Originální popis anglicky:
socket - Linux socket interface
Návod, kniha: Linux Programmer's Manual
#include <sys/socket.h>
mysocket = socket(int socket_family, int
socket_type, int protocol);
This manual page describes the Linux networking socket layer user interface. The
BSD compatible sockets are the uniform interface between the user process and
the network protocol stacks in the kernel. The protocol modules are grouped
into
protocol families like
PF_INET,
PF_IPX,
PF_PACKET and
socket types like
SOCK_STREAM or
SOCK_DGRAM. See
socket(2) for more information on families and
types.
These functions are used by the user process to send or receive packets and to
do other socket operations. For more information see their respective manual
pages.
socket(2) creates a socket,
connect(2) connects a socket to a
remote socket address, the
bind(2) function binds a socket to a local
socket address,
listen(2) tells the socket that new connections shall
be accepted, and
accept(2) is used to get a new socket with a new
incoming connection.
socketpair(2) returns two connected anonymous
sockets (only implemented for a few local families like
PF_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 non-zero position
is not supported on sockets.
It is possible to do non-blocking IO 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 |
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| Event |
Poll flag |
Occurrence |
| Read |
POLLIN |
New data arrived. |
| Read |
POLLIN |
A connection setup has been completed (for connection-oriented
sockets) |
| Read |
POLLHUP |
A disconnection request has been initiated by the other end. |
| Read |
POLLHUP |
A connection is broken (only for connection-oriented protocols). When
the socket is written SIGPIPE is also sent. |
| Write |
POLLOUT |
Socket has enough send buffer space for writing new data. |
| Read/Write |
POLLIN| POLLOUT |
An outgoing connect (2) finished. |
| Read/Write |
POLLERR |
An asynchronous error occurred. |
| Read/Write |
POLLHUP |
The other end has shut down one direction. |
| Exception |
POLLPRI |
Urgent data arrived. SIGURG is sent then. |
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An alternative to poll/select is to let the kernel inform the application about
events via a
SIGIO signal. For that the
FASYNC 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.
These socket options can be set by using
setsockopt(2) and read with
getsockopt(2) with the socket level set to
SOL_SOCKET for all
sockets:
- SO_KEEPALIVE
- Enable sending of keep-alive messages on
connection-oriented sockets. Expects an integer boolean flag.
- 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 only
passed when the MSG_OOB flag is set during receiving.
- 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 not
changeable in Linux and their argument size is always fixed to 1 byte.
getsockopt is able to read them; setsockopt will always
return ENOPROTOOPT.
- SO_RCVTIMEO and SO_SNDTIMEO
- Specify the receiving or sending timeouts until reporting
an error. The parameter 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
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.
- SO_BSDCOMPAT
- Enable BSD bug-to-bug compatibility. This is used only by
the UDP protocol module and scheduled to be removed in future. If enabled
ICMP errors received for a UDP socket will not be passed to the user
program. 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 has been removed in Linux 2.2. It is better to
fix the user programs than to enable this flag.
- SO_PASSCRED
- Enable or disable the receiving of the
SCM_CREDENTIALS control message. For more information see
unix(7).
- SO_PEERCRED
- Return the credentials of the foreign process connected to
this socket. Only useful for PF_UNIX sockets; see unix(7).
Argument is a ucred structure. Only valid as a
getsockopt.
- 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 only
works for some socket types, particularly AF_INET sockets. It is
not supported for packet sockets (use normal bind(8) there).
- SO_DEBUG
- Enable socket debugging. Only allowed for processes with
the CAP_NET_ADMIN capability or an effective user id of 0.
- SO_REUSEADDR
- Indicates that the rules used in validating addresses
supplied in a bind(2) call should allow reuse of local addresses.
For PF_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.
- SO_TYPE
- Gets the socket type as an integer (like
SOCK_STREAM). Can only be read with getsockopt.
- SO_ACCEPTCONN
- Returns a value indicating whether or not this socket has
been marked to accept connections with listen(). The value 0
indicates that this is not a listening socket, the value 1 indicates that
this is a listening socket. Can only be read with getsockopt.
- SO_DONTROUTE
- Don't send via a gateway, only send 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_BROADCAST
- Set or get the broadcast flag. When enabled, datagram
sockets receive packets sent to a broadcast address and they are allowed
to send packets to a broadcast address. This option has no effect on
stream-oriented sockets.
- SO_SNDBUF
- Sets or gets the maximum socket send buffer in bytes. The
default value is set by the wmem_default sysctl and the maximum
allowed value is set by the wmem_max sysctl.
- SO_RCVBUF
- Sets or gets the maximum socket receive buffer in bytes.
The default value is set by the rmem_default sysctl and the maximum
allowed value is set by the rmem_max sysctl.
- 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_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_ERROR
- Get and clear the pending socket error. Only valid as a
getsockopt. Expects an integer.
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 or
SIOCSPGRP ioctl,
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
realtime signal using the
F_SETSIG fcntl; the handler of the real time
signal will be called with the file descriptor in the
si_fd field of
its
siginfo_t. See
fcntl(2) for more information.
Under some circumstances (e.g. multiple processes accessing a single socket),
the condition that caused the
SIGIO may have already disappeared when
the process reacts to the signal. If this happens, the process should wait
again because Linux will resend the signal later.
The core socket networking sysctls can be accessed using the
/proc/sys/net/core/* files or with the
sysctl(2) interface.
- 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.
These ioctls 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.
- 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 signals unless it has the CAP_KILL
capability or an effective UID of 0.
- FIOASYNC
- Change the O_ASYNC flag to enable or disable
asynchronous IO mode of the socket. Asynchronous IO 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.
- SIOCGPGRP
- Get the current process or process group that receives
SIGIO or SIGURG signals, or 0 when none is set.
Valid fcntls:
- FIOGETOWN
- The same as the SIOCGPGRP ioctl.
- FIOSETOWN
- The same as the SIOCSPGRP ioctl
Linux assumes that half of the send/receive buffer is used for internal kernel
structures; thus the sysctls are twice what can be observed on the wire.
Linux will only allow port re-use with the SO_REUSEADDR option when this option
was set both in the previous program that performed a bind() to the port and
in the program that wants to re-use the port. This differs from some
implementations (e.g., FreeBSD) where only the later program needs to set the
SO_REUSEADDR option. Typically this difference is invisible, since, for
example, a server program is designed to always set this option.
The
CONFIG_FILTER socket options
SO_ATTACH_FILTER and
SO_DETACH_FILTER are not documented. The suggested interface to use
them is via the libpcap library.
SO_BINDTODEVICE was introduced in Linux 2.0.30.
SO_PASSCRED is new
in Linux 2.2. The sysctls are new 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.
getsockopt(2),
setsockopt(2),
socket(2),
capabilities(7),
ddp(7),
ip(7),
packet(7)
