socket(7)
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 stan
dard 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 get
peername(2) returns the remote socket address. getsock
opt(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 (usu
ally) return with EAGAIN; connect(2) returns an EIN
PROGRESS 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 connec |
| | | tion-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| | An outgoing connect(2) finished. |
| | POLLOUT | |
+-----------+-----------+--------------------------------------------+
|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 connec
tion-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. Oth
erwise 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; setsock
opt 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 com
patibility options (random header changing, skip
ping 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_CREDEN
TIALS control message. For more information see
unix(7).
SO_PEERCRED
Return the credentials of the foreign process con
nected to this socket. Only useful for PF_UNIX
sockets; see unix(7). Argument is a ucred struc
ture. Only valid as a getsockopt.
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.
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, data
gram 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 imme
diately 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 pos
sible to use poll(2) or select(2) in the signal handler to
find out which socket the event occurred on. An alterna
tive (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 access
ing 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 use
ful 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 opera
tion 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 asyn
chronous 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 inter
face to use them is via the libpcap library.
VERSIONS
SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASS
CRED 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)
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