Socket Programming and I/O Multiplexing in CPE 401/601, Slides of Computer Networks

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Lecture 4

Socket Programming Issues

slides are modified from Dave Hollinger Docsity.com

Debugging

• Debugging can be difficult
• Write routines to print out sockaddrs
• Use trace, strace, ptrace, truss, etc
• Include code that can handle unexpected situations

CPE 401/601 Lecture 4 : SocketProgramming Issues (^) Docsity.com 2

UDP Connected mode

• A UDP socket can be used in a call to connect()
• This simply tells the O.S. the address of the peer
• No handshake is made to establish that the peer exists
• No data of any kind is sent on the network as a result of calling connect() on a UDP socket CPE 401/601 Lecture 4 : SocketProgramming Issues (^) Docsity.com 4

Connected UDP

• Once a UDP socket is connected:
  • can use sendto() with a null dest address
  • can use write() and send()
  • can use read() and recv()
    • only datagrams from the peer will be returned
  • Asynchronous errors will be returned to the process

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OS Specific, some won’t do this!

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Back to UDP connect()

• Connect() is typically used with UDP when communication is with a single peer only
• It is possible to disconnect and connect the same socket to a new peer - More efficient to send multiple datagrams to the same user
• Many UDP clients use connect()
• Some servers (TFTP)

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I/O MULTIPLEXING

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Example - generic TCP client

• Input from standard input should be sent to a TCP socket
• Input from a TCP socket should be sent to standard output
• How do we know when to check for input from each source?

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STDIN

TCP SOCKET^ STDOUT Docsity.com

Options

• Use multiple processes/threads
• Use nonblocking I/O
  • use fcntl() to set O_NONBLOCK
• Use alarm and signal handler to interrupt slow system calls
• Use functions that support checking of multiple input sources at the same time CPE 401/601 Lecture 4 : I/O Multiplexing (^) Docsity.com 11

Non blocking I/O

while (! done) { if ( (n=read(STDIN_FILENO,…)<0)) if (errno != EWOULDBLOCK) / ERROR / else write(tcpsock,…)

if ( (n=read(tcpsock,…)<0)) if (errno != EWOULDBLOCK) / ERROR / else write(STDOUT_FILENO,…) }

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The problem with nonblocking I/O

• Using blocking I/O allows the OS to put your process to sleep when nothing is happening - Once input arrives, the OS will wake up your process and read() (or whatever) will return
• With nonblocking I/O, the process will chew up all available processor time!!!

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“Alarming” Issues

• What will happen to the response time?
• What is the ‘right’ value for MAX_TIME?

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Select()

• The select() system call allows us to use blocking I/O on a set of descriptors - file, socket, …
• We can ask select to notify us when data is available for reading on either STDIN or a socket

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struct timeval

struct timeval { long tv_sec; / seconds / long tv_usec; / microseconds / } struct timeval max = {1,0};

• To return immediately after checking descriptors - set timeout as {0, 0}
• To wait until I/O is ready
  • set timeout as a NULL pointer CPE 401/601 Lecture 4 : I/O Multiplexing (^) Docsity.com 19

fd_set

• Operations you can use with an fd_set:
  • Clear all bits in fd_set *void FD_ZERO(fd_set fdset);
  • Turn on the bit for fd in fd_set *void FD_SET(int fd, fd_set fdset);
  • Turn off the bit for fd in fd_set *void FD_CLR(int fd, fd_set fdset);
  • Check whether the bit for fd in fd_set is on *int FD_ISSET(int fd, fd_set fdset);

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