# 7.2 `socket` -- Low-level networking interface

This module provides access to the BSD socket interface. It is available on Unix systems that support this interface.

For an introduction to socket programming (in C), see the following papers: An Introductory 4.3BSD Interprocess Communication Tutorial, by Stuart Sechrest and An Advanced 4.3BSD Interprocess Communication Tutorial, by Samuel J. Leffler et al, both in the Unix Programmer's Manual, Supplementary Documents 1 (sections PS1:7 and PS1:8). The Unix manual pages for the various socket-related system calls are also a valuable source of information on the details of socket semantics.

The Python interface is a straightforward transliteration of the Unix system call and library interface for sockets to Python's object-oriented style: the `socket()` function returns a socket object whose methods implement the various socket system calls. Parameter types are somewhat higher-level than in the C interface: as with `read()` and `write()` operations on Python files, buffer allocation on receive operations is automatic, and buffer length is implicit on send operations.

Socket addresses are represented as a single string for the `AF_UNIX` address family and as a pair `(host, port)` for the `AF_INET` address family, where host is a string representing either a hostname in Internet domain notation like `'daring.cwi.nl'` or an IP address like `'100.50.200.5'`, and port is an integral port number. Other address families are currently not supported. The address format required by a particular socket object is automatically selected based on the address family specified when the socket object was created.

For IP addresses, two special forms are accepted instead of a host address: the empty string represents `INADDR_ANY`, and the string `'<broadcast>'` represents `INADDR_BROADCAST`.

All errors raise exceptions. The normal exceptions for invalid argument types and out-of-memory conditions can be raised; errors related to socket or address semantics raise the error `socket.error`.

Non-blocking mode is supported through the `setblocking()` method.

The module `socket` exports the following constants and functions:

`error`
  This exception is raised for socket- or address-related errors. The accompanying value is either a string telling what went wrong or a pair `(errno, string)`representing an error returned by a system call, similar to the value accompanying `os.error`. See the module `errno`, which contains names for the error codes defined by the underlying operating system.

`AF_UNIX`
`AF_INET`
  These constants represent the address (and protocol) families, used for the first argument to `socket()`. If the `AF_UNIX` constant is not defined then this protocol is unsupported.

`SOCK_STREAM`
`SOCK_DGRAM`
`SOCK_RAW`
`SOCK_RDM`
`SOCK_SEQPACKET`
  These constants represent the socket types, used for the second argument to `socket()`. (Only `SOCK_STREAM` and `SOCK_DGRAM` appear to be generally useful.)

`SO_*`
`SOMAXCONN`
`MSG_*`
`SOL_*`
`IPPROTO_*`
`IPPORT_*`
`INADDR_*`
`IP_*`
  Many constants of these forms, documented in the Unix documentation on sockets and/or the IP protocol, are also defined in the socket module. They are generally used in arguments to the `setsockopt()` and `getsockopt()` methods of socket objects. In most cases, only those symbols that are defined in the Unix header files are defined; for a few symbols, default values are provided.

`gethostbyname` (hostname)
  Translate a host name to IP address format. The IP address is returned as a string, e.g., `'100.50.200.5'`. If the host name is an IP address itself it is returned unchanged. See `gethostbyname_ex()` for a more complete interface.

`gethostbyname_ex` (hostname)
  Translate a host name to IP address format, extended interface. Return a triple `(hostname, aliaslist, ipaddrlist)` where `hostname` is the primary host name responding to the given ip_address, `aliaslist` is a (possibly empty) list of alternative host names for the same address, and `ipaddrlist` is a list of IP addresses for the same interface on the same host (often but not always a single address).

`gethostname` ()
  Return a string containing the hostname of the machine where the Python interpreter is currently executing. If you want to know the current machine's IP address, use `gethostbyname(gethostname())`. Note: `gethostname()` doesn't always return the fully qualified domain name; use `gethostbyaddr(gethostname())`(see below).

`gethostbyaddr` (ip_address)
  Return a triple `(hostname, aliaslist, ipaddrlist)` where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IP addresses for the same interface on the same host (most likely containing only a single address). To find the fully qualified domain name, check hostname and the items of aliaslist for an entry containing at least one period.

`getprotobyname` (protocolname)
  Translate an Internet protocol name (e.g. `'icmp'`) to a constant suitable for passing as the (optional) third argument to the `socket()` function. This is usually only needed for sockets opened in ``raw'' mode (`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen automatically if the protocol is omitted or zero.

`getservbyname` (servicename, protocolname)
  Translate an Internet service name and protocol name to a port number for that service. The protocol name should be `'tcp'` or `'udp'`.

`socket` (family, type[, proto])
  Create a new socket using the given address family, socket type and protocol number. The address family should be `AF_INET` or `AF_UNIX`. The socket type should be `SOCK_STREAM`, `SOCK_DGRAM` or perhaps one of the other "`SOCK_`" constants. The protocol number is usually zero and may be omitted in that case.

`fromfd` (fd, family, type[, proto])
  Build a socket object from an existing file descriptor (an integer as returned by a file object's `fileno()` method). Address family, socket type and protocol number are as for the `socket()` function above. The file descriptor should refer to a socket, but this is not checked -- subsequent operations on the object may fail if the file descriptor is invalid. This function is rarely needed, but can be used to get or set socket options on a socket passed to a program as standard input or output (e.g. a server started by the Unix inet daemon).

`ntohl` (x)
  Convert 32-bit integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

`ntohs` (x)
  Convert 16-bit integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

`htonl` (x)
  Convert 32-bit integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

`htons` (x)
  Convert 16-bit integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

`SocketType`
  This is a Python type object that represents the socket object type. It is the same as `type(socket(...))`.

- 7.2.1 Socket Objects
- 7.2.2 Example