#include <sys/socket.h> mysocket = socket(int socket_family, int socket_type, int protocol);
DESCRIPTION
This man page describes the BSD compatible Linux networking sockets layer.
The sockets layer is the uniform interface between the user process and the in kernel
network protocol stacks. The protocol modules are grouped
into
protocol families
like PF_INET, PF_IPX, PF_PACKET and socket types like
SOCK_RAW or SOCK_DGRAM. See
socket(2)
for more information.
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 man pages.
socket(2)
creates a socket,
connect(2)
connects a socket to a foreign socket address,
the
bind(2)
function binds a socket to a local socket address,
listen(2)
tells the socket that new connections will be accepted, and
accept(2)
is used to get a new socket with the new connection.
socketpair(2)
returns two connected anonymous sockets.
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),
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 foreign 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 is not supported on sockets.
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_KEEPALIVE
Enable sending of keep-alive messages on connection-oriented sockets. Expects
a integer boolean flag.
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_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 not changeable in Linux and their argument size
is always fixed
to 1 byte.
Getsockopt
is able to read them;
setsockopt
will always return
ENOPROTOOPT.
SO_RCVTIME0 and SO_SNDTIME0
Specify the sending or receiving timeouts until reporting an error.
They are fixed to a protocol specific setting in Linux and cannot be read
or written. They can be easily emulated using
alarm(2).
SO_BSDCOMPAT
Enable BSD bug-to-bug compatibility. This is used only by the UDP
protocol module and scheduled to be removed in future.
If enabled ICMP errors received for a UDP socket will not be passed
to the user program. Linux 2.0 enabled BSD bug-to-bug compatibility
options (random header changing, skipping of the broadcast flag) for the raw
sockets with this option too, but that has been removed with Linux 2.2. It is
better to fix the user programs than to enable this flag.
SO_PASSCRED
Enable or disable the receiving of the
SCM_CREDENTIALS
control message. For more information see
unix(4).
SO_PEERCRED
Return the credentials of the foreign process connected to this socket.
Only useful for
PF_UNIX
sockets; see
unix(4).
Argument is a
ucred
structure. Only valid as a
getsockopt.
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 is not bound.
The passed option is a variable-length interface name string (with
the maximum size of
IFNAMSIZ).
If a socket is bound to an interface,
only packets received from the bound interface are passed to the user.
SO_DEBUG
Enable socket debugging. Only allowed for processes with effective user id 0.
SO_REUSEADDR
Indicates that the rules used in validating addresses supplied in a
bind(2)
call should allow reuse of local addresses. For PF_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.
SO_TYPE
Gets the socket type as an integer (like
SOCK_STREAM).
Can be only read
with
getsockopt.
SO_DONTROUTE
Bypass the routing table and send directly to the interface specified by the
network part of the destination address. The same effect can be achieved by
setting the
MSG_DONTROUTE
flag on a socket
send
operation. Expects an integer boolean flag.
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_SNDBUF
Sets or gets the maximum socket send buffer in bytes. Default value is set
by the
wmem_default
sysctl. The maximum allowed value is set by the
wmem_max
sysctl.
SO_RCVBUF
Sets or gets the maximum socket receive buffer in bytes. Default value is set
by the
rmem_default
sysctl. The maximum allowed value is set by the
rmem_max
sysctl.
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 long to linger for in seconds */
};
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_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(4),
this also sets the IP type-of-service (TOS) field for outgoing packets.
SO_ERROR
Get and clear the pending socket error. Only valid as a
getsockopt.
Expects an integer.
SIGNALS
On connection-oriented sockets,
SIGPIPE
is sent when a disconnection
request has been received or the process writes to a socket that has been
locally shut down. In some cases,
SIGPIPE
is only send when the
SO_KEEPALIVE
option is enabled.
When requested with the
FIOCSETOWN
or
SIOCSPGRP
ioctl,
SIGIO
is sent when an I/O event occurs.
Valid I/O events include:
-
New data arrived.
The socket send buffer has enough room to queue new data.
-
A new connection request has completed (only for
connection-oriented protocols).
-
A disconnection request has been initiated.
-
A connection is broken (only for connection-oriented protocols).
SIGPIPE
might be sent also in this case.
-
An asynchronous error occured.
The other end has shut down one direction.
In some situations (multiple processes or the kernel sending data to a single socket) the condition that caused the
SIGIO
might already have disappeared when
the
SIGIO
is processed by the user process. When this happens the user process
should just wait again because Linux guarantees to resend a new
SIGIO
later.
SYSCTLS
The core socket networking sysctls can be accessed using the
/proc/sys/net/core/*
files or with the
sysctl(2)
interface.
rmem_default
contains the default setting in bytes of the socket receive buffer.
rmem_max
contains the maximum receive socket buffer size in bytes a user can set using
the
SO_RCVBUF
socket option.
wmem_default
contains the default setting in bytes of the socket send buffer.
wmem_max
contains the maximum send socket buffer size in bytes a user can set 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 a input queue.
BUGS
The
CONFIG_FILTER
socket options
SO_ATTACH_FILTER
and
SO_DETACH_FILTER
are
not documented.
VERSIONS
SO_BINDTODEVICE
was introduced in Linux 2.0.30.
SO_PASSCRED
is new in Linux 2.2.
The sysctls are new in Linux 2.2.
