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socket(7) — Linux manual page
NAME
SYNOPSIS
DESCRIPTION
VERSIONS
NOTES
SEE ALSO
COLOPHON
socket
(7) Miscellaneous Information Manual
socket
(7)
NAME
top
socket - Linux socket interface
SYNOPSIS
top
#include
sockfd
= socket(int
socket_family
, int
socket_type
, int
protocol
);
DESCRIPTION
top
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
such as
AF_INET
AF_IPX
, and
AF_PACKET
, and
socket types
such as
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)
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.
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)
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).
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 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_ATTACH_FILTER
(since Linux 2.2)
SO_ATTACH_BPF
(since Linux 3.19)
Attach a classic BPF (
SO_ATTACH_FILTER
) or an extended BPF
SO_ATTACH_BPF
) program to the socket for use as a filter
of incoming packets. A packet will be dropped if the
filter program returns zero. If the filter program returns
a nonzero value which is less than the packet's data size,
the packet will be truncated to the size returned. If the
value returned by the filter is greater than or equal to
the packet's data size, the packet is allowed to proceed
unmodified.
The argument for
SO_ATTACH_FILTER
is a
sock_fprog
structure, defined in
struct sock_fprog {
unsigned short len;
struct sock_filter *filter;
};
The argument for
SO_ATTACH_BPF
is a file descriptor
returned by the
bpf(2)
system call and must refer to a
program of type
BPF_PROG_TYPE_SOCKET_FILTER
These options may be set multiple times for a given socket,
each time replacing the previous filter program. The
classic and extended versions may be called on the same
socket, but the previous filter will always be replaced
such that a socket never has more than one filter defined.
Both classic and extended BPF are explained in the kernel
source file
Documentation/networking/filter.txt
SO_ATTACH_REUSEPORT_CBPF
SO_ATTACH_REUSEPORT_EBPF
For use with the
SO_REUSEPORT
option, these options allow
the user to set a classic BPF (
SO_ATTACH_REUSEPORT_CBPF
) or
an extended BPF (
SO_ATTACH_REUSEPORT_EBPF
) program which
defines how packets are assigned to the sockets in the
reuseport group (that is, all sockets which have
SO_REUSEPORT
set and are using the same local address to
receive packets).
The BPF program must return an index between 0 and N-1
representing the socket which should receive the packet
(where N is the number of sockets in the group). If the
BPF program returns an invalid index, socket selection will
fall back to the plain
SO_REUSEPORT
mechanism.
Sockets are numbered in the order in which they are added
to the group (that is, the order of
bind(2)
calls for UDP
sockets or the order of
listen(2)
calls for TCP sockets).
New sockets added to a reuseport group will inherit the BPF
program. When a socket is removed from a reuseport group
(via
close(2)
), the last socket in the group will be moved
into the closed socket's position.
These options may be set repeatedly at any time on any
socket in the group to replace the current BPF program used
by all sockets in the group.
SO_ATTACH_REUSEPORT_CBPF
takes the same argument type as
SO_ATTACH_FILTER
and
SO_ATTACH_REUSEPORT_EBPF
takes the
same argument type as
SO_ATTACH_BPF
UDP support for this feature is available since Linux 4.5;
TCP support is available since Linux 4.6.
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 size is zero, the socket device
binding is removed. The passed option is a variable-size
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
IFNAMSIZ
bytes. The real device name
length is reported back in the
optlen
argument.
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 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. Allowed only for processes with
the
CAP_NET_ADMIN
capability or an effective user ID of 0.
SO_DETACH_FILTER
(since Linux 2.2)
SO_DETACH_BPF
(since Linux 3.19)
These two options, which are synonyms, may be used to
remove the classic or extended BPF program attached to a
socket with either
SO_ATTACH_FILTER
or
SO_ATTACH_BPF
. The
option value is ignored.
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_INCOMING_CPU
(gettable since Linux 3.19, settable since Linux
4.4)
Sets or gets the CPU affinity of a socket. Expects an
integer flag.
int cpu = 1;
setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu,
sizeof(cpu));
Because all of the packets for a single stream (i.e., all
packets for the same 4-tuple) arrive on the single RX queue
that is associated with a particular CPU, the typical use
case is to employ one listening process per RX queue, with
the incoming flow being handled by a listener on the same
CPU that is handling the RX queue. This provides optimal
NUMA behavior and keeps CPU caches hot.
SO_INCOMING_NAPI_ID
(gettable since Linux 4.12)
Returns a system-level unique ID called NAPI ID that is
associated with a RX queue on which the last packet
associated with that socket is received.
This can be used by an application to split the incoming
flows among worker threads based on the RX queue on which
the packets associated with the flows are received. It
allows each worker thread to be associated with a NIC HW
receive queue and service all the connection requests
received on that RX queue. This mapping between an app
thread and a HW NIC queue streamlines the flow of data from
the NIC to the application.
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_LOCK_FILTER
When set, this option will prevent changing the filters
associated with the socket. These filters include any set
using the socket options
SO_ATTACH_FILTER
SO_ATTACH_BPF
SO_ATTACH_REUSEPORT_CBPF
, and
SO_ATTACH_REUSEPORT_EBPF
The typical use case is for a privileged process to set up
a raw socket (an operation that requires the
CAP_NET_RAW
capability), apply a restrictive filter, set the
SO_LOCK_FILTER
option, and then either drop its privileges
or pass the socket file descriptor to an unprivileged
process via a UNIX domain socket.
Once the
SO_LOCK_FILTER
option has been enabled, attempts
to change or remove the filter attached to a socket, or to
disable the
SO_LOCK_FILTER
option will fail with the error
EPERM
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
or
CAP_NET_RAW
(since
Linux 5.17) 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_PASSSEC
Enable or disable the receiving of the
SCM_SECURITY
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_PEERCRED
Return the credentials of the peer process connected to
this socket. For further details, see
unix(7)
SO_PEERSEC
(since Linux 2.6.2)
Return the security context of the peer socket connected to
this socket. For further details, see
unix(7)
and
ip(7)
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. 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
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.
Before Linux 2.6.28
select(2)
poll(2)
, and
epoll(7)
did
not respect the
SO_RCVLOWAT
setting on Linux, and indicated
a socket as readable when even a single byte of data was
available. A subsequent read from the socket would then
block until
SO_RCVLOWAT
bytes are available. Since Linux
2.6.28,
select(2)
poll(2)
, and
epoll(7)
indicate a socket
as readable only if at least
SO_RCVLOWAT
bytes are
available.
SO_RCVTIMEO
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 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.,
accept(2)
connect(2)
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_REUSEPORT
(since Linux 3.9)
Permits multiple
AF_INET
or
AF_INET6
sockets to be bound to
an identical socket address. This option must be set on
each socket (including the first socket) prior to calling
bind(2)
on the socket. To prevent port hijacking, all of
the processes binding to the same address must have the
same effective UID. This option can be employed with both
TCP and UDP sockets.
For TCP sockets, this option allows
accept(2)
load
distribution in a multi-threaded server to be improved by
using a distinct listener socket for each thread. This
provides improved load distribution as compared to
traditional techniques such using a single
accept(2)
ing
thread that distributes connections, or having multiple
threads that compete to
accept(2)
from the same socket.
For UDP sockets, the use of this option can provide better
distribution of incoming datagrams to multiple processes
(or threads) as compared to the traditional technique of
having multiple processes compete to receive datagrams on
the same socket.
SO_RXQ_OVFL
(since Linux 2.6.33)
Indicates that an unsigned 32-bit value ancillary message
(cmsg) should be attached to received skbs indicating the
number of packets dropped by the socket since its creation.
SO_SELECT_ERR_QUEUE
(since Linux 3.10)
When this option is set on a socket, an error condition on
a socket causes notification not only via the
exceptfds
set
of
select(2)
. Similarly,
poll(2)
also returns a
POLLPRI
whenever an
POLLERR
event is returned.
Background: this option was added when waking up on an
error condition occurred only via the
readfds
and
writefds
sets of
select(2)
. The option was added to allow
monitoring for error conditions via the
exceptfds
argument
without simultaneously having to receive notifications (via
readfds
) for regular data that can be read from the socket.
After changes in Linux 4.16, the use of this flag to
achieve the desired notifications is no longer necessary.
This option is nevertheless retained for backwards
compatibility.
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 a
cmsg_type
of
SCM_TIMESTAMP
. 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_TIMESTAMPNS
(since Linux 2.6.22)
Enable or disable the receiving of the
SO_TIMESTAMPNS
control message. The timestamp control message is sent
with level
SOL_SOCKET
and a
cmsg_type
of
SCM_TIMESTAMPNS
The
cmsg_data
field is a
struct timespec
indicating the
reception time of the last packet passed to the user in
this call. The clock used for the timestamp is
CLOCK_REALTIME
. See
cmsg(3)
for details on control
messages.
A socket cannot mix
SO_TIMESTAMP
and
SO_TIMESTAMPNS
: the
two modes are mutually exclusive.
SO_TYPE
Gets the socket type as an integer (e.g.,
SOCK_STREAM
).
This socket option is read-only.
SO_BUSY_POLL
(since Linux 3.11)
Sets the approximate time in microseconds to busy poll on a
blocking receive when there is no data. Increasing this
value requires
CAP_NET_ADMIN
. The default for this option
is controlled by the
/proc/sys/net/core/busy_read
file.
The value in the
/proc/sys/net/core/busy_poll
file
determines how long
select(2)
and
poll(2)
will busy poll
when they operate on sockets with
SO_BUSY_POLL
set and no
events to report are found.
In both cases, busy polling will only be done when the
socket last received data from a network device that
supports this option.
While busy polling may improve latency of some
applications, care must be taken when using it since this
will increase both CPU utilization and power usage.
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 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.
/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
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
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 size 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 options
SO_TIMESTAMP
and
SO_TIMESTAMPNS
are not set on the socket. Otherwise, it
returns the timestamp of the last packet that was received
while
SO_TIMESTAMP
and
SO_TIMESTAMPNS
were 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 that is to receive
SIGIO
or
SIGURG
signals when I/O becomes possible or urgent data
is available. The argument is a pointer to a
pid_t
. For
further details, see the description of
F_SETOWN
in
fcntl(2)
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).
VERSIONS
top
SO_BINDTODEVICE
was introduced in Linux 2.0.30.
SO_PASSCRED
is
new in Linux 2.2. The
/proc
interfaces were 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.
NOTES
top
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 allow port reuse only 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.
SEE ALSO
top
wireshark(1)
bpf(2)
connect(2)
getsockopt(2)
setsockopt(2)
socket(2)
pcap
(3),
address_families(7)
capabilities(7)
ddp(7)
ip(7)
ipv6(7)
packet(7)
tcp(7)
udp(7)
unix(7)
tcpdump(8)
COLOPHON
top
This page is part of the
man-pages
(Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟩. If you have a bug report
for this manual page, see
⟩.
This page was obtained from the tarball man-pages-6.16.tar.gz
fetched from
⟩ on
2026-01-16. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is
not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.16 2025-09-21
socket
(7)
Pages that refer to this page:
accept(2)
bind(2)
bpf(2)
getpeername(2)
getsockname(2)
getsockopt(2)
intro(2)
listen(2)
recv(2)
recvmmsg(2)
seccomp(2)
send(2)
sendmmsg(2)
shutdown(2)
socket(2)
socketpair(2)
cmsg(3)
sd_is_fifo(3)
sd_journal_print(3)
sockaddr(3type)
systemd.exec(5)
systemd.network(5)
systemd.socket(5)
address_families(7)
bpf-helpers(7)
ddp(7)
ip(7)
ipv6(7)
mctp(7)
packet(7)
raw(7)
sctp(7)
tcp(7)
udp(7)
udplite(7)
unix(7)
x25(7)
ping(8)
tc-etf(8)
tc-fq(8)
tc-mqprio(8)
tc-prio(8)
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