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SOCKET(7) Linux Programmer's Manual SOCKET(7)
NAME
socket - Linux socket interface
SYNOPSIS
#include <sys/socket.h>
mysocket = socket(int socket_family, int socket_type, int protocol);
DESCRIPTION
This manual page describes the Linux networking socket layer user interface. The BSD com-
patible sockets are the uniform interface between the user process and the network proto-
col 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.
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 (only imple-
mented 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 con-
nection.
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 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 (usu-
ally) return with EAGAIN (operation should be retried later); connect(2) will return EIN-
PROGRESS error. The user can then wait for various events via poll(2) or select(2).
+--------------------------------------------------------------------+
| I/O events |
+-----------+-----------+--------------------------------------------+
|Event | Poll flag | Occurrence |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | New data arrived. |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | A connection setup has been completed (for |
| | | connection-oriented sockets) |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A disconnection request has been initiated |
| | | by the other end. |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A connection is broken (only for connec- |
| | | tion-oriented protocols). When the socket |
| | | is written SIGPIPE is also sent. |
+-----------+-----------+--------------------------------------------+
|Write | POLLOUT | Socket has enough send buffer space for |
| | | writing new data. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLIN| | An outgoing connect(2) finished. |
| | POLLOUT | |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLERR | An asynchronous error occurred. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLHUP | The other end has shut down one direction. |
+-----------+-----------+--------------------------------------------+
|Exception | POLLPRI | Urgent data arrived. SIGURG is sent then. |
+-----------+-----------+--------------------------------------------+
An alternative to poll(2) and select(2) is to let the kernel inform the application about
events via a SIGIO signal. For that the O_ASYNC flag must be set on a socket file
descriptor via fcntl(2) and a valid signal handler for SIGIO must be installed via sigac-
tion(2). See the Signals discussion below.
Socket Options
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_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. Can only be read
with getsockopt(2).
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 ter-
minated 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, particu-
larly AF_INET sockets. It is not supported for packet sockets (use normal bind(8)
there).
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_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. Only allowed for processes with the CAP_NET_ADMIN capa-
bility or an effective user ID of 0.
SO_ERROR
Get and clear the pending socket error. Only valid as a getsockopt(2). Expects an
integer.
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_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_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_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. This is
only possible for connected PF_UNIX stream sockets and PF_UNIX stream and datagram
socket pairs created using socketpair(2); see unix(7). The returned credentials
are those that were in effect at the time of the call to connect(2) or socket-
pair(2). Argument is a ucred structure. Only valid as a getsockopt(2).
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_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 rmem_default sysctl and the maximum allowed value is set by the rmem_max
sysctl. 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. The select(2) and poll(2) system calls currently do not respect
the SO_RCVLOWAT setting on Linux, and mark a socket readable when even a single
byte of data is available. A subsequent read from the socket will block until
SO_RCVLOWAT bytes are available.
SO_RCVTIMEO and SO_SNDTIMEO
Specify the receiving or sending timeouts until reporting an error. The argument
is a struct timeval. If an input or output function blocks for this period of
time, and data has been sent or received, the return value of that function will be
the amount of data transferred; if no data has been transferred and the timeout has
been reached then -1 is returned with errno set to EAGAIN or EWOULDBLOCK just as if
the socket was specified to be non-blocking. If the timeout is set to zero (the
default) then the operation will never timeout. Timeouts only have effect for sys-
tem 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), etc.
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. Argument is an
integer boolean flag.
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 wmem_default sysctl and the maximum allowed value is set by the wmem_max
sysctl. 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 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_TYPE
Gets the socket type as an integer (like SOCK_STREAM). Can only be read with get-
sockopt(2).
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 condi-
tion 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 sig-
nal later.
Sysctls
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.
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 only be used if the socket
option SO_TIMESTAMP is not set on the socket. Otherwise, it returns the timestamp
of the last packet that was received while SO_TIMESTAMP was not set, or it fails if
no such packet has been received, (i.e., ioctl(2) returns -1 with errno set to
ENOENT).
SIOCSPGRP
Set the process or process group to send SIGIO or SIGURG signals to when an asyn-
chronous 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 effec-
tive UID of 0.
FIOASYNC
Change the O_ASYNC flag to enable or disable asynchronous I/O mode of the socket.
Asynchronous I/O mode means that the SIGIO signal or the signal set with F_SETSIG
is raised when a new I/O event occurs.
Argument is an integer boolean flag. (This operation is 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
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.
NOTES
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(2) 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.
BUGS
The CONFIG_FILTER socket options SO_ATTACH_FILTER and SO_DETACH_FILTER are not documented.
The suggested interface to use them is via the libpcap library.
SEE ALSO
getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7), packet(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2007-12-28 SOCKET(7)
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