SOCKET
NAME
socket - Linux socket interface
SYNOPSIS
#include <sys/socket.h>
mysocket = 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
PF_INET, PF_IPX, PF_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 incomming connection.
socketpair(2)
returns two connected anonymous sockets (only implemented for a few
local families like
PF_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 non-zero position is not supported on sockets.
It is possible to do non-blocking IO on sockets by setting the
O_NONBLOCK
flag on a socket file descriptor using
fcntl(2).
O_NONBLOCK
is inherited through an accept.
Then all operations that would normally block will (usually)
return with
EAGAIN;
connect(2)
returns an
EINPROGRESS
error in this case.
The user can then wait for various events via
poll(2)
or
select(2).
I/O events
|
Event | Poll flag | Occurrence
|
Read | POLLIN |
New data arrived.
|
Read | POLLIN |
A connection setup has been completed
(for connection-oriented sockets)
|
Read | POLLHUP |
A disconnection request has been initiated by the other end.
|
Read | POLLHUP |
A connection is broken (only for connection-oriented protocols).
When the socket is writen
SIGPIPE
is also sent.
|
Write | POLLOUT |
Socket has enough send buffer space for writing new data.
|
Read/Write |
POLLIN|
POLLOUT
|
An outgoing
connect(2)
finished.
|
Read/Write | POLLERR | An asynchronous error occured.
|
Read/Write | POLLHUP | The other end has shut down one direction.
|
Exception | POLLPRI |
Urgent data arrived.
SIGURG
is sent then.
|
An alternative to poll/select
is to let the kernel inform the application about events
via a
SIGIO
signal. For that the
FASYNC
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_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_RCVTIMEO and SO_SNDTIMEO
-
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. Their functionality can be emulated using
alarm(2)
or
setitimer(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 also enabled BSD bug-to-bug compatibility
options (random header changing, skipping of the broadcast flag) for raw
sockets with this option, but that has been removed in 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(7).
- SO_PEERCRED
-
Return the credentials of the foreign process connected to this socket.
Only useful for
PF_UNIX
sockets; see
unix(7).
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 null terminated 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 the
CAP_NET_ADMIN
capability or an effective user id of 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(2)
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. The default value is set
by the
wmem_default
sysctl and the maximum allowed value is set by the
wmem_max
sysctl.
- SO_RCVBUF
-
Sets or gets the maximum socket receive buffer in bytes. The default value is
set by the
rmem_default
sysctl and 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 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_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.
- SO_ERROR
-
Get and clear the pending socket error. Only valid as a
getsockopt.
Expects an integer.
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
FIOCSETOWN
fcntl or
SIOCSPGRP
ioctl,
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 realtime signal using the
F_SETSIG
fcntl; 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.
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 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 ioctls 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.
- 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 IO mode of the socket. Asynchronous IO
mode means that the
SIGIO
signal or the signal set with
F_SETSIG
is raised when a new I/O event occurs.
-
Argument is a integer boolean flag.
- SIOCGPGRP
-
Get the current process or process group that receives
SIGIO
or
SIGURG
signals,
or 0
when none is set.
Valid fcntls:
- FIOCGETOWN
-
The same as the SIOCGPGRP ioctl.
- FIOCSETOWN
-
The same as the SIOCSPGRP ioctl
NOTES
Linux assumes that half of the send/receive buffer is used for internal
kernel structures; thus the sysctls are twice than what can be observed
on the wire.
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.
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.
AUTHORS
This man page was writen by Andi Kleen.
SEE ALSO
socket(2), ip(7), setsockopt(2), getsockopt(2),
packet(7), ddp(7)