socket — Linux socket interface
#include <sys/socket.h>
mysocket = socket( |
int | socket_family, |
int | socket_type, | |
int | protocol) ; |
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.
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 I/O 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(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.
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 indicates that this is not a listening socket, the value 1 indicates that this is a listening socket. Can only be read with getsockopt(2).
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 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_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 program. 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 getsockopt(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 overhead) 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
initialised to 1. SO_SNDLOWAT
is not changeable on
Linux (setsockopt
fails
with the error ENOPROTOOPT). SO_RCVLOWAT
is changeable 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 output 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.
SO_SNDBUF
Sets or gets the maximum socket send buffer in
bytes. The kernel 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 message. The
timestamp control message is sent with level
SOL_SOCKET
and the
cmsg_data
field is a struct
timeval indicating 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).
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
disappeared when the process reacts to the signal. If this
happens, the process should wait again because Linux will
resend the signal later.
The core socket networking sysctls can be accessed using
the /proc/sys/net/core/*
files
or with the sysctl(2) interface.
contains the default setting in bytes of the socket receive buffer.
contains the maximum socket receive buffer size in
bytes which a user may set by using the SO_RCVBUF
socket option.
contains the default setting in bytes of the socket send buffer.
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.
Maximum number of packets in the global input queue.
Maximum length of ancillary data and user control data like the iovecs per socket.
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 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 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:
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.
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.
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_SNDTIMEO
are supported since Linux
2.3.41. Earlier, timeouts were fixed to a protocol specific
setting, and could not be read or written.
getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7), packet(7)
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