377 lines
16 KiB
TeX
377 lines
16 KiB
TeX
\section{Built-in Module \sectcode{socket}}
|
|
\label{module-socket}
|
|
|
|
\bimodindex{socket}
|
|
This module provides access to the BSD {\em socket} interface.
|
|
It is available on \UNIX{} systems that support this interface.
|
|
|
|
For an introduction to socket programming (in C), see the following
|
|
papers: \emph{An Introductory 4.3BSD Interprocess Communication
|
|
Tutorial}, by Stuart Sechrest and \emph{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 \code{socket()} function returns a
|
|
\dfn{socket object} whose methods implement the various socket system
|
|
calls. Parameter types are somewhat higher-level than in the C
|
|
interface: as with \code{read()} and \code{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
|
|
\code{AF_UNIX} address family and as a pair
|
|
\code{(\var{host}, \var{port})} for the \code{AF_INET} address family,
|
|
where \var{host} is a string representing
|
|
either a hostname in Internet domain notation like
|
|
\code{'daring.cwi.nl'} or an IP address like \code{'100.50.200.5'},
|
|
and \var{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 \code{INADDR_ANY}, and the string
|
|
\code{"<broadcast>"} represents \code{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 \code{socket.error}.
|
|
|
|
Non-blocking mode is supported through the \code{setblocking()}
|
|
method.
|
|
|
|
The module \code{socket} exports the following constants and functions:
|
|
|
|
\renewcommand{\indexsubitem}{(in module socket)}
|
|
\begin{excdesc}{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 \code{(\var{errno}, \var{string})}
|
|
representing an error returned by a system
|
|
call, similar to the value accompanying \code{posix.error}.
|
|
\end{excdesc}
|
|
|
|
\begin{datadesc}{AF_UNIX}
|
|
\dataline{AF_INET}
|
|
These constants represent the address (and protocol) families,
|
|
used for the first argument to \code{socket()}. If the \code{AF_UNIX}
|
|
constant is not defined then this protocol is unsupported.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{SOCK_STREAM}
|
|
\dataline{SOCK_DGRAM}
|
|
\dataline{SOCK_RAW}
|
|
\dataline{SOCK_RDM}
|
|
\dataline{SOCK_SEQPACKET}
|
|
These constants represent the socket types,
|
|
used for the second argument to \code{socket()}.
|
|
(Only \code{SOCK_STREAM} and
|
|
\code{SOCK_DGRAM} appear to be generally useful.)
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{SO_*}
|
|
\dataline{SOMAXCONN}
|
|
\dataline{MSG_*}
|
|
\dataline{SOL_*}
|
|
\dataline{IPPROTO_*}
|
|
\dataline{IPPORT_*}
|
|
\dataline{INADDR_*}
|
|
\dataline{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 \code{setsockopt} and
|
|
\code{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.
|
|
\end{datadesc}
|
|
|
|
\begin{funcdesc}{gethostbyname}{hostname}
|
|
Translate a host name to IP address format. The IP address is
|
|
returned as a string, e.g., \code{'100.50.200.5'}. If the host name
|
|
is an IP address itself it is returned unchanged.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{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
|
|
\code{socket.gethostbyname(socket.gethostname())}.
|
|
Note: \code{gethostname()} doesn't always return the fully qualified
|
|
domain name; use \code{socket.gethostbyaddr(socket.gethostname())}
|
|
(see below).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{gethostbyaddr}{ip_address}
|
|
Return a triple \code{(hostname, aliaslist, ipaddrlist)} where
|
|
\code{hostname} is the primary host name responding to the given
|
|
\var{ip_address}, \code{aliaslist} is a (possibly empty) list of
|
|
alternative host names for the same address, and \code{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 \var{hostname} and the
|
|
items of \var{aliaslist} for an entry containing at least one period.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{getprotobyname}{protocolname}
|
|
Translate an Internet protocol name (e.g. \code{'icmp'}) to a constant
|
|
suitable for passing as the (optional) third argument to the
|
|
\code{socket()} function. This is usually only needed for sockets
|
|
opened in ``raw'' mode (\code{SOCK_RAW}); for the normal socket modes,
|
|
the correct protocol is chosen automatically if the protocol is
|
|
omitted or zero.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{getservbyname}{servicename\, protocolname}
|
|
Translate an Internet service name and protocol name to a port number
|
|
for that service. The protocol name should be \code{'tcp'} or
|
|
\code{'udp'}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{socket}{family\, type\optional{\, proto}}
|
|
Create a new socket using the given address family, socket type and
|
|
protocol number. The address family should be \code{AF_INET} or
|
|
\code{AF_UNIX}. The socket type should be \code{SOCK_STREAM},
|
|
\code{SOCK_DGRAM} or perhaps one of the other \samp{SOCK_} constants.
|
|
The protocol number is usually zero and may be omitted in that case.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{fromfd}{fd\, family\, type\optional{\, proto}}
|
|
Build a socket object from an existing file descriptor (an integer as
|
|
returned by a file object's \code{fileno} method). Address family,
|
|
socket type and protocol number are as for the \code{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).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{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.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{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.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{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.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{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.
|
|
\end{funcdesc}
|
|
|
|
\begin{datadesc}{SocketType}
|
|
This is a Python type object that represents the socket object type.
|
|
It is the same as \code{type(socket.socket(...))}.
|
|
\end{datadesc}
|
|
|
|
\subsection{Socket Objects}
|
|
|
|
\noindent
|
|
Socket objects have the following methods. Except for
|
|
\code{makefile()} these correspond to \UNIX{} system calls applicable to
|
|
sockets.
|
|
|
|
\renewcommand{\indexsubitem}{(socket method)}
|
|
\begin{funcdesc}{accept}{}
|
|
Accept a connection.
|
|
The socket must be bound to an address and listening for connections.
|
|
The return value is a pair \code{(\var{conn}, \var{address})}
|
|
where \var{conn} is a \emph{new} socket object usable to send and
|
|
receive data on the connection, and \var{address} is the address bound
|
|
to the socket on the other end of the connection.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{bind}{address}
|
|
Bind the socket to \var{address}. The socket must not already be bound.
|
|
(The format of \var{address} depends on the address family --- see above.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{close}{}
|
|
Close the socket. All future operations on the socket object will fail.
|
|
The remote end will receive no more data (after queued data is flushed).
|
|
Sockets are automatically closed when they are garbage-collected.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{connect}{address}
|
|
Connect to a remote socket at \var{address}.
|
|
(The format of \var{address} depends on the address family --- see above.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{connect_ex}{address}
|
|
Likecode{connect(\var{address})}, but return an error indicator
|
|
instead of raising an exception. The error indicator is 0 if the
|
|
operation succeeded, otherwise the value of the \code{errno}
|
|
variable. This is useful e.g. for asynchronous connects.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{fileno}{}
|
|
Return the socket's file descriptor (a small integer). This is useful
|
|
with \code{select}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{getpeername}{}
|
|
Return the remote address to which the socket is connected. This is
|
|
useful to find out the port number of a remote IP socket, for instance.
|
|
(The format of the address returned depends on the address family ---
|
|
see above.) On some systems this function is not supported.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{getsockname}{}
|
|
Return the socket's own address. This is useful to find out the port
|
|
number of an IP socket, for instance.
|
|
(The format of the address returned depends on the address family ---
|
|
see above.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{getsockopt}{level\, optname\optional{\, buflen}}
|
|
Return the value of the given socket option (see the \UNIX{} man page
|
|
{\it getsockopt}(2)). The needed symbolic constants (\code{SO_*} etc.)
|
|
are defined in this module. If \var{buflen}
|
|
is absent, an integer option is assumed and its integer value
|
|
is returned by the function. If \var{buflen} is present, it specifies
|
|
the maximum length of the buffer used to receive the option in, and
|
|
this buffer is returned as a string. It is up to the caller to decode
|
|
the contents of the buffer (see the optional built-in module
|
|
\code{struct} for a way to decode C structures encoded as strings).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{listen}{backlog}
|
|
Listen for connections made to the socket. The \var{backlog} argument
|
|
specifies the maximum number of queued connections and should be at
|
|
least 1; the maximum value is system-dependent (usually 5).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{makefile}{\optional{mode\optional{\, bufsize}}}
|
|
Return a \dfn{file object} associated with the socket. (File objects
|
|
were described earlier under Built-in Types.) The file object
|
|
references a \code{dup()}ped version of the socket file descriptor, so
|
|
the file object and socket object may be closed or garbage-collected
|
|
independently. The optional \var{mode} and \var{bufsize} arguments
|
|
are interpreted the same way as by the built-in
|
|
\code{open()} function.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{recv}{bufsize\optional{\, flags}}
|
|
Receive data from the socket. The return value is a string representing
|
|
the data received. The maximum amount of data to be received
|
|
at once is specified by \var{bufsize}. See the \UNIX{} manual page
|
|
for the meaning of the optional argument \var{flags}; it defaults to
|
|
zero.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{recvfrom}{bufsize\optional{\, flags}}
|
|
Receive data from the socket. The return value is a pair
|
|
\code{(\var{string}, \var{address})} where \var{string} is a string
|
|
representing the data received and \var{address} is the address of the
|
|
socket sending the data. The optional \var{flags} argument has the
|
|
same meaning as for \code{recv()} above.
|
|
(The format of \var{address} depends on the address family --- see above.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{send}{string\optional{\, flags}}
|
|
Send data to the socket. The socket must be connected to a remote
|
|
socket. The optional \var{flags} argument has the same meaning as for
|
|
\code{recv()} above. Return the number of bytes sent.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{sendto}{string\optional{\, flags}\, address}
|
|
Send data to the socket. The socket should not be connected to a
|
|
remote socket, since the destination socket is specified by
|
|
\code{address}. The optional \var{flags} argument has the same
|
|
meaning as for \code{recv()} above. Return the number of bytes sent.
|
|
(The format of \var{address} depends on the address family --- see above.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{setblocking}{flag}
|
|
Set blocking or non-blocking mode of the socket: if \var{flag} is 0,
|
|
the socket is set to non-blocking, else to blocking mode. Initially
|
|
all sockets are in blocking mode. In non-blocking mode, if a
|
|
\code{recv} call doesn't find any data, or if a \code{send} call can't
|
|
immediately dispose of the data, a \code{socket.error} exception is
|
|
raised; in blocking mode, the calls block until they can proceed.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{setsockopt}{level\, optname\, value}
|
|
Set the value of the given socket option (see the \UNIX{} man page
|
|
{\it setsockopt}(2)). The needed symbolic constants are defined in
|
|
the \code{socket} module (\code{SO_*} etc.). The value can be an
|
|
integer or a string representing a buffer. In the latter case it is
|
|
up to the caller to ensure that the string contains the proper bits
|
|
(see the optional built-in module
|
|
\code{struct} for a way to encode C structures as strings).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{shutdown}{how}
|
|
Shut down one or both halves of the connection. If \var{how} is \code{0},
|
|
further receives are disallowed. If \var{how} is \code{1}, further sends are
|
|
disallowed. If \var{how} is \code{2}, further sends and receives are
|
|
disallowed.
|
|
\end{funcdesc}
|
|
|
|
Note that there are no methods \code{read()} or \code{write()}; use
|
|
\code{recv()} and \code{send()} without \var{flags} argument instead.
|
|
|
|
\subsection{Example}
|
|
\nodename{Socket Example}
|
|
|
|
Here are two minimal example programs using the TCP/IP protocol:\ a
|
|
server that echoes all data that it receives back (servicing only one
|
|
client), and a client using it. Note that a server must perform the
|
|
sequence \code{socket}, \code{bind}, \code{listen}, \code{accept}
|
|
(possibly repeating the \code{accept} to service more than one client),
|
|
while a client only needs the sequence \code{socket}, \code{connect}.
|
|
Also note that the server does not \code{send}/\code{receive} on the
|
|
socket it is listening on but on the new socket returned by
|
|
\code{accept}.
|
|
|
|
\bcode\begin{verbatim}
|
|
# Echo server program
|
|
from socket import *
|
|
HOST = '' # Symbolic name meaning the local host
|
|
PORT = 50007 # Arbitrary non-privileged server
|
|
s = socket(AF_INET, SOCK_STREAM)
|
|
s.bind(HOST, PORT)
|
|
s.listen(1)
|
|
conn, addr = s.accept()
|
|
print 'Connected by', addr
|
|
while 1:
|
|
data = conn.recv(1024)
|
|
if not data: break
|
|
conn.send(data)
|
|
conn.close()
|
|
\end{verbatim}\ecode
|
|
%
|
|
\bcode\begin{verbatim}
|
|
# Echo client program
|
|
from socket import *
|
|
HOST = 'daring.cwi.nl' # The remote host
|
|
PORT = 50007 # The same port as used by the server
|
|
s = socket(AF_INET, SOCK_STREAM)
|
|
s.connect(HOST, PORT)
|
|
s.send('Hello, world')
|
|
data = s.recv(1024)
|
|
s.close()
|
|
print 'Received', `data`
|
|
\end{verbatim}\ecode
|
|
%
|
|
\begin{seealso}
|
|
\seemodule{SocketServer}{classes that simplify writing network servers}
|
|
\end{seealso}
|