15-213 Recitation 9 Greg Reshko

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15-213 Recitation 9Greg Reshko

• Office Hours Wed 200-300PM
• April 21st, 2003

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Outline

• File I/O
• Networking
• Lab 7

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Unix File I/O

• All I/O devices are modeled as files
• Networks, disks, terminals, etc
• All input and output is performed by reading and
  writing the appropriate files
• Kernel maintains all information about files and
  just returns a file descriptor to an application
  that wants access to a given file

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Unix File I/O Read/Write
include ltunistd.hgt ssize_t read(int fd, void
buf, size_t count) ssize_t write(int fd, const
void buf, size_t count)
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Unix File I/O More Robust
Robust I/O (RIO) Package Unbuffered ssize_t
readn(int fd, void buf, size_t count) ssize_t
writen(int fd, const void buf, size_t
count) ssize_t readline(int fd, void buf,
size_t maxlen) Buffered (thread-safe) ssize_t
readnb(int fd, void buf, size_t count) ssize_t
writenb(int fd, const void buf, size_t
count) ssize_t readlineb(int fd, void buf,
size_t maxlen)
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Standard I/O vs. Unix I/O

• Standard I/O almost always preferred
• fopen, fclose, fseek, fread, fwrite, fscanf,
  fprintf, etc.
• A higher-level abstraction than UNIX I/O
• Models files as streams, like file descriptors in
  being full-duplex
• Restrictions exist that interact badly with
  restrictions on sockets
• Input function cannot follow an output function
  (or vice versa) without a call to fflush, fseek,
  fsetpos, rewind, or and end-of-file

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Networking

• Two sides to networking
• Client / Server
• Distinction is fading
• Peer-to-peer systems mean a node is both

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Client Code Path

• Clients typically connect to servers, perform a
  task, then close the connection
• Create a socket()
• Connect()
• Write()/read()
• Close()

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Server Code Path

• Servers wait for connections and handle them as
  they arrive
• Create a socket()
• Bind() to a port
• Listen() for connections
• Select() a ready connection
• Accept() a connection
• Read()/Write()
• Close() the connection
• Well focus on TCP, but UDP is just as important

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Server Init

• void CreateTCPServer()
• struct sockaddr_in myAddr
• int myAddrLen
• int reuseAddrFlg1
• tcpSocketfd socket(AF_INET,SOCK_STREAM,0)
• bzero((void )(myAddr),sizeof(myAddr))
• myAddr.sin_family AF_INET
• myAddr.sin_port htons((unsigned
  short)atoi(chatTCPServerPort))
• myAddr.sin_addr.s_addr htonl(INADDR_ANY)
• setsockopt(tcpSocketfd,SOL_SOCKET,SO_REUSEADDR,(
  void ) reuseAddrFlg,sizeof(reuseAddrFlg))
  
• bind(tcpSocketfd,(struct sockaddr
  )myAddr,sizeof(myAddr))
• listen(tcpSocketfd,MAX_BACKLOG)

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Server Loop

• while(1)
• readSetallSet
• numReadyDescriptorsselect(maxfd1,readSet,NUL
  L,NULL,NULL)
• if(FD_ISSET(tcpSocketfd,readSet))
• / Incoming request for new connection /
• struct sockaddr_in clientAddr
• int clientAddrLen
• slavefdaccept(tcpSocketfd,(struct sockaddr
  ) clientAddr, clientAddrLen)
• / Save slavefd so we know to check it
  later /
• FD_SET(slavefd,allSet)
• if(slavefd gt maxfd) maxfdslavefd
• fflush(stdout)
• numReadyDescriptors--
• if(numReadyDescriptors lt 0)
• continue

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Server The Rest

• Check whether interesting socket descriptors are
  in the read/write set
• Do appropriate action if they are (i.e., read()
  or write())
• Close() the connection when appropriate

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Logging and Network I/O

• Have a server set up to receive requests and
  respond to them
• Read in request, parse, and return response
• Need to read/write to a network stream, and log
  events to a file for record-keeping purposes
• Main loop opens file for logging and spawns
  children threads
• Each thread has own network streams, but shares
  logging file stream
• Use of mutex to lock resources glossed over for
  now

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Reading to/from Network File

• void main ()
• FILE log
• / open file /
• log fopen ( "log.txt, "a )
• while ( true )
• /
• Main Loop (inside thread)
• /
• fclose ( log )

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Reading to/from Network File

• void thread ( void vargp )
• FILE stream
• char methodBUFSIZE, uriBUFSIZE,
  versionBUFSIZE
• / create mutex /
• / given argument, set file descriptor for
  proper access to streams /
• childfd ((int )vargp)
• free(vargp)
• / open file descriptors for reading network
  socket, set buffer type /
• if ((stream fdopen(childfd, "r")) NULL)
• error("ERROR on fdopen")
• setvbuf(stream, NULL, _IONBF, 0)
• / get header off the stream and save /
• fgets(buf, BUFSIZE, stream)
• sscanf(buf, "s s s\n", method, uri, version)

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Reading to/from Network File

• /
• Parse, error check, ready for server
  communication.
• /
• /
• determine size of information to be written
  back
• in this case, copying from some other stream
  (buf)
• to the network stream (stream)
• /
• / get size of remaining buffer from stream /
• temp read ( serverfd, buf, BUFSIZE )
• while ( temp gt 0 )
• data.byte temp
• /
• write current buffered amount to
  network socket
• get size of remaining buffer from
  stream
• /

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Reading to/from Network File

• / init mutex /
• / lock mutex /
• / write to log file /
• fprintf(log, "s s s d\n", data.date,
  data.ip, data.address, data.byte)
• / unlock mutex /
• / close network buffer /
• fclose(stream)

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Lab 7 Logging Web Proxy

• Start early (yeah, thats a new one)
• Read the book carefully
• Everything you need for this lab is in the book
  make sure you understand it
• Concurrency issues
• Test it with your web browser
• i.e. Internet Explorer
• This is the coolest part of the lab