I/O multiplexing , select and poll function

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I/O multiplexing , select and poll function
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INPUT / OUTPUT MODEL
SYNCHRONOUS I/O
ASYNCHRONOUS I/O
SYNCHRONOUS I/O A
synchronous I/O causes the requesting process to
be blocked until the operation
Completes. ASYNCHRONOUS I/O
A asynchronous I/O operation does not
cause the requesting process to be blocked.

SYNCHRONOUS I/O
BLOCKING I/O
NON-BLOCKING I/O
I/O MULTIPLUXING
SIGNAL DRIVEN I/O
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Blocked I/O
kernel
Application
recvfrom
No data gram
Data gram ready
Copy data gram
Process data gram
Copy complete
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Non blocking I/O
Application
kernel
recvfrom
No data gram
recvfrom
No data gram
No data gram
Recvfrom
recvfrom
Data gram ready
Copy datagram
Process data gram
Copy complete
Return OK
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I/O MULTIPLEXING
Kernel
Application
System call
select
Data gram not ready
Return readable
Data gram ready
System call
recvfrom
Copy data gram
Return OK
Process datagram
Copy complete
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Signal driven I/O
Application
kernel
Establish SIGIO
signalhandler
Data gram ready
Signal handler
recvfrom
Copy data gram
Process data gram
Return OK
Copy complete
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Asynchronous I/O
kernel
Application
Aio_read
No data gram
Data gram ready
Copy data gram
Deliver signal
Process data gram
Copy complete
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I/O multiplexing
This is implemented in the program using the
select or the poll function. Now we will see the
select function in detail.   SYNTAX  
includeltsys/select.hgt
includeltsys/time.hgt  
int select(int maxfdp1,fd_set readset,
fd_set writeset, fd_se t exceptset ,
const struct timeval timeout)    
RETURNS
ve count of ready descripters
0    on time out
-1 on error  
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const struct timeval timeout
This
tells the kernel how long to wait for one of the
specified descriptor to become ready.
Timeval a structure specifies
the number of seconds and microseconds.

Struct timeval

long tv_sec long tv_usec
timeout may have
three values
NULL -----? returns only when one of the
specified descriptors is ready 0                 0
-----? returns immediately after checking ,
also called as polling. particular
time----?returns when one of the specified
descriptor is ready or after the specified time.
CONST Indicates
that the value of the timeout will not return the
number of seconds left if it has return before
the stipulated time.    
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Int maxfdp1 This gives the
total number of descriptors the select function
has to wait. Its value is maxfdp11.The
decriptors0,1,2,maxfd1-1 to be tested by the
select command.       Fd_set This
with the three cases i.e.,
Fd_set readset
Fd_set writeset
Fd_set exceptset Helps us to test the kernel
for reading , writing , and exception.    
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conditions for a socket to be ready for
reading and writing
condition
Exception ?
Readable ?
Writable ?
Data bytes in socket receive buffergt low-water
mark
yes
Read half of the connection closed
yes
New connection ready for listening
yes
Data bytes in socket send buffer gt low-water
mark
yes
Write half of the connection closed
yes
Socket error is pending
yes
yes
TCP out of band data
yes
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When select function is not used
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For ( ) select(maxfd1 , rset , NULL ,
NULL , NULL)
If(FD_ISSET(sockfd , rset))
If(readline(sockfd ,
recvline , MAXLINE) 0)
err_quit(servet terminate)
Fputs
(recvline , stdout)
if(DF_ISSET(fileno(fp), rset))
if(Fgets(sendline , MAXLINE , fp) NULL)
return write(sockfd
, sendline ,strlen (sendline))
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When select statement is used in the program
When connection is released at this point of time
the all the data grams at that Time in the
network will be lost so we have to introduce a
Shout down function which Will terminate the
connection in a proper manner
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calling shut down to close the connection
data
S E R V E R
data
FIN
C L I E N T
FIN - ACC
data
data
FIN
FIN -ACC
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Syntax for shutdown function  
Int shutdown( int sockfd , int howto )
  Returns 0 if OK
Returns -1 on error   It is found
in the header file of ltsocket.hgt    
the function of shutdown depends on the howto
argument. SHUT_RD The read half of
the connection is closed.   SHUT_WR
The write half the connection is
closed.   SHUT_RDWR The both half of the
connection are closed.
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If(FD_ISSET(sockfd ,
rset))
If(readline(sockfd ,
recvline , MAXLINE) 0) If (
stdinof 1) return else
err_quit(servet
terminate)
Fputs (recvline
, stdout) if(DF_ISSET(fileno(fp), rset))
if(Fgets(sendline , MAXLINE ,
fp) NULL) stdinof
1 shutdown ( sockfd , SHUT_WR)
FD_CLR ( fileno(fp) ,
rset) Continue
write(sockfd , sendline ,strlen (sendline))

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pselect Pselect function
syntax
includeltsys/time.hgt
includeltsignal.hgt
includelttime.hgt
int pselect (int maxfdp1, fd_set readset,
fd_set writeset, Fd_set exceptset, const struct
timespec timeout, const sigset_t
sigmask) Returns count of ready
descriptors if OK. 0 on time out.
-1 on error. Timespec gives the time in
seconds and nanoseconds. Sigmask allows the
program to disable the delivery of certain
signals.
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poll function Syntax
includeltpoll.hgt int
poll(struct pollfd fdarray , unsigned long nfds,
int timeout) Returns count
of the ready descriptors of OK 0 on
time out -1 on error.
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Arguments to poll The
first argument is a pointer to the first element
of an array of structures. Each element of the
array is a pollfd structure.this specifies the
conditions to be tested.
struct pollfd
int fd short
event short revent Time out is
INFTIM wait for ever. 0
return immediately. -1
error.
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