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 length, the packet will be truncated to the length
returned. If the value returned by the filter is greater
than or equal to the packet's data length, the packet is
allowed to proceed unmodified.
The argument for SO_ATTACH_FILTER is a sock_fprog
structure, defined in <linux/filter.h>:
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 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 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 a 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 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 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.
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 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
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_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 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 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).
