socket(7)

SECCIÓN: 7 - Miscelánea

socket(7) Miscellaneous Information 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 in‐

terface. 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 re‐

spective 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 ac‐

cepted, and accept(2) is used to get a new socket with a new incoming

connection. socketpair(2) returns two connected anonymous sockets (im‐

plemented 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) re‐

turns 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 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 do‐

main-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 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 do‐

main-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_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 in‐

coming 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, de‐

fined 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 ex‐

tended versions may be called on the same socket, but the previ‐

ous 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 ex‐

tended 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 repre‐

senting 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_AT‐

TACH_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 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 IFNAMSIZ 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 op‐

tion. Linux 2.0 also enabled BSD bug-to-bug compatibility op‐

tions (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 op‐

tion 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 pack‐

ets for the same 4-tuple) arrive on the single RX queue that is

associated with a particular CPU, the typical use case is to em‐

ploy 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 associ‐

ated 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 re‐

turns 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 associ‐

ated with the socket. These filters include any set using the

socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE‐

PORT_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 descrip‐

tor 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_AD‐

MIN 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 mes‐

sage. 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.

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 re‐

trieve 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 ca‐

pability.

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.

Before Linux 2.6.28 select(2), poll(2), and epoll(7) did not re‐

spect 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 er‐

ror. 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., 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 ad‐

dress. 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 hav‐

ing multiple threads that compete to accept(2) from the same

socket.

For UDP sockets, the use of this option can provide better dis‐

tribution of incoming datagrams to multiple processes (or

threads) as compared to the traditional technique of having mul‐

tiple 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 se‐

lect(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 se‐

lect(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 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 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 con‐

trolled 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 op‐

tion.

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 se‐

lect(2) in the signal handler to find out which socket the event oc‐

curred 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 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

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 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 were 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 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 in‐

visible, since, for example, a server program is designed to always set

this option.

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Índice de la Sección 7

Índice General

NAME

SYNOPSIS

DESCRIPTION

VERSIONS

NOTES

SEE ALSO

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