listen(int s, int backlog);
DESCRIPTION
To accept connections, a socket is first created with socket(2), a willingness to accept incoming connections and a queue limit for incom-
ing connections are specified with listen, and then the connections are accepted with accept(2). The listen call applies only to sockets
of type SOCK_STREAM or SOCK_SEQPACKET.
The backlog parameter defines the maximum length the queue of pending connections may grow to. If a connection request arrives with the
queue full the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the
request may be ignored so that retries succeed.
NOTES
The behaviour of the backlog parameter on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely established
sockets waiting to be accepted, instead of the number of incomplete connection requests. The maximum length of the queue for incomplete
sockets can be set using the tcp_max_syn_backlog sysctl. When syncookies are enabled there is no logical maximum length and this sysctl
setting is ignored. See tcp(7) for more information.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EADDRINUSE
Another socket is already listening on the same port.
EBADF The argument s is not a valid descriptor.
ENOTSOCK
The argument s is not a socket.
EOPNOTSUPP
The socket is not of a type that supports the listen operation.
CONFORMING TO
Single Unix, 4.4BSD, POSIX 1003.1g draft. The listen function call first appeared in 4.2BSD.
BUGS
If the socket is of type AF_INET, and the backlog argument is greater than the constant SOMAXCONN (128 in Linux 2.0 & 2.2), it is silently
truncated to SOMAXCONN. Don't rely on this value in portable applications since BSD (and some BSD-derived systems) limit the backlog to 5.
SEE ALSO
accept(2), connect(2), socket(2)
BSD Man Page 1993-07-23 LISTEN(2)