Example Client Software

1
Example Client Software

• Lecture 4

2
Start with Small Examples

• Learning from small examples
• Will learn
• Atomic algorithms
• Basic communication methods/protocols
• Understanding of how clients and servers
  communicate
• Reason for abstraction (even at a simple level)?
• Although they are trivial, they are a good place
  to start

3
Hiding Details (ABSTRACTION)?

• Need for reuse in other programs
• Also, valuable for portability to other
  computers/operating systems (not really covered
  in this course)?
• Ability to raise the level of the programmer by
  hiding the details of the program to FOCUS on the
  problem at hand
• Think of how to parameterize procedures to
  maximize your ability to focus on your problem at
  hand without having to ever go back and change
  procedure

4
Example Procedure Library

• Start simple
• Ask yourself, what do I NEED to create a
  connection?
• Answer
• IP Address
• Port
• Protocol
• Abstract procedure to accept these as arguments
• What are the options for any of these arguments?
• IP Address string
• Port integer (0 65535)?
• Protocol TCP/UDP (possibly a boolean, or short)?

5
Example Procedure Library

• Build a function to meet that requirement
• e.g.
• connectsocket(char ip address, int port, short
  protocol)?
• NOTE this is up to you....
• Think about how to abstract further
• Connect TCP
• connectTCP(char ipaddress, int port)?
• NOTE no protocol, that is hardcoded
• connectTCP will simply call
• connectsocket(ipaddress, port, TCP)?

6
connectTCP and connectsock

• Example code on page 84 86
• Things to note on code
• connectsock is atomic procedure
• accepts host, service, and transport all as
  pointers of characters
• This allows for
• IP address / hostname (FQDN)?
• Service as a name or char representation of
  integer
• Transport is either tcp or udp (case
  sensitive)?
• connectTCP and connectUDP are what are called
  wrapper functions - just a simplified call to a
  procedure with no real functionality

7
errexit.c code

• Page 87 code
• int errexit(const charformat, ....)
• va_list args
• va_start(args, format)
• vfprintf(stderr, format, args) / NOTE why
  stderr? /
• va_end(args)?
• exit(1) / NOTE why exit with 1? /

8
How to use example library

• A few ways
• Copy code in your source (not a good idea)?
• Add in through include directives (an ok idea)?
• Compile in library and link in at compile time
  (the correct way to abstract in C code)?
• To use as a library, simply
• compile non main code into object file
• use -c option to only compile source to object
  file
• use ltfilenamegt.o in compile

9
The DAYTIME Service

• Daytime is RFC 867
• TCP or UDP port 13
• No specific syntax required for day / time
  representation
• which is NOT good!
• Does recommend
• single line of output
• ASCII printing characters space and CRLF

10
DAYTIME example in code

• Page 89 in text
• Uses connectTCP abstraction for TCPDaytime.c
• NOTE first thing to work on for actual program
  is the USAGE of the program....
• Know what you need to send to connectTCP
  abstraction....
• If you do not get the correct arguments, tell
  user why (in ENGLISH words, give an example,
  reference documentation, do something, then
  exit), an error code of -6 is as useless as it
  gets!

11
TCPDaytime

• Also note that the main code abstracts even the
  WORK of the program to a different procedure, so
  the main procedure only deals with user
  interaction
• connectTCP will return a fully connected TCP
  socket
• So, what if it fails? What will happen?
• Once we have the socket, we simply need to use a
  form of read and/or write on the socket
• NOTE Daytime does not require a write, only a
  read
• (About as simple as it gets!)?

12
TCPDaytime read example

• Lets look more closely at this read example
• while (n read(s, buf, LINELEN)) gt 0 )
• bufn '\0'
• (void) fputs(buf, stdout)

13
TIME Service Example

• Time protocol is RFC 868
• Both TCP and UDP port 37
• ntohl used to convert from Network to Host Long
• 32bit integer long in C
• What is EPOCH?
• How are we converting/using it?
• NOTE how would this be different in another OS?
• What is the ctime function call?

14
ECHO Service

• Echo is RFC 862
• TCP and UDP port 7
• NOTE different from ping (ICMP)?
• Simply reads and writes back whatever the server
  reads

15
TCP Client for ECHO

• page 94-96
• Again, main procedure is for user interaction
• Even main function will call a procedure called
  TCPecho(host,service)?

16
Lets look closer at while() loop

• while (fgets(buf, sizeof(buf), stdin))
• bufLINELEN '\0' / insure
  line null-terminated /
• outchars strlen(buf)
• (void) write(s, buf, outchars)
• / read it back /
• for (inchars 0 inchars lt
  outchars incharsn )
• n read(s,
  bufinchars, outchars - inchars)
• if (n lt 0)?
• errexit("socket
  read failed s\n",
• 
  strerror(errno))
• fputs(buf, stdout)

17
UDP Version of Echo Client

• while (fgets(buf, sizeof(buf), stdin))
• bufLINELEN '\0' / insure
  null-terminated /
• nchars strlen(buf)
• (void) write(s, buf, nchars)
• if (read(s, buf, nchars) lt 0)?
• errexit("socket read
  failed s\n",
• 
  strerror(errno))
• fputs(buf, stdout)

18
EXERCISES

• 7.4 add time that elapses from call to write
  for more accurate time
• finger example
• NOTE focus on abstractions...
• possibly chargen