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Top Forums Programming Handle int listen(int sockfd, int backlog) in TCP Post 302481989 by Corona688 on Monday 20th of December 2010 10:31:28 AM
Old 12-20-2010
Quote:
Originally Posted by sehang
Hi,

from the manual
listen(2): listen for connections on socket - Linux man page

It has a parameter called backlog and it limits the maximum length of queue of pending list.

If I set backlog to 128, is it means no more than 128 packets can be handled by server?
No, it means that no more than 128 pending connections can be waiting at once. 128 people somewhere on the internet that are doing connect(my_fd, your_server); and the server has told them "OK, wait in line".
Quote:
Can you problem me a direction that how client knows the server has been reach the limit of backlog?
I think the client will just get "connection refused" when the server already has the maximum number of pending connections waiting. Note that this is pending connections: It won't limit established connections for you, you have to do that yourself.
 

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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)
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