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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), send
file(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 I/O on sockets by setting the O_NON
BLOCK flag on a socket file descriptor using fcntl(2). Then all opera‐
tions 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 connec‐ |
| | | tion-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| | An outgoing connect(2) finished. |
| | POLLOUT | |
+-----------+-----------+--------------------------------------------+
|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(2) and select(2) 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_ACCEPTCONN
Returns a value indicating whether or not this socket has been
marked to accept connections with listen(2). The value 0 indi‐
cates that this is not a listening socket, the value 1 indicates
that this is a listening socket. Can only be read with getsock
opt(2).
SO_BINDTODEVICE
Bind this socket to a particular device like “eth0”, as speci‐
fied 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 termi‐
nated 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 pro‐
gram. 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_ERROR
Get and clear the pending socket error. Only valid as a get
sockopt(2). 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 PF_UNIX stream
sockets and PF_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. Only valid as
a getsockopt(2).
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_RCVBUF
Sets or gets the maximum socket receive buffer in bytes. The
kernel doubles this value (to allow space for bookkeeping over‐
head) when it is set using setsockopt(2), and this doubled value
is returned by getsockopt(2). The default value is set by the
rmem_default sysctl and the maximum allowed value is set by the
rmem_max sysctl. 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 ini‐
tialized to 1. SO_SNDLOWAT is not changeable on Linux (setsock
opt fails with the error ENOPROTOOPT). SO_RCVLOWAT is change‐
able 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 parameter is a struct timeval. If an input or out‐
put 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.
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. Argument is an integer boolean flag.
SO_SNDBUF
Sets or gets the maximum socket send buffer in bytes. The ker‐
nel 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
wmem_default sysctl and the maximum allowed value is set by the
wmem_max sysctl. 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 mes‐
sage. The timestamp control message is sent with level
SOL_SOCKET and the cmsg_data field is a struct timeval indicat‐
ing 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 (like SOCK_STREAM). Can only
be read with getsockopt(2).
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 realtime 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 disap‐
peared 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 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 sig‐
nals 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.
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 sysctls are new in Linux 2.2. SO_RCVTIMEO and SO_SND
TIMEO 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 sysctls are twice what can be observed on
the wire.
Linux will only allow port re-use 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 re-use 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.
getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7),
ip(7), packet(7)