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SOCKET(7) Linux Programmer's Manual SOCKET(7)
socket - Linux socket interface
#include <sys/socket.h>
sockfd = 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 AF_INET, AF_IPX, AF_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 AF_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 nonzero position is not
supported on sockets.
It is possible to do nonblocking 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| | 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 O_ASYNC 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. This socket option is read-only.
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_DOMAIN (since Linux 2.6.32)
Retrieves the socket domain as an integer, returning a value such as
AF_INET6. See socket(2) for details. This socket option is read-only.
SO_ERROR
Get and clear the pending socket error. This socket option is read-
only. 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 AF_UNIX stream sockets and AF_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. This socket option is read-only.
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_PROTOCOL (since Linux 2.6.32)
Retrieves the socket protocol as an integer, returning a value such as
IPPROTO_SCTP. See socket(2) for details. This socket option is read-
only.
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
/proc/sys/net/core/rmem_default file, and the maximum allowed value is
set by the /proc/sys/net/core/rmem_max file. 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 initialized to 1.
SO_SNDLOWAT is not changeable on Linux (setsockopt(2) 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 argument 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. Timeouts only
have effect for system calls that perform socket I/O (e.g., read(2),
recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for
select(2), poll(2), epoll_wait(2), etc.
SO_REUSEADDR
Indicates that the rules used in validating addresses supplied in a
bind(2) call should allow reuse of local addresses. For AF_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 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
/proc/sys/net/core/wmem_default file and the maximum allowed value is
set by the /proc/sys/net/core/wmem_max file. 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 (e.g., SOCK_STREAM). This socket
option is read-only.
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 real-time 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 parameters can be accessed via files in the
directory /proc/sys/net/core/.
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.
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. (This operation is synonymous
with the use of fcntl(2) to set the O_ASYNC 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).
SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASSCRED is new in Linux
2.2. The /proc interfaces was introduced 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.
Linux assumes that half of the send/receive buffer is used for internal kernel
structures; thus the values in the corresponding /proc files are twice what
can be observed on the wire.
Linux will only allow port reuse 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 reuse 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.
getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7),
packet(7), tcp(7), udp(7), unix(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-06-13 SOCKET(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface