SOCKET(7) Linux Programmer's Manual SOCKET(7)
NAME
socket - Linux socket interface
SYNOPSIS
#include <sys/socket.h>
sockfd = 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 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.
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
(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), 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 nonzero position is
not supported on sockets.
It is possible to do nonblocking 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 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.
Socket address structures
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), getpeer-
name(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 mis-
matches in calls to the sockets API.
In addition, the sockets API provides the data type struct sock-
addr_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).
Socket options
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_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. This socket option is read-
only.
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 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 argu-
ment.
SO_BROADCAST
Set 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_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_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, 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_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_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_OOBINLINE
If 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_PASSCRED
Enable 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) sock-
ets, 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.i
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 off-
set" 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_PEERCRED
Return 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_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 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
/proc/sys/net/core/rmem_default file, and the maximum allowed
value is set by the /proc/sys/net/core/rmem_max file. The mini-
mum (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(2) fails with the error ENOPROTOOPT). SO_RCVLOWAT is
changeable only since Linux 2.4. The select(2) and poll(2) sys-
tem 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, or EINPROGRESS (for connect(2)) just
as if the socket was specified to be nonblocking. If the time-
out is set to zero (the default) then the operation will never
timeout. Timeouts only have effect for system calls that per-
form 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_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 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
/proc/sys/net/core/wmem_default file and the maximum allowed
value is set by the /proc/sys/net/core/wmem_max file. The mini-
mum (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 (e.g., SOCK_STREAM). This
socket option is read-only.
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 real-time sig-
nal 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 sig-
info_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.
/proc interfaces
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 buf-
fer.
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 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).
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. (This operation is synony-
mous 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).
VERSIONS
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_RCV-
TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41. Earlier, time-
outs 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 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.
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.
SEE ALSO
getsockopt(2), connect(2), setsockopt(2), socket(2), capabilities(7),
ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7)
COLOPHON
This page is part of release 3.53 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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