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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 incoming 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)
. 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 |
| 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 written 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 occurred. |
| 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 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_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
- 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_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
FIOSETOWN 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:
- FIOGETOWN
- The
same as the SIOCGPGRP ioctl.
- FIOSETOWN
- 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 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 written by Andi Kleen.
See Also
socket(2)
, ip(7)
, setsockopt(2)
, getsockopt(2)
,
packet(7)
, ddp(7)
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