Overview
• Last Lecture
– Daemon processes and advanced I/O functions
• This Lecture
– Unix domain protocols and non-blocking I/O
– Source: Chapters 15&16&17 of Stevens’ book
• Next Lecture
– Advanced UDP sockets and Threads
– Source: Chapters 22&26 of Stevens’ book
TELE 402 Lecture 10: Unix domain … 1
Unix domain sockets
• A way of performing client-server communication
on a single host using the same socket API
• Two types: stream and datagram
• Why use Unix domain sockets?
– Unix domain sockets are twice as fast as a TCP socket
• Example: X Window System
– Can be used to pass descriptors between processes on
the same host
– Can provide the client’s credentials (user ID and group
IDs) to the server for additional security check (newer
implementations)
TELE 402 Lecture 10: Unix domain … 2
Unix domain socket protocol
address
• Are pathnames within the normal filesystem
• Cannot read from or write to these files
except as a socket
TELE 402 Lecture 10: Unix domain … 3
Socket address structure
struct sockaddr_un {sa_family_t sun_family;
char sun_path[104]; }
• sun_family should be AF_LOCAL
• sun_path is a pathname string terminated with a \0.
The unspecified address is indicated by a null string as
the pathname.
• The pathname should be an absolute pathname, not a
relative pathname.
• The macro SUN_LEN calculates the length of a
sockaddr_un structure.
TELE 402 Lecture 10: Unix domain … 4
sockpair function 1
• Creates two sockets that are then connected together
int socketpair(int
int
int
int
family,
type,
protocol,
sockfd[2])
• family must be AF_LOCAL
• protocol must be 0
• type can be either SOCK_STREAM or SOCK_DGRAM.
TELE 402 Lecture 10: Unix domain … 5
sockpair function 2
• The two sockets created are returned as sockfd[0] and
sockfd[1], which are unnamed.
• There is no implicit bind involved.
• They form a stream pipe if their type is SOCK_STREAM.
The pipe is full-duplex.
TELE 402 Lecture 10: Unix domain … 6
Differences from inet sockets 1
– Default file permissions for a pathname created by bind
should be 0777, modified by umask value.
– The pathname associated with a Unix domain socket
should be an absolute pathname, not a relative name.
– The pathname specified in a call to connect must be a
pathname currently bound to an open Unix domain
socket of the same type.
– A bind will fail if the pathname already exists (use
unlink before bind)
– The permission testing for connect of a Unix domain
socket is the same as if open had been called for writeonly access.
TELE 402 Lecture 10: Unix domain … 7
Differences from inet sockets 2
– Unix domain stream sockets are similar to TCP sockets
• They provide a byte stream with no record boundaries.
– If a call to connect finds that the listening socket’s
queue is full, ECONNREFUSED is returned
immediately
– DGRAM sockets are similar to UDP sockets
• They provide an unreliable datagram service that preserves
record boundaries.
– Sending a datagram on an unbound Unix domain
datagram socket does not bind a pathname to the socket
(bind must be called).
TELE 402 Lecture 10: Unix domain … 8
Passing descriptors 1
• Descriptors can be shared between processes in
the following ways
– A child process shares all the open descriptors with the
parent after a call to fork
– All descriptors normally remain open when exec is
called
– Pass descriptors using Unix domain sockets and
recvmsg
TELE 402 Lecture 10: Unix domain … 9
Passing descriptors 2
• Steps involved in passing a descriptor
– Create Unix domain sockets (preferably SOCK_STREAM) and
connect them for communication between a server and a client
– One process opens a descriptor. Any type of descriptor can be
exchanged.
– Sender builds a msghdr structure containing the descriptor to be
passed, and calls sendmsg with the structure across one of the Unix
domain sockets
– Reciever calls recvmsg to receive the descriptor from the other
Unix domain socket.
• Client and server must have an application protocol so they
know when the descriptor is to be passed.
TELE 402 Lecture 10: Unix domain … 10
Example 1
• Refer to unixdomain/mycat.c, unixdomain /myopen.c,
unixdomain/openfile.c, lib/read_fd.c, and lib/write_fd.c
TELE 402 Lecture 10: Unix domain … 11
Example 2
TELE 402 Lecture 10: Unix domain … 12
Passing user credentials 1
• User credentials (user ID, group IDs) can be
passed along a Unix domain socket as the fcred
structure
struct fcred { uid_t fc_ruid;
gid_t fc_rgid;
char fc_login[MAXLOGNAME];
uid_t fc_uid;
short fc_ngroups;
gid_t fc_groups[NGROUPS];}
TELE 402 Lecture 10: Unix domain … 13
Passing user credentials 2
• The above information is always available on a Unix
domain socket, subject to the following conditions
– The credentials are sent as ancillary data when data is sent on the
Unix domain socket, but only if the receiver of the data has
enabled the LOCAL_CREDS socket option. The level for this
option is 0.
– On a datagram socket, the credentials accompany every datagram.
On a stream socket, the credentials are sent only once (the first
time data is sent)
– Credentials cannot be sent along with a descriptor
– Users are not able to forge credentials
TELE 402 Lecture 10: Unix domain … 14
Distributed Shared Memory
• Use local duplicate of the shared memory
• Consistency maintenance
–
–
–
–
Message passing based on UDP
Stop and wait protocol
Client/server model
Two connections between any pair of nodes
TELE 402 Lecture 10: Unix domain … 15
Blocking and nonblocking 1
• Input operations: read, readv, recv,
recvfrom, and recvmsg
– Blocking: if there is no data available in the
socket receive buffer, the process is put to sleep
– Nonblocking: if there is no data available, the
process is returned an error of
EWOULDBLOCK
TELE 402 Lecture 10: Unix domain … 16
Blocking and nonblocking 2
• Output operations: write, writev, send, sendto, and
sendmsg
– Blocking: if there is no room in the socket send buffer,
the process is put to sleep
– Nonblocking: if there is no room at all in the socket
send buffer, the process is returned an error of
EWOULDBLOCK
– In general UDP does not block since it does not have a
socket send buffer. Some implementations might block
in the kernel due to buffering and flow control.
TELE 402 Lecture 10: Unix domain … 17
Blocking and nonblocking 3
• Accepting incoming connections: accept
– Blocking: if there is no new connection available, the
process is put to sleep
– Nonblocking: if there is no new connection available,
the process is returned an error of EWOULDBLOCK
• Initiating outgoing connections: connect
– Blocking: the process is blocked for at least the round
trip time (RTT) to the server
– Nonblocking: if a connection cannot be established
immediately, the connection establishment is initiated
but the error of EINPROGRESS is returned
• Some connections can be established immediately, e.g. when
the server and the client are on the same host
TELE 402 Lecture 10: Unix domain … 18
Example1
• Nonblocking reads and writes for str_cli
function
• refer to strclinonb.c
TELE 402 Lecture 10: Unix domain … 19
Example1 (cont.)
• Buffer for data from standard input going to the
socket
TELE 402 Lecture 10: Unix domain … 20
Example1 (cont.)
• Buffer for data from the socket going to standard
output
TELE 402 Lecture 10: Unix domain … 21
Example Nonblocking Timeline
TELE 402 Lecture 10: Unix domain … 22
Simple version of example1
• Use fork to remove blocking factors
– Refer to strclifork.c
TELE 402 Lecture 10: Unix domain … 23
Nonblocking connect 1
• To set a socket nonblocking, use fcntl to set
O_NONBLOCK flag
• Three uses for nonblocking connect
– Overlap other processing with the three-way handshake
• Should use select to test the connection later
– Establish multiple connections at the same time
– Shorten the timeout for connect using select with a
specified time limit
• Example
– Overlap other processing with three-way handshake
– Use select to shorten timeout.
TELE 402 Lecture 10: Unix domain … 24
Nonblocking connect 2
• There are a couple of details to attend if we
use this technique:
– If the server is on the same host, the connection
is normally established immediately. We need
to handle this.
– Berkeley derived have the following rules:
• The descriptor is writable when the connection
completes successfully
• If connection establishment encounters an error, the
descriptor becomes both readable and writable
TELE 402 Lecture 10: Unix domain … 25
Web client
• Use multiple connections to send requests and to
receive response. (refer to nonblock/web.c)
• Control flow
– Associate each request with a nonblocking socket
whose connection is initiated, depending on the
maximum allowable connections.
– Use select to wait for any socket to be ready
– Scan the request array to find out if their sockets are
readable or writable, and react to each situation
accordingly
• Writable: send request.
• readable: receive response.
– Repeat the above until all requests are processed
TELE 402 Lecture 10: Unix domain … 26
Nonblocking accept
• Normally nonblocking accept is not necessary if
we use select, since when select returns there must
be a completed connection
• However, there is a possibility (because the server
is doing something else) that between the call
select and the call accept, the client sends a RST
to close the connection, which will cause accept to
block
• To fix the problem
– Always set a listening socket nonblocking
– Ignore the following errors on the call to accept:
EWOULDBLOCK, ECONNABORTED, EPROTO,
and EINTR
TELE 402 Lecture 10: Unix domain … 27
ioctl
• ioctl has traditionally been the system interface
used for everything that did not fit into some other
nicely defined category
• Posix is getting rid of ioctl
• However, numerous ioctls remain for
implementation-dependent features related to
network programming
– Obtaining the interface information
– Accessing the routing table
– Accessing the ARP cache
• The ioctls introduced here are implementation
dependent and may not be supported by Linux
TELE 402 Lecture 10: Unix domain … 28
ioctl
int ioctl(int fd,
int request,
void *arg)
• Requests can be divided into six categories
–
–
–
–
–
–
Socket operations
File operations
Interface operations
ARP cache operations
Routing table operations
Streams system
TELE 402 Lecture 10: Unix domain … 29
Interface configuration
• Get interface configuration information
– Use SIOCGIFCONF, SIOCGIFFLAGS, and
SIOCGIFBRDADDR requests
– And others
• ifconf structure is used as the argument
TELE 402 Lecture 10: Unix domain … 30
SIOCGIFCONF
TELE 402 Lecture 10: Unix domain … 31
SIOCGIFCONF
TELE 402 Lecture 10: Unix domain … 32