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Linux Programming Interprocess Communication

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Title: Linux Programming Interprocess Communication


1
Linux Programming Interprocess Communication
  • CS 230
  • ???

2
Overview
  • IPC is a mechanism to transfer data between
    processes
  • methods overview
  • shared memory
  • uncontrolled accesses to memory
  • mapped file
  • similar to shared memory, but with file
    operations
  • pipes
  • sequential communication
  • FIFO
  • unnamed pipe unrelated processes can communicate
  • socket
  • communication between different computers

3
Shared Memory
addr space of process I
addr space of process J
DRAM
main() attach()
main() attach()
page tbl
page tbl
  • the fastest method
  • write by a process can be seen by another process
    instantly
  • no syscall intervention
  • no data copying
  • no synchronization is provided
  • it is up to programmers responsibility

4
Programming Shared Memory
  • segment_id shmget (key, size, IPC_CREAT
    IPC_EXCL S_IRUSR S_IWUSR)
  • a brand new segement is created if
  • key is IPC_PRIVATE or
  • IPC_CREAT is asserted
  • otherwise, segment_id of existing segment is
    returned
  • you have to know the key
  • size is rounded up to multiple of the page size
  • S_Ixxx
  • for read/write
  • xxx for USR(owner) OTH(others) GRP(group)
  • IPC_EXCL
  • for segment creation(IPC_CREAT option), it
    guarantees the key is unique (not existing one)

5
Program Example
define KEY ((key_t)(1234)) define SEGSIZE
sizeof(struct some_data_structure)) struct
some_data_structure ap int id shmget(KEY,
SEGSIZE, IPC_CREAT 0666) if (id lt 0)
error_rtn(id) ap (struct some_data_structure
) shmat(id, 0, 0)
let the system choose the location of the segment
define KEY ((key_t)(1234)) define SEGSIZE
sizeof(struct some_data_structure)) struct
some_data_structure ap2 int id shmget(KEY,
SEGSIZE, 0) if (id lt 0) error_rtn(id) ap2
(struct some_data_structure ) shmat(id, 0, 0)
0 for R/W SHM_RDONLY
6
Shared Memory Control
  • shmctl()
  • provides information about the segment
  • delete the segment when the last process finishes
  • every segment should be deleted when it is done
  • ipcs
  • provides information about IPC resources
  • -m for shared memory
  • -s for semaphore
  • -a for all
  • ipcrm
  • removes IPC resource
  • discussions
  • fastest way to communicate
  • key should be known to processes
  • synchronization should be arranged

7
Mapped File
  • ptr mmap( / map the file to ptr /
  • addr, / suggest ptr valur. 0 means let system
    choose /
  • length, / size of mapped region /
  • prot, / access PROT_READ PROT_WRITE /
  • flags, / mapping type MAP_SHARED /
  • fd, / file /
  • offset) / location within the file /
  • fd can be a device
  • a device can be accessed without full blown
    device driver
  • operations are more familiar than shared memory
  • open, close, chmod, unlink, .

8
Network Communication Primer
  • Protocol Stacks
  • The data pass down the protocol stack at the
    sending end, and back up the stack at the
    receiving end.
  • Entities at the same level in the protocol stack
    are called peers.
  • The communication between peers is called peer to
    peer communication.

Peer-to-peer communication
Application protocol
User code
Application layer
Application layer
UDP/TCP protocol
Transport layer
Transport layer
Kernel code
IP protocol
IP layer
IP layer
Ethernet frames
Ethernet layer
Ethernet layer
Hardware
Bits
9
Data Encapsulation
  • Addition of header information to a packet of
    data by a layer.
  • The header contains just the information needed
    for that layer to do its job.
  • Each layer of a protocol stack
  • not aware of the header added by the lower layer.
  • not attempt to interpret the data that have been
    handed down from the layer above.
  • Process of Encapsulation

Application data
Application data
UDP header
Application data
UDP header
IP header
Application data
UDP header
IP header
Ethernet header
10
What is a Socket?
  • An abstraction for connecting to a network
  • Different flavors TCP and UDP
  • TCP is a connection-oriented byte-stream protocol
    with reliable transmission (an end to end
    protocol)
  • UDP is a connectionless best-effort datagram
    service (not much beyond IP itself)
  • Both require specification of an IP address and
    port number
  • When a socket is created, it has an associated
    protocol (TCP or UDP) but no address or port
    number
  • The socket must be bound to an
    address/portthere are several ways more later

11
Sockets (cont.)
  • Create a socket with the socket() call
  • socket(int protocolFamily, int type, int proto)
  • protocolFamily PF_INET for IP
  • type is SOCK_STREAM (TCP) or SOCK_DGRAM (UDP)
  • proto is end-to-end protocol IPPROTO_TCP or
    IPPROTO_UDP should be used (0 gives default,
    which is these anyway at the moment)

12
Sockets (cont.)
  • The socket() call returns a descriptor
  • A descriptor here is an integer which gives a
    handle to the socket
  • Just like file handles returned from open()
  • Its a reference that you store away and then use
    whenever you need to reference that socket in any
    future systems calls
  • It should be nonnegative if socket() returns 1
    this indicates an error creating the socket
  • close(int socket) when youre done

13
Socket Addresses
  • Special datatype called sockaddr
  • struct sockaddr
  • unsigned short sa_family / AF_INET /
  • char sa_data14 /
    Protocol-specific address formation /
  • This is the generic datatype containing only
    the address family (AF_INET for IP) and the
    address specific to the protocol
  • Of the 14 bytes, well use 4 bytes for the IP
    address and 2 for the port number

14
Socket Addresses (cont.)
  • IP Specific version of the sockaddr struct
  • struct sockaddr_in
  • unsigned short sin_family / AF_INET /
  • unsigned short sin_port / Port
    (16-bits) /
  • struct in_addr sin_addr / Internet
    address (32-bits) /
  • char sin_zero8 / Not used /
  • The sockaddr_in is the internet version of the
    sockaddr struct. It can be cast to a generic
    sockaddr
  • Its the right size and has a compatible first
    field

15
Connection-Oriented Client Server
  • Asymmetric Client-Server Relationship
  • Server
  • passively sit waiting for work.
  • not know where that work will come from.
  • Client
  • actively go out and connect to the server it
    requires.

16
Connection-Oriented Client Server
  • Connection-Oriented Client and Server Operations.

Server operations
create SOCKET
Client operations
BIND a well-known port number to the socket
Establish a LISTEN queue for connections
create SOCKET
ACCEPT a connection
CONNECT to servers port
READ from connection
WRITE to connection
As required by application
As required by application
WRITE to connection
READ from connection
END-OF-FILE
CLOSE connection
17
Connection Oriented Client
  • We know the steps
  • 1) Create a socket
  • 2) Establish a connection with connect()
  • 3) Communicate with send() and recv()
  • 4) Use close() to destroy the socket
  • Calling connect
  • int connect(int socket, struct sockaddr
    destAddress, unsigned int addressLen)
  • socket is a socket descriptor,
  • the sockaddr is generic,
  • len is sizeof(struct sockaddr_in)

18
Calling send/recv
  • int send(int socket, const void msg, unsigned
    int msgLen, int flags)
  • int recv(int socket, void rcvBuffer, unsigned
    int bufferLen, int flags)
  • msg is a pointer to the message, and msgLen is
    its length
  • rcvBuffer captures the incoming bytes up to a max
    of bufferLen
  • flags0 gives default behavior (more later)
  • send blocks until ALL bytes transferred
  • recv blocks until at least SOME bytes received
  • send and recv return the number of bytes sent or
    received if 1 we have an error
  • If recv returns 0, it means connection was closed
    at other end

19
Connection-Oriented Client (contd)
  • Obtaining the IP address and the port number of
    the server
  • server machine can be changed
  • hard-wiring the IP address is clearly
    unacceptable
  • typically, the client read a host name from its
    command line arguments
  • gethostbyname() provide a mapping from machine
    names to IP address by using /etc/hosts (or NIS
    hosts map)
  • port numbers rarely change
  • hard-wiring the port number into the code is
    often satisfactory
  • Alternatively, getservbyname() can be used to
    obtain the port number of a given named service
    by using /etc/services (or NIS servicess map)

20
A Client Program
main() int sockid struct sockaddr_in
serv_addr bzero((char ) serv_addr,
sizeof(serv_addr)) serv_addr.sin-family
AF_INET serv_addr.sin_addr.s_addr
inet_addr(SERVER_ADDR) serv_addr.sin_port
htons(SERVER_PORT) sockid socket(IF_INET,
SOCK_STREAM, 0) connect(sockid, serv_addr,
sizeof(serv_addr)) send(sockid, msg,
sizeof(msg),0) recv(sockid, buffer,
sizeof(buffer), 0) close(sockid) exit(0)
21
Constructing the Server
  • Lets do TCP for now (UDP later)
  • We know the method here as well
  • 1) Create a TCP/IP socket
  • 2) Assign a port number with bind()
  • 3) Allow incoming connections with listen()
  • 4) Repeat the following
  • a) Call accept() to get a new socket for each
    connection
  • b) Use send() and recv() to communicate via each
    socket
  • c) Close the client connection when finished with
    close()

22
The bind() Call
  • int bind(int socket, struct sockaddr localAddr,
    unsigned int addressLen)
  • assigns a port number to the socket
  • Once bind() succeeds, all connect() requests to
    this host for this address/port will go to this
    socket
  • If the address is set to INADDR_ANY then all
    connections to this port will go to this socket,
    regardless of the Internet address used (useful
    if host happens to have multiple Internet
    addresses)

23
The listen() Call
  • Once socket is created and bound, we need to
    begin listening for connection requests
  • int listen(int socket, int queueLimit)
  • socket is the socket descriptor
  • queueLimit is the largest number of incoming
    connections which can be waiting (or queued)
  • this socket is for setting up a connection only
  • We never use send() or recv() on this socket once
    it is used for listen() calls instead we use
    this socket as a queue for incoming connections,
    and use accept() to dequeue the requests

24
The accept() Call
  • accept() dequeues the next connection request
    from the listen sockets queue
  • int accept(socket, struct sockaddr
    clientAddr, unsigned int addrLen)
  • If queue is empty, accept() blocks until
    connection request is made
  • socket is the listen socket
  • clientAddr is filled in with clients address
  • addrLen is filled in by you initially with max
    size of clientAddr, and changed by accept() to
    actual size of clientAddr
  • accept returns new socket descriptor on success,
    -1 on failure

25
Server Interaction
  • Once we have a connection (via accept()) we may
    interact with this client via send() and recv()
  • Same as before for client-side
  • Use the socket returned by accept() not the
    listen socket
  • If we have multiple clients, we interact
    separately by referencing the appropriate socket
  • When a particular client interaction is complete,
    use close() on the appropriate socket
  • When server no longer wishes to accept incoming
    connections, call close on the listen socket

26
A Server Program
  • main()
  • int sockid, newsockid, childpid
  • struct sockaddr_in cli_addr, serv_addr
  • sockid socket(AF_INET, SOCK_STREAM, 0)
  • bzero((char ) serv_addr, sizeof(serv_addr))
  • serv_addr.sin-family AF_INET
  • serv_addr.sin_addr.s_addr htonl(INADDR_ANY)
  • serv_addr.sin_port htons(SERVER_PORT)
  • bind(sockid, serv_addr, sizeof(serv_addr))
  • listen(sockid, 5)
  • for ( )
  • / wait for a client connects /
  • newsockid accept(sockid, cli_addr,
    sizeof(cli_addr))
  • if ((childpid fork()) 0) / child
    process /
  • close(sockid)

27
Interoperability
  • All network info is big endian
  • htons() host to network short
  • htonl() host to network long
  • ntohl() network to host long
  • ntohs() network to host short
  • Use these to send a multibyte binary value from
    one machine to another
  • Any time you send an externally significant
    value (like an IP address or port number) the
    API expects network order
  • Apply these funcs as the last operation before
    sending data, and as the first when receiving data

28
Alignment Issues
  • Different architectures do alignment in various
    ways
  • Ex Suppose I have a struct
  • a (4 byte int), b (2 byte int), c (4 byte int)
  • This may take 10 bytes on one machine, then I
    send (as a struct) to another machine which
    declares the same struct but pads b. We get
  • a (4 byte int), b (2 byte int), 2 bytes of pad, c
    (4 byte int)
  • This occurs because the machine may want c at an
    address which is a multiple of its size (4)
  • Solutions
  • Manually rearrange things so this doesnt happen
    (put 2 byte quantities at the end of the struct)
  • Pad manually when sending to other machines

29
Framing and Parsing
  • With TCP (and not UDP) one send() call does not
    necessary translate to one recv() call
  • This means that the data sent must have framing
    info embedded within it (eg, spaces, nulls, etc)
  • The receiving end must know how to retrieve the
    relevant info based on the framing data

30
Example Iterative Server
  • char inbuf1000 / Data from client /
  • char outbuf1000 / data to client /
  • while(1)
  • fd accept (sock, client, client_len)
  • while( ( read (fd, inbuf, 1000) gt 0)
  • / Process the data in inbuf, placing the
    result in outbuf /
  • ...
  • write (fd, outbuf, 1000)
  • / When client close its end of the connection,
    we close our end.
  • Otherwise we would eventually run out of
    file descriptors.
  • Then we loop round to pick up another
    connection. /
  • close (fd)

31
Concurrent Server
  • Problem of Iterative Servers
  • When Iterative servers maintain long-lived
    connections to their clients, new clients may
    wait for an arbitrarily long time
  • Concurrent Server
  • Server that can support multiple clients at the
    same time
  • Two Approaches for Concurrent Server
  • one child server process per client
  • use the fork() system call.
  • take care of zombies!
  • single server process
  • use the select() system call.

32
Concurrent Server - one child per client
  • sock socket ( ... )
  • bind (sock, ... )
  • listen (sock, 2)
  • while (1)
  • fd accept ( sock, ... )
  • if (fork() 0)
  • / Child - process the request /
  • ... / use fd to say to the client /
  • exit(0) / Child is done /
  • else
  • close (fd) / Parent does not use the
    connection /

33
Single Process Server
  • Single Process Server with select( )
  • the server multiplex between the messages being
    received on each client connection
  • The server return to the main processing loop
    after every interaction with client
  • Advantage
  • uses less system resources (memory and process
    table slots) since there is a single process.
  • Disadvantage
  • the server may be required to keep state
    information of each client
  • the concurrent server keeps this state
    information in each child process

34
select() syscall
  • select(int nfds, readfds, writefds,
    exceptfds, struct timeval timeout)
  • readfds, writefds and exceptfds point to
    descriptor sets
  • nfds is the highest fd number plus one
  • return if any descriptor in the readfds writefds
    exceptfds set is ready for reading (or is
    ready for writing, or has an exceptional
    condition pending)
  • else blocks until timeout
  • if timeout 0, return immediately
  • if timeoutNULL, block indefinitely
  • Macros for clearing, setting, testing individual
    descriptors within a set
  • FD_ZERO(fdset) / Remove all descriptor
    from set fdset /
  • FD_CLR(fd, fdset) / Remove descriptor fd from
    set fdset /
  • FD_SET(fd, fdset) / Add descriptor fd to set
    fdset /
  • FD_ISSET(fd, fdset) / True if fd is present in
    the set fdset /

35
select() example
  • fd_set myset
  • FD_ZERO(myset) / Put file descriptors 3 and 4
    into myset /
  • FD_SET(3, myset)
  • FD_SET(4, myset)
  • select (5, myset, NULL, NULL, NULL)
  • if (FD_ISSET(3, myset))
  • / read from descriptor 3 ... /
  • else if (FD_ISSET(4, myset))
  • / read from descriptor 4 ... /
  • else
  • printf(This should never happen)

36
select() syscall (contd)
  • Because your descriptor sets get overwritten by
    the call, you usually have to keep a separate
    copy of them
  • (ex) after the call, only file descriptors ready
    for reading remains in the descriptor set
  • use memcpy(set1, set2, sizeof set2)
  • When the call returns, there is no immediate
    indication of which file descriptor has woken you
    up. There will probably only be one, but to find
    it you have to poll each in turn

37
Connectionless Client and Server
  • The server has a socket on which it receives
    datagrams
  • Many clients may send datagrams to this socket,
    and in general the server will retrieve datagrams
    from several clients in an arbitrary, interleaved
    order
  • Server that does not maintain state (stateless
    server)
  • server simply reads the datagram, formulates a
    reply, sends it back to the client and forgets
    about it
  • (ex) daytime, echo server
  • Server that maintain state (stateful server)
  • One approach is to parcel the per client state
    information into a structure
  • Another approach is for the server to create a
    child process for each client
  • easy to keep track of the state information of
    each server
  • (ex) TFTP server

38
Connectionless Client and Server (2)
  • Unreliability of connectionless client server
    (e.g., UDP)
  • means that there is no mechanism built into the
    protocol to detect (and correct) problems such as
    dropped or misordered packet deliveries
  • Assume that the underlying delivery system is
    reliable enough, and accept that the application
    will fail if the network fails.
  • a timeout/retransmit mechanism may be built in at
    the application level.
  • Syntactic difference between client and server is
    not much
  • Both client and server simply create a socket,
    bind a port, and then send and receive datagrams
  • just a matter of who goes first
  • the server does not call listen() and accept()
  • the client does not call connect()

39
Connectionless Client and Server (3)
  • Connectionless Client and Server Operations

Server
Client 1
Client 2
create SOCKET
create SOCKET
BIND well-known port number
BIND any port number
SEND datagram
RECEIVE datagram
create SOCKET
SEND datagram
BIND any port number
RECEIVE datagram
SEND datagram
RECEIVE datagram
SEND datagram
RECEIVE datagram
40
Connectionless Client and Server (4)
  • Creating a datagram socket
  • int sock
  • sock socket (AF_INET, SOCKDGRAM, 0)
  • server bind its well-known port number to this
    socket
  • client does not need to make a call to bind().
    The system will automatically bind one.
  • Sending and Receiving a Datagram
  • read() and write() calls cannot be used with
    connectionless sockets
  • use
  • sendto(sock, buff, count, flags, addr,
    addrlen)
  • recvfrom(sock, buff, count, flags, addr,
    addrlen)
  • first three arguments are as for write() and
    read()
  • flags argument can be used to specify special
    delivery options, but is usually zero
  • preserves message boundaries each recvfrom()
    corresponds to one sendto() no breaking messages
    into pieces allowed

41
Connectionless Client and Server
  • send() and recv()
  • send(sock, data_addr, data_len, flags)
  • recv(sock, data_addr, data_len, flags)
  • do not supply a peer address in their argument
    list.
  • only be used on datagram sockets to which a
    connect() call has been applied to specify the
    peer address in advance.
  • sendmsg() and recvmsg()
  • sendmsg(sock, msg_info, flags)
  • recvmsg(sock, msg_info, flags)
  • msg_info points to a structure that supports
    gather-write and scatter-read operation
  • gather-write collection of separate data
    buffers, at different places in memory, can be
    gathered together into a single datagram
  • scatter-read single datagram may be split up and
    delivered in to several separate buffers

42
UDP details
  • sendto() causes a port to be bound to the socket
    used
  • client never knows (nor needs to know) what the
    port number is, but the socket becomes bound
  • recvfrom() uses same socket, it is therefore
    monitoring the same port used for the sendto()
  • Since datagrams could be lost, the call to
    recvfrom() might never complete and our program
    would just hang better to use timeouts
  • Server blocks on call to recvfrom() instead of to
    accept()
  • Only one socket is used by UDP server (unlike TCP
    server which had a listen socket and a new
    per-client socket for each client)
  • Single send and receive, unlike TCP server where
    we had to continue receiving until the client
    closed the connection

43
UDP Details (cont.)
  • Two calls to recvfrom() will never return data
    from the same single sendto()
  • UDP does not buffer for retransmission like TCP
    (because there is no error recovery)
  • Once a call to sendto() returns, the msg is out
    the door
  • When data arrive via TCP or UDP, they are queued
    in a FIFO structure awaiting a call to recv() or
    recvfrom()
  • For a TCP connection, we accumulate all data into
    an ever-growing queue until recv() retrieves them
  • For UDP we might be getting several msgs from
    different source addresses we cant concatenate
    these!
  • If you call recvfrom() with size parameter n, and
    first chunk in FIFO buffer is larger than n, it
    returns first n bytes and quietly discards the
    rest!

44
TCP and UDP Ports
  • These are disjoint!
  • Can have a TCP port 8888 and a UDP port 8888
  • They are different and so do not conflict
  • If you try to connect() to port 1234 on machine
    134.197.40.62 and there is only a UDP port 1234
    on this machine, the connection request will fail
  • Ie, You must use TCP to talk to TCP ports and UDP
    to talk to UDP ports
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