SOCKET(4) Linux Programmer's Manual SOCKET(4)
NAME
socket - Linux socket interface
SYNOPSIS
#include <sys/socket.h>
mysocket = socket(int socket_family, int socket_type, int
protocol);
DESCRIPTION
This man page describes the BSD compatible Linux network-
ing sockets layer. The sockets layer is the uniform
interface between the user process and the in kernel net-
work 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 connec-
tion. socketpair(2) returns two connected anonymous sock-
ets.
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 stan-
dard 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 get-
peername(2) returns the foreign socket address. getsock-
opt(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 connec-
tion-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. Oth-
erwise 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; setsock-
opt 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 compati-
bility 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_CREDEN-
TIALS control message. For more information see
unix(4).
SO_PEERCRED
Return the credentials of the foreign process con-
nected to this socket. Only useful for PF_UNIX
sockets; see unix(4). Argument is a ucred struc-
ture. Only valid as a getsockopt.
SO_BINDTODEVICE
Bind this socket to a particular device like
"eth0", 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 des-
tination 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, data-
gram 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 imme-
diately 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 con-
nection-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 send-
ing 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_PASS-
CRED is new in Linux 2.2. The sysctls are new in Linux
2.2.
SEE ALSO
socket(2), ip(4), setsockopt(2), getsockopt(2), packet(4),
ddp(4)
Linux Man Page 3 Oct 1998 1