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socket

create an endpoint for communication


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  2. socket.7.man
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1. socket.2.man

Manpage of SOCKET

SOCKET

Section: Linux Programmer's Manual (2)
Updated: 2009-01-19
Index Return to Main Contents
 

NAME

socket - create an endpoint for communication  

SYNOPSIS

#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>

int socket(int domain, int type, int protocol);  

DESCRIPTION

socket() creates an endpoint for communication and returns a descriptor.

The domain argument specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in <sys/socket.h>. The currently understood formats include:

NamePurposeMan page
AF_UNIX, AF_LOCAL Local communication unix(7)
AF_INET IPv4 Internet protocols ip(7)
AF_INET6 IPv6 Internet protocols ipv6(7)
AF_IPX IPX - Novell protocols
AF_NETLINK Kernel user interface device netlink(7)
AF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
AF_AX25 Amateur radio AX.25 protocol
AF_ATMPVC Access to raw ATM PVCs
AF_APPLETALK Appletalk ddp(7)
AF_PACKET Low level packet interface packet(7)

The socket has the indicated type, which specifies the communication semantics. Currently defined types are:

SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
SOCK_SEQPACKET
Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each input system call.
SOCK_RAW
Provides raw network protocol access.
SOCK_RDM
Provides a reliable datagram layer that does not guarantee ordering.
SOCK_PACKET
Obsolete and should not be used in new programs; see packet(7).

Some socket types may not be implemented by all protocol families; for example, SOCK_SEQPACKET is not implemented for AF_INET.

Since Linux 2.6.27, the type argument serves a second purpose: in addition to specifying a socket type, it may include the bitwise OR of any of the following values, to modify the behavior of socket():

SOCK_NONBLOCK
Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result.
SOCK_CLOEXEC
Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.

The protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family, in which case protocol can be specified as 0. However, it is possible that many protocols may exist, in which case a particular protocol must be specified in this manner. The protocol number to use is specific to the ``communication domain'' in which communication is to take place; see protocols(5). See getprotoent(3) on how to map protocol name strings to protocol numbers.

Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. They do not preserve record boundaries. A stream socket must be in a connected state before any data may be sent or received on it. A connection to another socket is created with a connect(2) call. Once connected, data may be transferred using read(2) and write(2) calls or some variant of the send(2) and recv(2) calls. When a session has been completed a close(2) may be performed. Out-of-band data may also be transmitted as described in send(2) and received as described in recv(2).

The communications protocols which implement a SOCK_STREAM ensure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered to be dead. When SO_KEEPALIVE is enabled on the socket the protocol checks in a protocol-specific manner if the other end is still alive. A SIGPIPE signal is raised if a process sends or receives on a broken stream; this causes naive processes, which do not handle the signal, to exit. SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets. The only difference is that read(2) calls will return only the amount of data requested, and any data remaining in the arriving packet will be discarded. Also all message boundaries in incoming datagrams are preserved.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in sendto(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram along with the address of its sender.

SOCK_PACKET is an obsolete socket type to receive raw packets directly from the device driver. Use packet(7) instead.

An fcntl(2) F_SETOWN operation can be used to specify a process or process group to receive a SIGURG signal when the out-of-band data arrives or SIGPIPE signal when a SOCK_STREAM connection breaks unexpectedly. This operation may also be used to set the process or process group that receives the I/O and asynchronous notification of I/O events via SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call with the FIOSETOWN or SIOCSPGRP argument.

When the network signals an error condition to the protocol module (e.g., using a ICMP message for IP) the pending error flag is set for the socket. The next operation on this socket will return the error code of the pending error. For some protocols it is possible to enable a per-socket error queue to retrieve detailed information about the error; see IP_RECVERR in ip(7).

The operation of sockets is controlled by socket level options. These options are defined in <sys/socket.h>. The functions setsockopt(2) and getsockopt(2) are used to set and get options, respectively.  

RETURN VALUE

On success, a file descriptor for the new socket is returned. On error, -1 is returned, and errno is set appropriately.  

ERRORS

EACCES
Permission to create a socket of the specified type and/or protocol is denied.
EAFNOSUPPORT
The implementation does not support the specified address family.
EINVAL
Unknown protocol, or protocol family not available.
EINVAL
Invalid flags in type.
EMFILE
Process file table overflow.
ENFILE
The system limit on the total number of open files has been reached.
ENOBUFS or ENOMEM
Insufficient memory is available. The socket cannot be created until sufficient resources are freed.
EPROTONOSUPPORT
The protocol type or the specified protocol is not supported within this domain.

Other errors may be generated by the underlying protocol modules.  

CONFORMING TO

4.4BSD, POSIX.1-2001.

The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.

socket() appeared in 4.2BSD. It is generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants).  

NOTES

POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it.

The manifest constants used under 4.x BSD for protocol families are PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for address families. However, already the BSD man page promises: "The protocol family generally is the same as the address family", and subsequent standards use AF_* everywhere.  

EXAMPLE

An example of the use of socket() is shown in getaddrinfo(3).  

SEE ALSO

accept(2), bind(2), connect(2), fcntl(2), getpeername(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getprotoent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)

lqAn Introductory 4.3BSD Interprocess Communication Tutorialrq is reprinted in UNIX Programmer's Supplementary Documents Volume 1.

lqBSD Interprocess Communication Tutorialrq is reprinted in UNIX Programmer's Supplementary Documents Volume 1.  

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
EXAMPLE
SEE ALSO
COLOPHON

This document was created by man2html using the manual pages.
Time: 17:32:31 GMT, October 23, 2013

2. socket.7.man

Manpage of SOCKET

SOCKET

Section: Linux Programmer's Manual (7)
Updated: 2010-06-13
Index Return to Main Contents
 

NAME

socket - Linux socket interface  

SYNOPSIS

#include <sys/socket.h>

sockfd = socket(int socket_family, int socket_type, int protocol);  

DESCRIPTION

This manual page describes the Linux networking socket layer user interface. The BSD compatible sockets are the uniform interface between the user process and the network protocol stacks in the kernel. The protocol modules are grouped into protocol families like AF_INET, AF_IPX, AF_PACKET and socket types like SOCK_STREAM or SOCK_DGRAM. See socket(2) for more information on families and types.  

Socket Layer Functions

These functions are used by the user process to send or receive packets and to do other socket operations. For more information see their respective manual pages.

socket(2) creates a socket, connect(2) connects a socket to a remote socket address, the bind(2) function binds a socket to a local socket address, listen(2) tells the socket that new connections shall be accepted, and accept(2) is used to get a new socket with a new incoming connection. socketpair(2) returns two connected anonymous sockets (only implemented for a few local families like AF_UNIX)

send(2), sendto(2), and sendmsg(2) send data over a socket, and recv(2), recvfrom(2), recvmsg(2) receive data from a socket. poll(2) and select(2) wait for arriving data or a readiness to send data. In addition, the standard I/O operations like write(2), writev(2), sendfile(2), read(2), and readv(2) can be used to read and write data.

getsockname(2) returns the local socket address and getpeername(2) returns the remote socket address. getsockopt(2) and setsockopt(2) are used to set or get socket layer or protocol options. ioctl(2) can be used to set or read some other options.

close(2) is used to close a socket. shutdown(2) closes parts of a full-duplex socket connection.

Seeking, or calling pread(2) or pwrite(2) with a nonzero position is not supported on sockets.

It is possible to do nonblocking I/O on sockets by setting the O_NONBLOCK flag on a socket file descriptor using fcntl(2). Then all operations that would block will (usually) return with EAGAIN (operation should be retried later); connect(2) will return EINPROGRESS error. The user can then wait for various events via poll(2) or select(2).

I/O events
EventPoll flagOccurrence
ReadPOLLIN New data arrived.
ReadPOLLIN A connection setup has been completed (for connection-oriented sockets)
ReadPOLLHUP A disconnection request has been initiated by the other end.
ReadPOLLHUP A connection is broken (only for connection-oriented protocols). When the socket is written SIGPIPE is also sent.
WritePOLLOUT Socket has enough send buffer space for writing new data.
Read/Write POLLIN|
POLLOUT
An outgoing connect(2) finished.
Read/WritePOLLERRAn asynchronous error occurred.
Read/WritePOLLHUPThe other end has shut down one direction.
ExceptionPOLLPRI Urgent data arrived. SIGURG is sent then.

An alternative to poll(2) and select(2) is to let the kernel inform the application about events via a SIGIO signal. For that the O_ASYNC flag must be set on a socket file descriptor via fcntl(2) and a valid signal handler for SIGIO must be installed via sigaction(2). See the Signals discussion below.  

Socket Options

These socket options can be set by using setsockopt(2) and read with getsockopt(2) with the socket level set to SOL_SOCKET for all sockets:
SO_ACCEPTCONN
Returns a value indicating whether or not this socket has been marked to accept connections with listen(2). The value 0 indicates that this is not a listening socket, the value 1 indicates that this is a listening socket. This socket option is read-only.
SO_BINDTODEVICE
Bind this socket to a particular device like lqeth0rq, as specified in the passed interface name. If the name is an empty string or the option length is zero, the socket device binding is removed. The passed option is a variable-length null-terminated interface name string with the maximum size of IFNAMSIZ. If a socket is bound to an interface, only packets received from that particular interface are processed by the socket. Note that this only works for some socket types, particularly AF_INET sockets. It is not supported for packet sockets (use normal bind(8) there).
SO_BROADCAST
Set or get the broadcast flag. When enabled, datagram sockets receive packets sent to a broadcast address and they are allowed to send packets to a broadcast address. This option has no effect on stream-oriented sockets.
SO_BSDCOMPAT
Enable BSD bug-to-bug compatibility. This is used by the UDP protocol module in Linux 2.0 and 2.2. If enabled ICMP errors received for a UDP socket will not be passed to the user program. In later kernel versions, support for this option has been phased out: Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning (printk()) if a program uses this option. Linux 2.0 also enabled BSD bug-to-bug compatibility options (random header changing, skipping of the broadcast flag) for raw sockets with this option, but that was removed in Linux 2.2.
SO_DEBUG
Enable socket debugging. Only allowed for processes with the CAP_NET_ADMIN capability or an effective user ID of 0.
SO_DOMAIN (since Linux 2.6.32)
Retrieves the socket domain as an integer, returning a value such as AF_INET6. See socket(2) for details. This socket option is read-only.
SO_ERROR
Get and clear the pending socket error. This socket option is read-only. Expects an integer.
SO_DONTROUTE
Don't send via a gateway, only send to directly connected hosts. The same effect can be achieved by setting the MSG_DONTROUTE flag on a socket send(2) operation. Expects an integer boolean flag.
SO_KEEPALIVE
Enable sending of keep-alive messages on connection-oriented sockets. Expects an integer boolean flag.
SO_LINGER
Sets or gets the SO_LINGER option. The argument is a linger structure.

struct linger {
    int l_onoff;    /* linger active */
    int l_linger;   /* how many seconds to linger for */
};
When enabled, a close(2) or shutdown(2) will not return until all queued messages for the socket have been successfully sent or the linger timeout has been reached. Otherwise, the call returns immediately and the closing is done in the background. When the socket is closed as part of exit(2), it always lingers in the background.
SO_OOBINLINE
If this option is enabled, out-of-band data is directly placed into the receive data stream. Otherwise out-of-band data is only passed when the MSG_OOB flag is set during receiving.
SO_PASSCRED
Enable or disable the receiving of the SCM_CREDENTIALS control message. For more information see unix(7).
SO_PEERCRED
Return the credentials of the foreign process connected to this socket. This is only possible for connected AF_UNIX stream sockets and AF_UNIX stream and datagram socket pairs created using socketpair(2); see unix(7). The returned credentials are those that were in effect at the time of the call to connect(2) or socketpair(2). Argument is a ucred structure. This socket option is read-only.
SO_PRIORITY
Set the protocol-defined priority for all packets to be sent on this socket. Linux uses this value to order the networking queues: packets with a higher priority may be processed first depending on the selected device queueing discipline. For ip(7), this also sets the IP type-of-service (TOS) field for outgoing packets. Setting a priority outside the range 0 to 6 requires the CAP_NET_ADMIN capability.
SO_PROTOCOL (since Linux 2.6.32)
Retrieves the socket protocol as an integer, returning a value such as IPPROTO_SCTP. See socket(2) for details. This socket option is read-only.
SO_RCVBUF
Sets or gets the maximum socket receive buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/rmem_default file, and the maximum allowed value is set by the /proc/sys/net/core/rmem_max file. The minimum (doubled) value for this option is 256.
SO_RCVBUFFORCE (since Linux 2.6.14)
Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_RCVBUF, but the rmem_max limit can be overridden.
SO_RCVLOWAT and SO_SNDLOWAT
Specify the minimum number of bytes in the buffer until the socket layer will pass the data to the protocol (SO_SNDLOWAT) or the user on receiving (SO_RCVLOWAT). These two values are initialized to 1. SO_SNDLOWAT is not changeable on Linux (setsockopt(2) fails with the error ENOPROTOOPT). SO_RCVLOWAT is changeable only since Linux 2.4. The select(2) and poll(2) system calls currently do not respect the SO_RCVLOWAT setting on Linux, and mark a socket readable when even a single byte of data is available. A subsequent read from the socket will block until SO_RCVLOWAT bytes are available.
SO_RCVTIMEO and SO_SNDTIMEO
Specify the receiving or sending timeouts until reporting an error. The argument is a struct timeval. If an input or output function blocks for this period of time, and data has been sent or received, the return value of that function will be the amount of data transferred; if no data has been transferred and the timeout has been reached then -1 is returned with errno set to EAGAIN or EWOULDBLOCK just as if the socket was specified to be nonblocking. If the timeout is set to zero (the default) then the operation will never timeout. Timeouts only have effect for system calls that perform socket I/O (e.g., read(2), recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for select(2), poll(2), epoll_wait(2), etc.
SO_REUSEADDR
Indicates that the rules used in validating addresses supplied in a bind(2) call should allow reuse of local addresses. For AF_INET sockets this means that a socket may bind, except when there is an active listening socket bound to the address. When the listening socket is bound to INADDR_ANY with a specific port then it is not possible to bind to this port for any local address. Argument is an integer boolean flag.
SO_SNDBUF
Sets or gets the maximum socket send buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/wmem_default file and the maximum allowed value is set by the /proc/sys/net/core/wmem_max file. The minimum (doubled) value for this option is 2048.
SO_SNDBUFFORCE (since Linux 2.6.14)
Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_SNDBUF, but the wmem_max limit can be overridden.
SO_TIMESTAMP
Enable or disable the receiving of the SO_TIMESTAMP control message. The timestamp control message is sent with level SOL_SOCKET and the cmsg_data field is a struct timeval indicating the reception time of the last packet passed to the user in this call. See cmsg(3) for details on control messages.
SO_TYPE
Gets the socket type as an integer (e.g., SOCK_STREAM). This socket option is read-only.
 

Signals

When writing onto a connection-oriented socket that has been shut down (by the local or the remote end) SIGPIPE is sent to the writing process and EPIPE is returned. The signal is not sent when the write call specified the MSG_NOSIGNAL flag.

When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO is sent when an I/O event occurs. It is possible to use poll(2) or select(2) in the signal handler to find out which socket the event occurred on. An alternative (in Linux 2.2) is to set a real-time signal using the F_SETSIG fcntl(2); the handler of the real time signal will be called with the file descriptor in the si_fd field of its siginfo_t. See fcntl(2) for more information.

Under some circumstances (e.g., multiple processes accessing a single socket), the condition that caused the SIGIO may have already disappeared when the process reacts to the signal. If this happens, the process should wait again because Linux will resend the signal later.  

/proc interfaces

The core socket networking parameters can be accessed via files in the directory /proc/sys/net/core/.
rmem_default
contains the default setting in bytes of the socket receive buffer.
rmem_max
contains the maximum socket receive buffer size in bytes which a user may set by using the SO_RCVBUF socket option.
wmem_default
contains the default setting in bytes of the socket send buffer.
wmem_max
contains the maximum socket send buffer size in bytes which a user may set by using the SO_SNDBUF socket option.
message_cost and message_burst
configure the token bucket filter used to load limit warning messages caused by external network events.
netdev_max_backlog
Maximum number of packets in the global input queue.
optmem_max
Maximum length of ancillary data and user control data like the iovecs per socket.
 

Ioctls

These operations can be accessed using ioctl(2):

error = ioctl(ip_socket, ioctl_type, &value_result);
SIOCGSTAMP
Return a struct timeval with the receive timestamp of the last packet passed to the user. This is useful for accurate round trip time measurements. See setitimer(2) for a description of struct timeval. This ioctl should only be used if the socket option SO_TIMESTAMP is not set on the socket. Otherwise, it returns the timestamp of the last packet that was received while SO_TIMESTAMP was not set, or it fails if no such packet has been received, (i.e., ioctl(2) returns -1 with errno set to ENOENT).
SIOCSPGRP
Set the process or process group to send SIGIO or SIGURG signals to when an asynchronous I/O operation has finished or urgent data is available. The argument is a pointer to a pid_t. If the argument is positive, send the signals to that process. If the argument is negative, send the signals to the process group with the ID of the absolute value of the argument. The process may only choose itself or its own process group to receive signals unless it has the CAP_KILL capability or an effective UID of 0.
FIOASYNC
Change the O_ASYNC flag to enable or disable asynchronous I/O mode of the socket. Asynchronous I/O mode means that the SIGIO signal or the signal set with F_SETSIG is raised when a new I/O event occurs.
Argument is an integer boolean flag. (This operation is synonymous with the use of fcntl(2) to set the O_ASYNC flag.)
SIOCGPGRP
Get the current process or process group that receives SIGIO or SIGURG signals, or 0 when none is set.

Valid fcntl(2) operations:

FIOGETOWN
The same as the SIOCGPGRP ioctl(2).
FIOSETOWN
The same as the SIOCSPGRP ioctl(2).
 

VERSIONS

SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASSCRED is new in Linux 2.2. The /proc interfaces was introduced in Linux 2.2. SO_RCVTIMEO and SO_SNDTIMEO are supported since Linux 2.3.41. Earlier, timeouts were fixed to a protocol-specific setting, and could not be read or written.  

NOTES

Linux assumes that half of the send/receive buffer is used for internal kernel structures; thus the values in the corresponding /proc files are twice what can be observed on the wire.

Linux will only allow port reuse with the SO_REUSEADDR option when this option was set both in the previous program that performed a bind(2) to the port and in the program that wants to reuse the port. This differs from some implementations (e.g., FreeBSD) where only the later program needs to set the SO_REUSEADDR option. Typically this difference is invisible, since, for example, a server program is designed to always set this option.  

BUGS

The CONFIG_FILTER socket options SO_ATTACH_FILTER and SO_DETACH_FILTER are not documented. The suggested interface to use them is via the libpcap library.  

SEE ALSO

getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7)  

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
Socket Layer Functions
Socket Options
Signals
/proc interfaces
Ioctls
VERSIONS
NOTES
BUGS
SEE ALSO
COLOPHON

This document was created by man2html using the manual pages.
Time: 17:32:31 GMT, October 23, 2013

3. socket.9.man

Manpage of socket

socket

Section: Tcl Built-In Commands (n)
Updated: 8.0
Index Return to Main Contents



 

NAME

socket - Open a TCP network connection  

SYNOPSIS

socket ?options? host port

socket -server command ?options? port




 

DESCRIPTION

This command opens a network socket and returns a channel identifier that may be used in future invocations of commands like read, puts and flush. At present only the TCP network protocol is supported; future releases may include support for additional protocols. The socket command may be used to open either the client or server side of a connection, depending on whether the -server switch is specified.

Note that the default encoding for all sockets is the system encoding, as returned by encoding system. Most of the time, you will need to use fconfigure to alter this to something else, such as utf-8 (ideal for communicating with other Tcl processes) or iso8859-1 (useful for many network protocols, especially the older ones).  

CLIENT SOCKETS

If the -server option is not specified, then the client side of a connection is opened and the command returns a channel identifier that can be used for both reading and writing. Port and host specify a port to connect to; there must be a server accepting connections on this port. Port is an integer port number (or service name, where supported and understood by the host operating system) and host is either a domain-style name such as www.tcl.tk or a numerical IP address such as 127.0.0.1. Use localhost to refer to the host on which the command is invoked.

The following options may also be present before host to specify additional information about the connection:

-myaddr addr
Addr gives the domain-style name or numerical IP address of the client-side network interface to use for the connection. This option may be useful if the client machine has multiple network interfaces. If the option is omitted then the client-side interface will be chosen by the system software.
-myport port
Port specifies an integer port number (or service name, where supported and understood by the host operating system) to use for the client's side of the connection. If this option is omitted, the client's port number will be chosen at random by the system software.
-async
The -async option will cause the client socket to be connected asynchronously. This means that the socket will be created immediately but may not yet be connected to the server, when the call to socket returns. When a gets or flush is done on the socket before the connection attempt succeeds or fails, if the socket is in blocking mode, the operation will wait until the connection is completed or fails. If the socket is in nonblocking mode and a gets or flush is done on the socket before the connection attempt succeeds or fails, the operation returns immediately and fblocked on the socket returns 1. Synchronous client sockets may be switched (after they have connected) to operating in asynchronous mode using:

fconfigure chan -blocking 0

See the fconfigure command for more details.

 

SERVER SOCKETS

If the -server option is specified then the new socket will be a server for the port given by port (either an integer or a service name, where supported and understood by the host operating system; if port is zero, the operating system will allocate a free port to the server socket which may be discovered by using fconfigure to read the -sockname option). Tcl will automatically accept connections to the given port. For each connection Tcl will create a new channel that may be used to communicate with the client. Tcl then invokes command with three additional arguments: the name of the new channel, the address, in network address notation, of the client's host, and the client's port number.

The following additional option may also be specified before port:

-myaddr addr
Addr gives the domain-style name or numerical IP address of the server-side network interface to use for the connection. This option may be useful if the server machine has multiple network interfaces. If the option is omitted then the server socket is bound to the special address INADDR_ANY so that it can accept connections from any interface.

Server channels cannot be used for input or output; their sole use is to accept new client connections. The channels created for each incoming client connection are opened for input and output. Closing the server channel shuts down the server so that no new connections will be accepted; however, existing connections will be unaffected.

Server sockets depend on the Tcl event mechanism to find out when new connections are opened. If the application does not enter the event loop, for example by invoking the vwait command or calling the C procedure Tcl_DoOneEvent, then no connections will be accepted.

If port is specified as zero, the operating system will allocate an unused port for use as a server socket. The port number actually allocated may be retrieved from the created server socket using the fconfigure command to retrieve the -sockname option as described below.  

CONFIGURATION OPTIONS

The fconfigure command can be used to query several readonly configuration options for socket channels:
-error
This option gets the current error status of the given socket. This is useful when you need to determine if an asynchronous connect operation succeeded. If there was an error, the error message is returned. If there was no error, an empty string is returned.
-sockname
This option returns a list of three elements, the address, the host name and the port number for the socket. If the host name cannot be computed, the second element is identical to the address, the first element of the list.
-peername
This option is not supported by server sockets. For client and accepted sockets, this option returns a list of three elements; these are the address, the host name and the port to which the peer socket is connected or bound. If the host name cannot be computed, the second element of the list is identical to the address, its first element.

 

EXAMPLES

Here is a very simple time server:

proc Server {channel clientaddr clientport} {
   puts "Connection from $clientaddr registered"
   puts $channel [clock format [clock seconds]]
   close $channel
}

socket -server Server 9900
vwait forever

And here is the corresponding client to talk to the server:


set server localhost
set sockChan [socket $server 9900]
gets $sockChan line
close $sockChan
puts "The time on $server is $line"

 

SEE ALSO

fconfigure(n), flush(n), open(n), read(n)

 

KEYWORDS

bind, channel, connection, domain name, host, network address, socket, tcp


 

Index

NAME
SYNOPSIS
DESCRIPTION
CLIENT SOCKETS
SERVER SOCKETS
CONFIGURATION OPTIONS
EXAMPLES
SEE ALSO
KEYWORDS

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