08 Io Multiplexing

7/31/2019 08 Io Multiplexing

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ApplicationsforI/OMultiplexing

Clienthandlingmultipledescriptorse.g.theTCPechoclient

e.g.webclient

Serverhandlinglistenting,connectedsockets

,

Serverhandlingmultipleservices(xinetd)

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I/OModels

BlockingI/O

Nonblocking I/O

,

SignaldrivenI/O(SIGIO)

AsynchronousI/O

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BlockingI/OModel

Allexamplesseensofar

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Nonblocking I/OModel

Dontputprocesstosleep

Returnerror(EWOULDBLOCK)instead

Needtocheckperiodically(polling) WasteofCPU

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I/OMultiplexing

Blockinselectorpoll,notinactualI/Ocall

7

SignalDrivenI/OModel

InstallhandlerforSIGIO(signal,sigaction)

,

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AsynchronousI/O

Similartosignaldrivenexcept

Notified after data is read in buffer

POSIXspec,stillnotwidelysupported

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Comparison

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select Function

#include

#include

int select(int maxfdp1, fd_set *readset, fd_set *writeset,

fd_set *exceptset, const struct timeval *timeout);

Returns: positive count of ready descriptors, 0 on timeout, 1 on error

struct timeval {

long tv_sec; /* seconds */

long tv_usec; /* microseconds */

};

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select Function

Waitforanyormultipleevents

Returnwhenoneormoreoccurortimeout

,

anyofdescriptors{1,4,5}isreadyforreading

anyofdescriptors{2,7}isreadyforwriting

an ofdescri tors 1 4 haveexce tion endin

10.2secondshaveelapsed

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select Function

maxfdp1numberofdescriptorstobetested

maximum descri tor number + 1

FD_SETSIZE constantdefinedin

numberofdescriptorsinfd_set type

often1024,fewprogramsusethatmany maxfdp1isspecifiedforefficiency

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readset,writeset,exceptset

Valueresultarguments

Oncalling,specifydescriptorsofinterest

,

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SpecifyingDescriptorValues

fd_set rset;

_

FD_SET(1, &rset); /* turn on bit for fd 1 */

FD_SET(4, &rset); /* turn on bit for fd 4 */

* *_ ,

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ReadyforReadingConditions

Bytesinsocketreceivebuffer>lowwatermark

defaultis1forTCP,UDPsocket

can be set with SO RCVLOWAT _

readwillnotblock(return>0)

ea a o t econnect onc ose

TCPconnectionreceivedFIN

read willnotblock(return0)

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ReadyforReadingConditions

Listeningsocket:completedconnections>0accept willnotblock

read willnotblock(return 1)

errno issettospeci iccondition

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ReadyforWritingConditions

Availablespaceinsendbuffer>lowwatermark

socketconnected(TCP)ornotrequireconn (UDP)

can be set with SO SNDLOWAT _

defaultto2048forTCP,UDP

r te a o connect onc ose

writeoperationwill generateSIGPIPE (errno)

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ReadyforWritingConditions

Nonblocking socketconnectfunctioncompletedorfailed

write willnotblock(return 1)

errno issettospecificcondition

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DescriptorConditionsSummary

Condition Readable? Writable?

Read halfoftheconnectionclosedNewconnectionreadyforlisteningsocket

Writehalfoftheconnectionclosed

Nonblocking connect() completedorfailed

Pendingerror

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TCPEchoClientusingselect

tcpechosel1_cli.c

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TCPEchoClientusingselect

str_cli() rewrittenusingselect

Blockedinselect insteadoffgets

Notifiedassoonasserverterminates

Onlyneedonedescriptorsetfor readability

wr ese an excep onse

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ConditionsHandledusingselect

PeerTCP(server)senddataread returns>0

read returns0(EOF)

PeerTCPsendRST(crashed)

read returns 1errno =specificerror

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ProblemwithBatchInput

Previouscodeworksokforinteractiveinput

Forbatchinput(e.g.redirectedfromfile)

atEOF,willreturntomain,exit

becauseofspeed,willnotreceiveserverresponse

Needtoclosehalfconnection write

leavereadopentoreceiveserverresponse

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Limitationsofclose Function

Onlyclosessocketifrefcountreaches0sometimesweneedtoclosesocketregardless

sometimeswewanttocloseonedirectiononly

asencounteredinpreviousexample

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shutdown Function

#include

,Returns: 0 if OK, 1 on error

howto Value Action

SHUT_RD readhalfof connectionclosed

nomore a acan erece ve

anydataonreceivebufferisdiscarded

SHUT_WR writehalfofconnectionclosed

nomoredatacanbesent

any datacurrentlyinsentbufferwillbesentfollowedbyTCP

connectionterminationsequence

SHUT_RDWR combines bothactionsabove

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TCPEchoServerusingselect

tcpechosel1_srv.c

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TCPEchoServerusingselect

Onlyusereadfd_set

Assumeserverstartedinforeground

, ,

listeningsocketwillusefd =3

Maintainarrayof

clientfds

initiallyall= 1

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Handlingselect

Newconnectioncallaccept,updatedatastructuresandarrays

normaldata:echoback

FIN(connectionclosing):close,updatedata

RST(readerror):close,updatedata,reporterror

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poll Function

#include

int poll (struct pollfd *fdarray, unsigned long nfds, int

timeout);

Returns: count of ready descriptors, 0 on timeout, 1 on error

struct pollfd {

int fd; /* descriptor to check */

short events; /* events of interest on fd */

short revents; /* events that occurred on fd */

;

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poll Function

Similarfunctionalitytoselect MoreinfoonSTREAMdevices(TCP)

fd:descriptorstobepolled

events:conditionstobechecked

revents:conditionsactuall occurred

Moresystemssupportselect thanpoll

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InputeventsandReturnedrevents

Constant Inputtoevents?

Resultfromrevents?

Description

POLLINPOLLRDNORM

POLLRDBAND

NormalorprioritydatacanbereadNormaldatacanberead

Priorit band data can be read

POLLPRI

Highprioritydatacanberead

POLLOUT Normaldatacanbewritten

POLLWRBAND

Prioritybanddatacanbewritten

POLLERR Errorhasoccurred

POLLNVAL

angup asoccurre

Descriptor isnotanopenfile

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TCPEchoServerusingpoll

tcpechopoll1_srv.c

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AdditionalReferences

select

ttp: opengroup.org on inepu s 007908775 xs seect.html

p: ee .us gu e gne ou pu m mu page advanced.html#select

http://opengroup.org/onlinepubs/007908799/xsh/pol.

http://beej.us/guide/bgnet/output/html/multipage/p

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File: /home/mostafa/netprog/tcpsockex/tcpechosel1_cli.c Page 1 of 2

/* TCP echo client using selectworks good only for interactive inputcannot handle batch input redirected from another file

*/#include /* printf, fgets, fputs and standard i/o*/#include /* socket, bind, listen, accept, socklen_t */#include /* sockaddr_in, inet_ntop */

#include /* memset, strlen */#include /* exit */#include /* close, read */#include /* select */

#define SRV_PORT 5105 /* default port number */#define LISTEN_ENQ 5 /* for listen backlog */#define MAX_SEND_BUF 256#define MAX_RECV_BUF MAX_SEND_BUF

intstr_cli(FILE*,

int); /* prototype for function str_cli */

int main(int argc, char* argv[]){int sock_fd; /* client socket */struct sockaddr_in srv_addr; /* server address structure */

if (argc < 2) { /* user entered no arguments */printf("usage: %s \n", argv[0]); /* arg[0] is prog name*/

return -1;}

/* create a client socket */sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);srv_addr.sin_port = htons(SRV_PORT);srv_addr.sin_family = AF_INET;/* convert command line argument to numeric IP */if ( inet_pton(AF_INET, argv[1], &(srv_addr.sin_addr)) < 1 ){printf("Invalid IP address\n");return -1;

}

if( connect(sock_fd, (struct sockaddr*) &srv_addr, sizeof(srv_addr)) < 0 ){perror("connect error"); return -1;

}/* run the client using standard input */if (str_cli(stdin, sock_fd) < 0 )exit(-1);

else

exit(0);}

int str_cli(FILE* fp, int sock_fd){char recv_str[MAX_RECV_BUF]; /* buffer to hold received data */char send_str[MAX_SEND_BUF]; /* buffer to hold data to be sent */


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fd_set read_set; /* only read (from input or from socket) */int fp_no, /* file descriptor number of fp */

max_fd; /* maximum file descriptor */ssize_t rcvd_bytes, sent_bytes;

fp_no = fileno(fp);FD_ZERO(&read_set); /* initialize read_set to zero */

for(;;){FD_SET(fp_no, &read_set); /* set bit corresponding to fp */FD_SET(sock_fd, &read_set); /* set bit corresponding to sock_fd */max_fd = ( (fp_no > sock_fd) ? fp_no : sock_fd );

select(max_fd+1, &read_set, NULL, NULL, NULL);

if (FD_ISSET(sock_fd, &read_set)) /* socket is readable */

{ memset(recv_str, 0, MAX_RECV_BUF); // clear recv_str /* read back from socket into recv_str */ if ( (rcvd_bytes = read(sock_fd, recv_str, MAX_RECV_BUF)) < 0 ) {

perror("read error"); return -1;}

if (rcvd_bytes == 0) { /* server has been shutdown */printf("Server terminated prematurely\n"); return -1;

}/* write received line to standard output */

fputs(recv_str, stdout);} // if

if (FD_ISSET(fp_no, &read_set)) /* fp (stdin) is readable */{ /* get input from file fp into send_str buffer */

/* fgets returns NULL when EOF, EOL or MAX_SEND_BUF reached */if( fgets(send_str,MAX_SEND_BUF, fp) == NULL ){puts("EOF");

return0; /* reached EOF ^D */

}

/* else, write send_str contents to socket */ if ( (sent_bytes = write(sock_fd, send_str, strlen(send_str))) < 0 ) {

perror("write error"); return -1;}

} //if

} // forreturn0;

}


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/* TCP echo client using selectcan handle batch input redirected from another file

*/

#include /* printf, fgets, fputs and standard i/o*/#include /* socket, bind, listen, accept, socklen_t */#include /* sockaddr_in, inet_ntop */

#include /* memset, strlen */#include /* exit */#include /* close, read */#include /* select */

#define SRV_PORT 5105 /* default port number */#define LISTEN_ENQ 5 /* for listen backlog */#define MAX_SEND_BUF 256#define MAX_RECV_BUF MAX_SEND_BUF

intstr_cli(FILE*,

int); /* prototype for function str_cli */

int main(int argc, char* argv[]){int sock_fd; /* client socket */struct sockaddr_in srv_addr; /* server address structure */

if (argc < 2) { /* user entered no arguments */printf("usage: %s \n", argv[0]); /* arg[0] is prog name*/

return -1;}

/* create a client socket */sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);srv_addr.sin_port = htons(SRV_PORT);srv_addr.sin_family = AF_INET;/* convert command line argument to numeric IP */if ( inet_pton(AF_INET, argv[1], &(srv_addr.sin_addr)) < 1 ){printf("Invalid IP address\n");return -1;

}

if( connect(sock_fd, (struct sockaddr*) &srv_addr, sizeof(srv_addr)) < 0 ){perror("connect error"); return -1;

}/* run the client using standard input */if (str_cli(stdin, sock_fd) < 0 )exit(-1);

else

exit(0);}

int str_cli(FILE* fp, int sock_fd){char recv_str[MAX_RECV_BUF]; /* buffer to hold received data */char send_str[MAX_SEND_BUF]; /* buffer to hold data to be sent */


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fd_set read_set; /* only read (from input or from socket) */int fp_no, /* file descriptor number of fp */

max_fd; /* maximum file descriptor */ssize_t rcvd_bytes, sent_bytes, read_bytes;int stdin_eof; /* flag indicate eof from stdin */

fp_no = fileno(fp);

stdin_eof = 0;FD_ZERO(&read_set); /* initialize read_set to zero */

for(;;){if(stdin_eof == 0)FD_SET(fp_no, &read_set); /* set bit corresponding to fp */

FD_SET(sock_fd, &read_set); /* set bit corresponding to sock_fd */max_fd = ( (fp_no > sock_fd) ? fp_no : sock_fd );

select(max_fd+1, &read_set, NULL, NULL, NULL);

if (FD_ISSET(sock_fd, &read_set)) /* socket is readable */{memset(recv_str, 0, MAX_RECV_BUF); // clear recv_str

/* read back from socket into recv_str */ if ( (rcvd_bytes = read(sock_fd, recv_str, MAX_RECV_BUF)) < 0 ) {

perror("read error"); return -1;}

if (rcvd_bytes == 0)

{ if (stdin_eof == 1) return0; /* normal termination */ else

{ /* server has been shutdown */printf("Server terminated prematurely\n");

return -1;}

}/* write received line to standard output */fputs(recv_str, stdout);

} // if

if (FD_ISSET(fp_no, &read_set)) /* fp (stdin) is readable */{ /* read input from file fp into send_str buffer */ if ( (read_bytes = read(fp_no, send_str, MAX_SEND_BUF)) == 0){stdin_eof = 1;shutdown(sock_fd, SHUT_WR); /* send FIN, can still recv */FD_CLR(fp_no, &read_set);puts("EOF");

continue

;}

/* else, write send_str contents to socket */ if ( (sent_bytes = write(sock_fd, send_str, read_bytes)) < 0 ) {


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perror("write error"); return -1;}

} //if} // for

return0;}


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File: /home/mostafa/netprog/tcpsockex/tcpechosel1_srv.c Page 1 of 3

/*TCP echo server using selectbased on example in figure 6.21 in the textbook*/

#include #include

#include #include /* memset */#include /* close, read, write */#include /* select */

#define SRV_PORT 5105 /* default port number */#define LISTEN_ENQ 5 /* for listen backlog */#define MAX_RECV_BUF 256#define MAX_CLIENTS FD_SETSIZE

intmain(

intargc,

char* argv[]){

int listen_fd, conn_fd,clients [MAX_CLIENTS], /* array of client sockets */client_fd, /* current client being serviced */max_fd, /* maximum file descriptor */i, /* index to use with clients[] array */max_i, /* max index of connected client */n_ready; /* number of ready fds returned from select() */

fd_set read_set, /* readable file descriptors to use with select() */all_set; /* all file descriptors, to prepare read_set */

char recv_str[MAX_RECV_BUF];ssize_t rcvd_bytes, sent_bytes;struct sockaddr_in srv_addr, cli_addr;socklen_t cli_len;

/* start */if ( (listen_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0 ) {perror("listen error"); return -1;

}memset(&srv_addr, 0, sizeof(srv_addr));memset(&cli_addr, 0, sizeof(cli_addr));

srv_addr.sin_family = AF_INET;srv_addr.sin_addr.s_addr = htonl(INADDR_ANY);srv_addr.sin_port = htons(SRV_PORT);

if ( bind(listen_fd, (struct sockaddr*) &srv_addr, sizeof(srv_addr))< 0 ){perror("bind error"); return -1;

}if ( listen(listen_fd, LISTEN_ENQ) < 0 ) {perror("listen error"); return -1;

}

max_fd = listen_fd; /* initially, maximum fd is listening socket fd */max_i = -1; /* no connected clients yet */

for(i=0; i < MAX_CLIENTS; i++)


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clients[i] = -1; /* -1 indicates available entry */

FD_ZERO(&all_set); /* clear all_set */FD_SET(listen_fd, &all_set); /* set entry for listen_fd */

for(;;) /* main loop */{

read_set = all_set; /* copy all_set to read_set */

/* block in select until a fd is ready, no timeout */n_ready = select(max_fd + 1, &read_set, NULL, NULL, NULL);

/* once select returns, check which fds are ready */

if (FD_ISSET(listen_fd, &read_set)) /* new client connection */{cli_len = sizeof(cli_addr);

if

( (conn_fd = accept(listen_fd, (struct

sockaddr*) &cli_addr,&cli_len) ) < 0 ) {perror("accept error");

continue; /* go back to main loop */}

for (i=0; i < MAX_CLIENTS ; i++)if (clients[i] < 0){clients[i] = conn_fd; /* assign conn_fd to first available */break; /* slot in clients array */

}

if ( i == MAX_CLIENTS ){fputs("too many clients", stderr);close (conn_fd); /* disconnect this client */continue; /* go back to main loop */

}

FD_SET(conn_fd, &all_set); /* add new fd to next loop select */

if( conn_fd > max_fd) max_fd = conn_fd; /* update max_fd */if (i > max_i) max_i = i; /* update max_i */

if (--n_ready


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if (rcvd_bytes


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/*TCP echo server using select (more compact)based on example from Beej's tutorial to network programming*/

#include #include

#include #include /* memset */#include /* close, read, write */#include /* select */

#define SRV_PORT 5105 /* default port number */#define LISTEN_ENQ 5 /* for listen backlog */#define MAX_RECV_BUF 256

int main(int argc, char* argv[])

{ int listen_fd, conn_fd,max_fd, /* maximum file descriptor */i; /* index */

fd_set read_set, /* readable file descriptors to use with select() */all_set; /* all file descriptors, to prepare read_set */

char recv_str[MAX_RECV_BUF];ssize_t rcvd_bytes, sent_bytes;struct sockaddr_in srv_addr, cli_addr;socklen_t cli_len;

/* start */if ( (listen_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0 ) {perror("listen error"); return -1;

}memset(&srv_addr, 0, sizeof(srv_addr));memset(&cli_addr, 0, sizeof(cli_addr));srv_addr.sin_family = AF_INET;srv_addr.sin_addr.s_addr = htonl(INADDR_ANY);srv_addr.sin_port = htons(SRV_PORT);

if ( bind(listen_fd, (struct sockaddr*) &srv_addr, sizeof(srv_addr))< 0 ){

perror("bind error"); return -1;}if ( listen(listen_fd, LISTEN_ENQ) < 0 ) {perror("listen error"); return -1;

}

max_fd = listen_fd; /* initially, maximum fd is listening socket fd */FD_ZERO(&all_set); /* clear all_set */FD_SET(listen_fd, &all_set); /* set entry for listen_fd */

for(;;) /* main loop */{read_set = all_set; /* copy all_set to read_set */

/* block in select until a fds is ready, no timeout */


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if( select(max_fd + 1, &read_set, NULL, NULL, NULL) < 0 ) {perror("select error"); return -1;

}

/* once select returns, check which fds is ready */for (i=0; i max_fd)max_fd = conn_fd; /* update max_fd */

}}//end if new connection

else/* it is data for client */{rcvd_bytes = read(i, recv_str, MAX_RECV_BUF);

if (rcvd_bytes


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File: /home/mostafa/netprog/tcpsockex/tcpechopoll1_srv.c Page 1 of 3

/*TCP echo server using pollbased on example in figure 6.25 in the textbook*/

#include #include

#include #include /* memset */#include /* close, read, write */#include /* poll */

#define SRV_PORT 5105 /* default port number */#define LISTEN_ENQ 5 /* for listen backlog */#define MAX_RECV_BUF 256#define MAX_CLIENTS 256

intmain(

intargc,

char* argv[]){

int listen_fd, conn_fd, client_fd, i, max_i, n_ready;char recv_str[MAX_RECV_BUF];ssize_t rcvd_bytes, sent_bytes;struct sockaddr_in srv_addr, cli_addr;socklen_t cli_len;struct pollfd clients[MAX_CLIENTS]; /* array of pollfd structures */

/* start */if ( (listen_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0 ) {

perror("listen error"); return -1;}memset(&srv_addr, 0, sizeof(srv_addr));memset(&cli_addr, 0, sizeof(cli_addr));srv_addr.sin_family = AF_INET;srv_addr.sin_addr.s_addr = htonl(INADDR_ANY);srv_addr.sin_port = htons(SRV_PORT);

if ( bind(listen_fd, (struct sockaddr*) &srv_addr, sizeof(srv_addr))< 0 ){perror("bind error"); return -1;

}

if ( listen(listen_fd, LISTEN_ENQ) < 0 ) {perror("listen error"); return -1;

}

clients[0].fd = listen_fd; /* first element is listening socket */clients[0].events = POLLIN; /* poll for input data read */

for (i=1; i< MAX_CLIENTS; i++)clients[i].fd = -1; /* initialize the rest of the array */

max_i = 0; /* max index of the array */

for(;;) /* main loop*/{n_ready = poll(clients, max_i + 1, -1); /* -1 means no timeout */if (n_ready < 0) {


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perror("poll"); return -1;}

if (clients[0].revents & POLLIN){/* listening socket data, i.e. new connection */cli_len = sizeof(cli_addr);

if ( (conn_fd = accept(listen_fd, (struct sockaddr*) &cli_addr,

&cli_len) ) < 0 ) {perror("accept error");

continue; /* go back to main loop */}

for (i=1; i < MAX_CLIENTS ; i++)if (clients[i].fd < 0){clients[i].fd = conn_fd; /* assign conn_fd to first available */break; /* slot in clients array */

}

if ( i == MAX_CLIENTS ){fputs("too many clients", stderr);close (conn_fd); /* disconnect this client */continue; /* go back to main loop */

}

clients[i].events = POLLIN; /* add events for poll to new client */

if (i > max_i) max_i = i; /* update max_i */

if (--n_ready


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{close(client_fd);clients[i].fd = -1;perror("write error");

}}

if (--n_ready

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08 Io Multiplexing