Prof. Edmundo R. M. MadeiraCarlos R. Senna

MC823MC823Laboratório de TeleprocessamentoLaboratório de Teleprocessamento

e Redese Redes Primeiro Semestre 2007

2Programa da DisciplinaA

pre

senta

ção

• Tecnologia de Comunicação:

1. Sockets

2. RMI

3. Web Services

3Critério de AvaliaçãoA

pre

senta

ção

• Quatro projetos com relatórios de comparação (pesos iguais).

• Todos os projetos devem ter notas superiores ou iguais a 5. Se não, a média final é o menor valor entre 4,9 e a média dos quatro projetos.

4Bibliografia

• Stevens, R. W. “Unix Network Programming – Networking APIs: Sockets and XTI” – Vol. 1, Second Edition, Prentice-Hall, 1998. • BIMECC 005.43St47u

• Tutoriais sobre RMI e Web Services.

Ap

rese

nta

ção

5Tecnologias de Comunicação

Sockets

6OutlineS

ock

ets

• Socket basics• TCP sockets• Socket details and options• Socket functions• Concurrent server• Example: Echo Server and Echo Client• Signals and Zumbies• I/O Multiplexing• UDP Sockets

7BasicsS

ock

ets

Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Fig. 2.20

8BasicsS

ock

ets

• An end-point for a IP network connection• what the application layer “plugs into”• programmer cares about Application Programming Interface (API)

• End point determined by two things:• Host address: IP address is Network Layer• Port number: is Transport Layer

• Two end-points determine a connection: socket pair

• ex: 206.62.226.35,p21 + 198.69.10.2,p1500• ex: 206.62.226.35,p21 + 198.69.10.2,p1499

9PortsS

ock

ets

• Numbers (vary in BSD, Solaris):

• 0-1023 “reserved”, must be root

• 1024 – 49151 (registered with IANA)

• 49152 – 65535 “ephemeral”

• /etc/services:

• ftp 21/tcp

• telnet 23/tcp

• finger 79/tcp

• snmp 161/udp

10Sockets and the OSS

ock

ets

• User Socket Operating System (Transport Layer)

• User sees “descriptor”, integer index

• like: FILE *, or file index

• returned by socket() call (more later)

11Transport LayerS

ock

ets

• UDP: User Datagram Protocol• no acknowledgements• no retransmissions• out of order, duplicate possible• connectionless

• TCP: Transmission Control Protocol• reliable (in order, all arrive, no

duplicates)• flow control• connection• duplex

12Socket DetailsS

ock

ets

Unix Network Programming, W. Richard Stevens, Second Edition, 1998, Prentice Hall, BIMECC 005.43St47u

• Socket address structure (Chap. 3 )• TCP client-server (Chap. 4-5 )• UDP client server (Chap. 8 )• Misc stuff (Chap. 7 )

• setsockopt(), getsockopt()

13Socket Address Structure

#include <sys/types.h>#include <sys/socket.h>struct in_addr {

/* 32-bit IPv4 addresses */

in_addr_t s_addr; };struct sock_addr_in { unit8_t sin_len; /* length of structure */

sa_family_t sin_family; /* AF_INET */

in_port_t sin_port; /* TCP/UDP Port num */

struct in_addr sin_addr; /* IPv4 address */

char sin_zero[8]; /* unused */

}

Sock

ets

14Tamanhos e LimitaçõesS

ock

ets • Tamanho máximo IPv4: 65535 bytes incluindo

cabeçalho• Tamanho máximo IPv6• IPv6: opção Jumbo Payload – 32 bits• MTU mínimo de um enlace para:

• IPv4: 68 bytes; IPv6: 576 bytes• Path MTU• Tamanho mínimo do buffer de remontagem – tamanho

mínimo do datagrama que qualquer implementação deve suportar:• IPv4: 576 bytes

15TCP Client-ServerS

ock

ets

socket( )

bind( )

listen( )

accept( )

socket( )

connect( )

write( )

read( )

write( )

read( )

close( )

Blocks until conection

from client

Process request

read( )

close( )

TCP Server

TCP Client

Connection establishment

(TCP) three-way handshake

data (request)

data (reply)

End-of-file notification

well-known

port

16socket FunctionS

ock

ets

• family is one of• AF_INET (IPv4), AF_INET6 (IPv6), AF_LOCAL (local Unix),• AF_ROUTE (access to routing tables), AF_KEY (new, for

encryption)

• type is one of• SOCK_STREAM (TCP), SOCK_DGRAM (UDP)• SOCK_RAW (for special IP packets, PING, etc. Must be root)

• protocol is 0 (used for some raw socket options)• upon success returns socket descriptor

• like file descriptor => -1 if failure

int socket(int family, int type, int protocol);Create a socket, giving access to transport layer service.

Example:if (( sockfd = socket (AF_INET, SOCK_STREAM, 0)) < 0)

err_sys (“socket call error”);

17socket OptionsS

ock

ets • Many socket( ) options

• Set/lookup using• setsockopt(), getsockopt()

• Examples:• SO_LINGER• SO_RCVBUF, SO_SNDBUF (modify buffer sizes)• SO_RCVLOWAT, SO_SNDLOWAT• SO_RCVTIMEO, SO_SNDTIMEO (Timeouts)• TCP_KEEPALIVE (idle time before close (2 hours,

default))• TCP_MAXRT (set timeout value)• TCP_NODELAY (disable Nagle Algorithm)

• See man pages for details• man socket on any Unix machine

18connect FunctionS

ock

ets

• sockfd is socket descriptor from socket()• servaddr is a pointer to a structure with:

• Server port number and IP address• must be specified (unlike bind())

• addrlen is length of structure• client doesn’t need bind()

• OS will pick ephemeral port• returns socket descriptor if ok, -1 on error

int connect( int sockfd, const struct sockaddr *servaddr,

socklen_t addrlen );Connect to server.

Example:if (connect (sockfd, (struct sockaddr *) &servaddr, sizeof

(servaddr)) != 0)err_sys(“connect call error”);

19bind FunctionS

ock

ets

• sockfd is socket descriptor from socket()• myaddr is a pointer to address struct with:

• port number and IP address• addrlen is length of structure• returns 0 if ok, -1 on error

• EADDRINUSE (“Address already in use”)

int bind( int sockfd, const struct sockaddr *myaddr, socklen_t

addrlen);Assign a local protocol address (“name”) to a socket.

Example:if (bind (sd, (struct sockaddr *) &servaddr, sizeof (servaddr))

!= 0)errsys (“bind call error”);

20listen FunctionS

ock

ets

• sockfd is socket descriptor from socket()• backlog is maximum number of incomplete connections

• historically 5• rarely above 15 on a even moderate web server!

• Sockets default to active (for client)

int listen(int sockfd, int backlog);Announce willingness to accept connections, give queue size,

change socket state for TCP server.

Example:if (listen (sd, 2) != 0)

errsys (“listen call error”);

21listen FunctionS

ock

ets

22accept FunctionS

ock

ets

• sockfd is socket descriptor from socket()• cliaddr and addrlen return protocol address from client• returns brand new descriptor, created by OS• if used with fork(), can create concurrent server (more later)

int accept( int sockfd, struct sockaddr cliaddr,

socklen_t *addrlen );Return next completed connection.

Example:sfd = accept (s, NULL, NULL);if (sfd == -1) err_sys (“accept error”);

23close FunctionS

ock

ets

• sockfd is socket descriptor from socket()• closes socket for reading/writing

• returns (doesn’t block)• attempts to send any unsent data• -1 if error

int close(int sockfd);Close socket for use.

24read and write FunctionsS

ock

ets • Pode ler/escrever menos do que necessita devido ao

tamanho restrito do buffer.• É necessário chamar a função novamente.• Somente o write é nonblocking

• Tamanho máximo IPv4: 65535 bytes incluindo cabeçalho

• Tamanho máximo IPv6• IPv6: opção Jumbo Payload – 32 bits• MTU mínimo de um enlace para:

• IPv4: 68 bytes; IPv6: 576 bytes• Path MTU• Tamanho mínimo do buffer de remontagem – tamanho

mínimo do datagrama que qualquer implementação deve suportar:• IPv4: 576 bytes

25write FunctionS

ock

ets Aplicação

TCP

IP

Fila saída

Enlace

Buffer da aplicação (qualquer tamanho)

write

Buffer de envio do socket (SO_SNDBUF)

Processo do usuário

Kernel

Segmentos TCP do tamanho do MSS

MSS normalmente ≤ MTU – 40 (IPv4) ou MTU – 60 (IPv6)

Pacotes IPv4 ou IPv6 do tamanho da MTU

• Kernel copia dados do buffer da aplicação para buffer do socket.

• Se não há espaço, o processo é bloqueado até a transferência do último byte.

• Sucesso da operação write não significa que o outro lado do TCP recebeu os dados.

26write Function (lib/writen.c)S

ock

ets 1 #include "unp.h"

2 ssize_t /* Write "n" bytes to a descriptor. */ 3 writen(int fd, const void *vptr, size_t n) 4 { 5 size_t nleft; 6 ssize_t nwritten; 7 const char *ptr; 8 ptr = vptr; 9 nleft = n; 10 while (nleft > 0) { 11 if ( (nwritten = write(fd, ptr, nleft)) <= 0) { 12 if (nwritten < 0 && errno == EINTR) 13 nwritten = 0; /* and call write() again */ 14 else 15 return (-1); /* error */ 16 } 17 nleft -= nwritten; 18 ptr += nwritten; 19 } 20 return (n); 21 }

27read Function (lib/readn.c)S

ock

ets 1 #include "unp.h"

2 ssize_t /* Write "n" bytes to a descriptor. */ 3 readn(int fd, void *vptr, size_t n) 4 { 5 size_t nleft; 6 ssize_t nread; 7 char *ptr; 8 ptr = vptr; 9 nleft = n; 10 while (nleft > 0) { 11 if ( (nread = read(fd, ptr, nleft)) < 0) { 12 if (errno == EINTR) 13 nread = 0; /* and call read() again */ 14 else 15 return (-1); 16 } else if (nread == 0) 17 break; /* EOF */ 18 nleft -= nread; 19 ptr += nread; 20 } 21 return (n - nleft); /* return >= 0 */ 22 }

28Sending and ReceivingS

ock

ets

• Same as read() and write() but for flags

• flags for I/O functions (see man pages)• MSG_DONTWAIT (this send non-blocking)• MSG_OOB (out of band data, 1 byte sent ahead)• MSG_PEEK • MSG_WAITALL (don’t give me less than max)• MSG_DONTROUTE (bypass routing table)

int recv(int sockfd, void *buff, size_t mbytes, int flags);

int send(int sockfd, void *buff, size_t mbytes, int flags);

29Servidor ConcorrenteS

ock

ets

- T

CP • Após accept e fork, o processo filho executa

no CONNFD e o pai continua a escutar no LISTENFD.

• close decrementa contador de referências.

• FYN é enviado somente quando contador de referências possui valor zero.

30Servidor Concorrente – forkS

ock

ets

- T

CP • fork executa uma cópia idêntica do processo.

• Retorna o identificador do processo filho ao pai (process ID) e o valor 0 (process ID) ao filho.

• Os descritores abertos antes do fork são compartilhados com os filhos.

• o “connected socked” após um accept seguido de fork é compartilhado com o filho.

# include <unistd.h>pid_t fork(void);

Returns: 0 in child, process ID of child in parent, -1 on error

31Servidor Concorrente – execS

ock

ets

- T

CP • A função exec executa outro programa.

#include <unistd.h>

int execl ( const char *pathname, const char *arg0,... /* (char *) 0

*/ );int execv ( const char *pathname, char *const argv[ ] );int execle ( const char *pathname, const char *arg0, ...

/* (char *) 0, char *const envp[ ] */ );int execve ( const char *pathname, char *const argv[ ],

char *const envp[ ] );int execlp ( const char *filename, const char *arg0, ... /* (char *) 0

*/ );int execvp ( const char *filename, char *const argv[ ] );

All six return: -1 on error, no return on success

32Servidor ConcorrenteS

ock

ets

- T

CP pid_t pid;

int listenfd, connfd; /* fill in sockaddr_in{ } with server's well-known port */ listenfd = socket( ... ); bind(listenfd, ... ); listen(listenfd, LISTENQ);

for ( ; ; ) { connfd = accept (listenfd, ... ); /* probably blocks */

if( (pid = fork() ) == 0) {

close(listenfd); /* child closes listening socket */ doit(connfd); /* process the request */ close(connfd); /* done with this client */ exit(0); /* child terminates */

}

close(connfd); /* parent closes connected socket */ }

33Servidor ConcorrenteS

ock

ets

- T

CP

connect( )

Client

listenfd

Serverconnection

request

connect( ) listenfd

connfd

connection

connect( ) listenfd

connfd

connection

listenfd

connfd

fork

Server

(parent)

Server

(child)

connect( )listenfd

connection

connfd

Server

(parent)

Server

(child)

1

2

3

4

Stevens Vol. 1 Fig. 4.14 a 4.17

34Servidor Concorrente - ExemploS

ock

ets

- T

CP • Questões: o que acontece...

• Quando o cliente e o servidor iniciam?• Se o servidor “terminar” antes do cliente finalizar a

sessão?

• Servidor Echo

• Cliente Echo

35Servidor Echo (main)S

ock

ets

- T

CP 1 #include "unp.h"

2 int 3 main(int argc, char **argv) 4 { 5 int listenfd, connfd; 6 pid_t childpid; 7 socklen_t clilen; 8 struct sockaddr_in cliaddr, servaddr; 9 listenfd = Socket (AF_INET, SOCK_STREAM, 0); 10 bzero(&servaddr, sizeof(servaddr)); 11 servaddr.sin_family = AF_INET; 12 servaddr.sin_addr.s_addr = htonl (INADDR_ANY); 13 servaddr.sin_port = htons (SERV_PORT); 14 Bind(listenfd, (SA *) &servaddr, sizeof(servaddr)); 15 Listen(listenfd, LISTENQ); 16 for ( ; ; ) { 17 clilen = sizeof(cliaddr); 18 connfd = Accept(listenfd, (SA *) &cliaddr, &clilen); 19 if ( (childpid = Fork()) == 0) { /* child process */ 20 Close(listenfd); /* close listening socket */

21 str_echo(connfd); /* process the request */

22 exit (0);23 } 24 Close(connfd); /* parent closes connected socket */ 25 } 26 }

Stevens Vol. 1 Fig. 5.2 TCP echo server

36Servidor Echo (str_echo)S

ock

ets

- T

CP 1 #include "unp.h"

2 3 void str_echo(int sockfd) 4 { 5 ssize_t n; 6 char buf[MAXLINE];

7 again: 8 while ( (n = read(sockfd, buf, MAXLINE)) > 0) 9 writen(sockfd, buf, n); 10 if (n < 0 && errno == EINTR) 11 goto again; 12 else if (n < 0) 13 err_sys("str_echo: read error"); 14 }

37Cliente Echo (main)S

ock

ets

- T

CP 1 #include "unp.h"

2 3 int main(int argc, char **argv) 4 { 5 int sockfd; 6 struct sockaddr_in servaddr; 7 if (argc != 2) 8 err_quit("usage: tcpcli <IPaddress>"); 9 sockfd = Socket(AF_INET, SOCK_STREAM, 0); 10 bzero(&servaddr, sizeof(servaddr)); 11 servaddr.sin_family = AF_INET; 12 servaddr.sin_port = htons(SERV_PORT); 13 Inet_pton(AF_INET, argv[1], &servaddr.sin_addr); 14 Connect(sockfd, (SA *) &servaddr, sizeof(servaddr));

15 str_cli(stdin, sockfd); /* do it all */

16 exit(0); 17 }

Stevens Vol. 1 Fig. 5.4 TCP echo client

38Cliente Echo (str_cli)S

ock

ets

- T

CP 1 #include "unp.h"

2 3 str_cli(FILE *fp, int sockfd) 4 { 5 char sendline[MAXLINE], recvline[MAXLINE]; 6 while (Fgets(sendline, MAXLINE, fp) != NULL) { 7 Writen(sockfd, sendline, strlen (sendline)); 8 if (Readline(sockfd, recvline, MAXLINE) == 0) 9 rr_quit("str_cli: server terminated

prematurely"); 10 Fputs(recvline, stdout); 11 } 12 }

Stevens Vol. 1 Fig. 5.5 str_cli function: client processing loop

39Netstat (início normal)S

ock

ets

- T

CP linux % tcpserv01 &

[1] 17870

linux % netstat -a

Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 *:9877 *:* LISTEN

linux % tcpcli01 127.0.0.1

linux % netstat -a

Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 local host:9877 localhost:42758 ESTABLISHED

tcp 0 0 local host:42758 localhost:9877 ESTABLISHED

tcp 0 0 *:9877 *:* LISTEN

linux % ps -t pts/6 -o pid,ppid,tty,stat,args,wchan

PID PPID TT STAT COMMAND WCHAN

22038 22036 pts/6 S -bash wait4

17870 22038 pts/6 S ./tcpserv01 wait_for_connect

19315 17870 pts/6 S ./tcpserv01 tcp_data_wait

19314 22038 pts/6 S ./tcpcli01 127.0 read_chan

40Netstat (finalização normal)S

ock

ets

- T

CP linux % netstat -a | grep 9877

tcp 0 0 *:9877 *:* LISTEN

tcp 0 0 localhost:42758 localhost:9877 TIME_WAIT

linux % ps -t pts/6 -o pid,ppid,tty,stat,args,wchan

PID PPID TT STAT COMMAND WCHAN

22038 22036 pts/6 S -bash read_chan

17870 22038 pts/6 S ./tcpserv01 wait_for_connect

19315 17870 pts/6 Z [tcpserv01 <defu do_exit

41Finalização NormalS

ock

ets

- T

CP • Ao digitar EOF, fget retorna um ponteiro nulo e

a função str_cli retorna;• O cliente executa exit;• Descriptors no cliente são fechados e um FYN

é enviado ao servidor. Servidor no estado CLOSE_WAIT e cliente FIN_WAIT-2;

• O filho servidor estava bloqueado em readline (str_echo) e retorna para main servidor. Filho servidor termina executando exit;

• Todos os descritores do filho servidor são fechados, um FIN do servidor é enviado e um ACK do cliente;

• Conexão terminada; e• Sinal SIGCHLD é enviado.

42signal FunctionS

ock

ets

- T

CP Sigfunc *signal(int signo, Sigfunc *func);

• signo is a signal number.• *func is a pointer to a signal handling function, as well

as the return value from the function.

• A signal is a notification to a process that ana event has ocurred.

• Are sometimes called software interrupts.• Usually occur asynchronously.• Signals can be sent:

• By one process to another process (or to itself), or• By the Kernel to a process.

43signal FunctionS

ock

ets

- T

CP 1 #include "unp.h"

2 3 Sigfunc *signal (int signo, Sigfunc *func) 4 { 5 struct sigaction act, oact; 6 act.sa_handler = func; 7 sigemptyset (&act.sa_mask); 8 act.sa_flags = 0; 9 if (signo == SIGALRM) { 10 #ifdef SA_INTERRUPT 11 act.sa_flags |= SA_INTERRUPT; /* SunOS 4.x */ 12 #endif 13 } else { 14 #ifdef SA_RESTART 15 act.sa_flags |= SA_RESTART; /* SVR4, 4.4BSD */ 16 #endif 17 } 18 if (sigaction (signo, &act, &oact) < 0) 19 return (SIG_ERR); 20 return (oact.sa_handler); 21 }

Stevens Vol. 1 Fig. 5.6 signal function

44Handling SIGCHLD Signals and Zombies

Sock

ets

- T

CP • The purpose of the zombies state is to maintain

information about the child for the parent to fetch at some later time.

• The information includes:• The process ID of the child;• Its termination status, and• Information on the resource utilization of the child

(CPU Time, memory, etc).• If a process terminates, and the process has children in

the zombie state, the parent process ID of all the zombie children is set to 1 (the init process), which will inherit the children and clean them up.

• Whenever we fork children, we must wait for them to prevent them from becoming zombies.

45Handling SIGCHLD Signals and Zombies

Sock

ets

- T

CP Signal (SIGCHLD, sig_chld);

...1 #include “unp.h”23 void sig_chld( int signo )4 {5 pid_t pid;6 int stat;7 pid = wait(&stat);8 printf(“child %d terminated\n”, pid);9 return;10 }

46wait and waitpid Functions#include <sys/wait.h>pid_t wait (int *statloc);pid_t waitpid (pid_t pid, int *statloc, int options);

Both return: process ID if OK, 0 or–1 on error

Sock

ets

- T

CP

Stevens Vol. 1 Fig. 5.10

Client terminates, closing all five connections, terminating all five children.

Server

parent

Server

child #1

Server

child #2

Server

child #3

Server

child #4

Server

child #5

4 3 2 1 0

client exit

FIN

FIN

FIN

FIN

FIN

SIGCHLD

SIGCHLD

SIGCHLD

SIGCHLD

SIGCHLD

47wait and waitpid Functions

• waitpid fetching the status of any of our children that have terminated.

• WNOHANG option specify waitpid not to block.

Sock

ets

- T

CP

Stevens Vol. 1 Fig. 5.11

1 #include "unp.h" 2 3 void sig_chld(int signo) 4 { 5 pid_t pid; 6 int stat; 7 while ( (pid = waitpid(-1, &stat, WNOHANG)) > 0) 8 printf("child %d terminated\n", pid); 9 return; 10 }

48TCP server that handles na error of EINTRS

ock

ets

- T

CP

Stevens Vol. 1 Fig. 5.11

1 #include "unp.h" 3 int main(int argc, char **argv) 4 { 5 int listenfd, connfd; 6 pid_t childpid; 7 socklen_t clilen; 8 struct sockaddr_in cliaddr, servaddr; 9 void sig_chld(int); 10 listenfd = Socket (AF_INET, SOCK_STREAM, 0); 11 bzero (&servaddr, sizeof(servaddr)); 12 servaddr.sin_family = AF_INET; 13 servaddr.sin_addr.s_addr = htonl(INADDR_ANY); 14 servaddr.sin_port = htons(SERV_PORT); 15 Bind(listenfd, (SA *) &servaddr, sizeof(servaddr)); 16 Listen(listenfd, LISTENQ); 17 Signal (SIGCHLD, sig_chld); /* must call waitpid() */ 18 for ( ; ; ) { 19 clilen = sizeof(cliaddr); 20 if ( (connfd = accept (listenfd, (SA *) &cliaddr, &clilen)) < 0) { 21 if (errno == EINTR) 22 continue; /* back to for() */ 23 else 24 err_sys("accept error"); 25 } 26 if ( (childpid = Fork()) == 0) { /* child process */ 27 Close(listenfd); /* close listening socket */ 28 str_echo(connfd); /* process the request */ 29 exit(0); 30 } 31 Close (connfd); /*parent closes connected socket*/ 32 } 33 }

49Tecnologias de Comunicação

Sockets

Multiplexação de E/S

50IntroduçãoM

ult

iple

xaçã

o d

e E

/S • Capacidade de avisar Núcleo que se deseja ser notificado quando condições de E/S estejam válidas.• Ex: dados para leitura estão disponíveis.

• Funções relacionadas:• select• poll• shutdown

51Quando usarM

ult

iple

xaçã

o d

e E

/S • Quando cliente está manipulando vários descritores.• Ex: descritor do socket é de entrada interativa.

• Quando o cliente manipula vários sockets ao mesmo tempo.

• Quando TCP manipula listening sockets e outros sockets conectados

• Quando o servidor trabalha com TCP e com UDP simultaneamente.

• Quando o servidor manipula vários protocolos e serviços simultaneamente.

52Tipos e OperaçõesM

ult

iple

xaçã

o d

e E

/S • Tipos:• E/S bloqueante;• E/S não bloqueante ;• Multiplexação de E/S;• E/S orientada a sinal; e• E/S assíncrona.

• Operações em duas fases:• Espera para dados estarem disponíveis e• Cópia dos dados do Kernel para o processo.

53ModeloM

ult

iple

xaçã

o d

e E

/S

Stevens Vol. 1 Fig. 6.3

54select FunctionM

ult

iple

xaçã

o d

e E

/S #include <sys/select.h>#include <sys/time.h>

struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */

};

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

fd_set *exceptset, const struct timeval *timeout);

Return: positive count of ready descriptors, 0 on timeout and –1 on error.

55select FunctionM

ult

iple

xaçã

o d

e E

/S • Instrui o núcleo para acordar o processo quando:• Um conjunto de eventos ocorrer ou • Um certo intervalo de tempo tiver ocorrido.

• Pode especificar o descritor:• Reading, writing ou exception.

• Três possibilidades de espera:• Espera até condição ser verdadeira – null pointer;• Espera por um valor máximo de intervalo; e• Não espera. Retorna após verificar os descritores

(polling).

• Os valores de temporização em timeout devem ser zero.

56poll FunctionM

ult

iple

xaçã

o d

e E

/S #include <poll.h>

struct pollfd { int fd; /* descriptor to check */ short events; /* events of interest on fd */ short revents; /* events that occurred on fd */

};

int poll ( struct pollfd *fdarray, unsigned long nfds, int timeout);

Return: -1 if an error occurred, 0 if no descriptors are ready before the timer

expires, Otherwise it is the number of descriptors that

have a nonzero revents member.

57shutdown FunctionM

ult

iple

xaçã

o d

e E

/S #include <sys/socket.h>

int shutdown(int sockfd, int howto);

Return: 0 if OK or -1 on error.

Argumento howto:

• SHUT_RD – fecha para leitura (read-halt). Conteúdo do buffer e futuras recepções são

descartadas.

• SHUT_WR – qualquer dado no buffer de escrita será enviado. Não pode mais escrever.

• SHUT_RDWR – efeito de fazer SHUT_WR após o SHUT_RD.

58shutdown FunctionM

ult

iple

xaçã

o d

e E

/S

Stevens Vol. 1 Fig. 6.12

59Tecnologias de ComunicaçãoM

C82

3

Sockets

UDP