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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 (implemented only 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|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
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.
Each socket domain has its own format for socket
addresses, with a domain-specific address structure. Each
of these structures begins with an integer "family" field
(typed as sa_family_t) that indicates
the type of the address structure. This allows the various
system calls (e.g., connect(2), bind(2), accept(2), getsockname(2), getpeername(2)), which
are generic to all socket domains, to determine the domain
of a particular socket address.
To allow any type of socket address to be passed to interfaces in the sockets API, the type struct sockaddr is defined. The purpose of this type is purely to allow casting of domain-specific socket address types to a "generic" type, so as to avoid compiler warnings about type mismatches in calls to the sockets API.
In addition, the sockets API provides the data type struct sockaddr_storage. This type is suitable to accommodate all supported domain-specific socket address structures; it is large enough and is aligned properly. (In particular, it is large enough to hold IPv6 socket addresses.) The structure includes the following field, which can be used to identify the type of socket address actually stored in the structure:
sa_family_t ss_family;
The sockaddr_storage structure
is useful in programs that must handle socket addresses in
a generic way (e.g., programs that must deal with both IPv4
and IPv6 socket addresses).
The socket options listed below can be set by using
setsockopt(2) and read
with getsockopt(2) with the
socket level set to SOL_SOCKET for all sockets. Unless
otherwise noted, optval is a pointer to an
int.
SO_ACCEPTCONNReturns 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_BINDTODEVICEBind 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 works only for some socket types,
particularly AF_INET
sockets. It is not supported for packet sockets (use
normal bind(2) there).
Before Linux 3.8, this socket option could be set,
but could not retrieved with getsockopt(2).
Since Linux 3.8, it is readable. The optlen argument
should contain the buffer size available to receive
the device name and is recommended to be IFNAMSZ bytes. The real device name
length is reported back in the optlen argument.
SO_BROADCASTSet or get the broadcast flag. When enabled, datagram sockets are allowed to send packets to a broadcast address. This option has no effect on stream-oriented sockets.
SO_BSDCOMPATEnable 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_DEBUGEnable 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_ERRORGet and clear the pending socket error. This socket option is read-only. Expects an integer.
SO_DONTROUTEDon't send via a gateway, send only 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_KEEPALIVEEnable sending of keep-alive messages on connection-oriented sockets. Expects an integer boolean flag.
SO_LINGERSets or gets the SO_LINGER option. The argument is a
linger
structure.
struct linger { int l_onoff;int l_linger;};
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_MARK (since Linux
2.6.25)Set the mark for each packet sent through this
socket (similar to the netfilter MARK target but
socket-based). Changing the mark can be used for
mark-based routing without netfilter or for packet
filtering. Setting this option requires the
CAP_NET_ADMIN
capability.
SO_OOBINLINEIf this option is enabled, out-of-band data is
directly placed into the receive data stream.
Otherwise out-of-band data is passed only when the
MSG_OOB flag is set
during receiving.
SO_PASSCREDEnable or disable the receiving of the
SCM_CREDENTIALS control
message. For more information see unix(7).
SO_PEEK_OFF (since Linux
3.4)This option, which is currently supported only for
unix(7) sockets,
sets the value of the "peek offset" for the recv(2) system call
when used with MSG_PEEK
flag.
When this option is set to a negative value (it is
set to −1 for all new sockets), traditional
behavior is provided: recv(2) with the
MSG_PEEK flag will peek
data from the front of the queue.
When the option is set to a value greater than or equal to zero, then the next peek at data queued in the socket will occur at the byte offset specified by the option value. At the same time, the "peek offset" will be incremented by the number of bytes that were peeked from the queue, so that a subsequent peek will return the next data in the queue.
If data is removed from the front of the queue via
a call to recv(2) (or
similar) without the MSG_PEEK flag, the "peek offset"
will be decreased by the number of bytes removed. In
other words, receiving data without the MSG_PEEK flag will cause the "peek
offset" to be adjusted to maintain the correct
relative position in the queued data, so that a
subsequent peek will retrieve the data that would
have been retrieved had the data not been
removed.
For datagram sockets, if the "peek offset" points
to the middle of a packet, the data returned will be
marked with the MSG_TRUNC flag.
The following example serves to illustrate the use
of SO_PEEK_OFF. Suppose
a stream socket has the following queued input
data:
aabbccddeeff
The following sequence of recv(2) calls would have the effect noted in the comments:
int ov = 4; // Set peek offset to 4 setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov)); recv(fd, buf, 2, MSG_PEEK); // Peeks "cc"; offset set to 6 recv(fd, buf, 2, MSG_PEEK); // Peeks "dd"; offset set to 8 recv(fd, buf, 2, 0); // Reads "aa"; offset set to 6 recv(fd, buf, 2, MSG_PEEK); // Peeks "ee"; offset set to 8
SO_PEERCREDReturn the credentials of the foreign process
connected to this socket. This is possible only 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). The
argument is a ucred structure;
define the GNU_SOURCE
feature test macro to obtain the definition of that
structure from <sys/socket.h> This socket option is
read-only.
SO_PRIORITYSet 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_RCVBUFSets 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_SNDLOWATSpecify 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_SNDTIMEOSpecify 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, or EINPROGRESS (for connect(2)) 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), and
so on.
SO_REUSEADDRIndicates 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_SNDBUFSets 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_TIMESTAMPEnable 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_TYPEGets 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_defaultcontains the default setting in bytes of the socket receive buffer.
rmem_maxcontains the maximum socket receive buffer size in
bytes which a user may set by using the SO_RCVBUF socket option.
wmem_defaultcontains the default setting in bytes of the socket send buffer.
wmem_maxcontains the maximum socket send buffer size in
bytes which a user may set by using the SO_SNDBUF socket option.
message_cost and
message_burstconfigure the token bucket filter used to load limit warning messages caused by external network events.
netdev_max_backlogMaximum number of packets in the global input queue.
optmem_maxMaximum 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);
SIOCGSTAMPReturn 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 be used only
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).
SIOCSPGRPSet 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.
FIOASYNCChange 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.)
SIOCGPGRPGet the current process or process group that
receives SIGIO or
SIGURG signals, or 0
when none is set.
Valid fcntl(2) operations:
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), connect(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 3.54 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/.
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t This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. and copyright (c) 1999 Matthew Wilcox. %%%LICENSE_START(VERBATIM_ONE_PARA) Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. %%%LICENSE_END 2002-10-30, Michael Kerrisk, <mtk.manpagesgmail.com> Added description of SO_ACCEPTCONN 2004-05-20, aeb, added SO_RCVTIMEO/SO_SNDTIMEO text. Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements A few small grammar fixes 2010-06-13 Jan Engelhardt <jengelhmedozas.de> Documented SO_DOMAIN and SO_PROTOCOL. FIXME The following are not yet documented: SO_PEERNAME (2.4?) get only Seems to do something similar to getpeernam(), but then why is it necessary / how does it differ? SO_TIMESTAMPNS (2.6.22) Documentation/networking/timestamping.txt commit 92f37fd2ee805aa77925c1e64fd56088b46094fc Author: Eric Dumazet <dada1cosmosbay.com> SO_TIMESTAMPING (2.6.30) Documentation/networking/timestamping.txt commit cb9eff097831007afb30d64373f29d99825d0068 Author: Patrick Ohly <patrick.ohlyintel.com> SO_RXQ_OVFL (2.6.33) commit 3b885787ea4112eaa80945999ea0901bf742707f Author: Neil Horman <nhormantuxdriver.com> SO_WIFI_STATUS (3.3) commit 6e3e939f3b1bf8534b32ad09ff199d88800835a0 Author: Johannes Berg <johannes.bergintel.com> Also: SCM_WIFI_STATUS SO_NOFCS (3.4) commit 3bdc0eba0b8b47797f4a76e377dd8360f317450f Author: Ben Greear <greearbcandelatech.com> |