npc13

UNIX Network Programming 1

Chapter 13.

Advanced

I / O Functions


13.1 Introduction13.1 Introduction

• Socket Timeouts

• recv and send Functions

• readv and writev Functions

• recvmsg and sendmsg Function

• Ancillary Data

• How much Data is Queued?

• Sockets and Standard I/O

• T/TCP


13.2 Socket Timeouts13.2 Socket Timeouts

• Three ways to place a timeout on an I/O operation involving a socket

– Call alarm, which generates the SIGALRM signal when the specified time has expired.

– Block waiting for I/O in select, which has a time limit built in, instead of blocking in a call to read or write.

– Use the newer SO_RCVTIMEO and SO_SNDTIMEO socket options.


Connect with a Timeout Using SIGALRM (figure 13.1)Connect with a Timeout Using SIGALRM (figure 13.1)

#include "unp.h"

static voidconnect_alarm(int);

int connect_timeo(int sockfd, const SA *saptr, socklen_t salen, int nsec){

Sigfunc *sigfunc;int n;

sigfunc = Signal(SIGALRM, connect_alarm);if (alarm(nsec) != 0)

err_msg("connect_timeo: alarm was already set");

if ( (n = connect(sockfd, (struct sockaddr *) saptr, salen)) < 0) {close(sockfd);if (errno == EINTR)

errno = ETIMEDOUT;}alarm(0); /* turn off the alarm */Signal(SIGALRM, sigfunc); /* restore previous signal handler */

return(n);}

static voidconnect_alarm(int signo){

return; /* just interrupt the connect() */}


recvfrom with a Timeout Using SIGALRM (figure 13.2)recvfrom with a Timeout Using SIGALRM (figure 13.2)

#include "unp.h"

static void sig_alrm(int);

void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;char sendline[MAXLINE], recvline[MAXLINE + 1];

Signal(SIGALRM, sig_alrm);

while (Fgets(sendline, MAXLINE, fp) != NULL) {

Sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

alarm(5);if ( (n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL)) < 0)

{if (errno == EINTR)

fprintf(stderr, "socket timeout\n");else

err_sys("recvfrom error");} else {

alarm(0);recvline[n] = 0; /* null terminate */Fputs(recvline, stdout);

}}

}

static void sig_alrm(int signo){

return; /* just interrupt the recvfrom() */}


recvfrom with a Timeout Using select (figure 13.3)recvfrom with a Timeout Using select (figure 13.3)

#include "unp.h"

int

readable_timeo(int fd, int sec)

{

fd_set rset;

struct timeval tv;

FD_ZERO(&rset);

FD_SET(fd, &rset);

tv.tv_sec = sec;

tv.tv_usec = 0;

return(select(fd+1, &rset, NULL, NULL, &tv));

/* 4> 0 if descriptor is readable */

}


recvfrom with a Timeout Using the SO_RCVTIMEO Socket recvfrom with a Timeout Using the SO_RCVTIMEO Socket Option (figure 13.5)Option (figure 13.5)voiddg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen){

int n;char sendline[MAXLINE], recvline[MAXLINE + 1];struct timeval tv;

tv.tv_sec = 5;tv.tv_usec = 0;Setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));

while (Fgets(sendline, MAXLINE, fp) != NULL) {

Sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);if (n < 0) {

if (errno == EWOULDBLOCK) {fprintf(stderr, "socket timeout\n");continue;

} elseerr_sys("recvfrom error");

}recvline[n] = 0; /* null terminate */Fputs(recvline, stdout);

}}


13.3 recv and send Functions13.3 recv and send Functions

#include <sys/socket.h>

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

ssize_t send (int sockfd, const void *buff, size_t nbytes, int flags);

Flags Description recv send

MSG_DONTROUTEMSG_DONTWAITMSG_OOBMSG_PEEKMSG_WAITALL

bypass routing table lookuponly this operation is nonblockingsend or receive out-of-band datapeek at incoming messagewait for all the data

Figure 13.6 flags for I/O functions


13.4 readv and writev Functions13.4 readv and writev Functions

– readv and writev let us read into or write from one or more buffers with a single function call.

• are called scatter read and gather write.

#include <sys/uio.h>

ssize_t readv (int filedes, const struct iovec *iov, int iovcnt);

ssize_t writev (int filedes, const struct iovec *iov, int iovcnt);

Struct iovec {

void *iov_base; /* starting address of buffer */

size_t iov_len; /* size of buffer */

};


13.5 recvmsg and sendmsg Functions13.5 recvmsg and sendmsg Functions

#include <sys/socket.h>

ssize_t recvmsg (int sockfd, struct msghdr *msg, int flags);

ssize_t sendmsg (int sockfd, struct msghdr *msg, int flags);

Struct msghdr {

void *msg_name; /* starting address of buffer */

socklen_t msg_namelen; /* size of protocol address */

struct iovec *msg_iov; /* scatter/gather array */

size_t msg_iovlen; /* # elements in msg_iov */

void *msg_control; /* ancillary data; must be aligned

for a cmsghdr structure */

socklen_t msg_controllen; /* length of ancillary data */

int msg_flags; /* flags returned by recvmsg() */

};


13.5 recvmsg and sendmsg Functions 13.5 recvmsg and sendmsg Functions (cont.)(cont.)

Flag

Examined by:Send flags

Sendto flagsSendmsg flags

Examined by:recv flags

recvfrom flagsrecvmsg flags

Returned by:

Recvmsg msg_flagsMSG_DONTROUTEMSG_DONTWAITMSG_PEEKMSG_WAITALL

MSG_EORMSG_OOB

MSG_BCASTMSG_MCASTMSG_TRUNCMSG_CTRUNC

Figure 13.7 Summary of input and output flags by various I/O functions



16

0

20

3

msg_name

msg_ flagsmsg_con tro llenmsg_con tro lm sg_ iov lenmsg_ iovmsg_name len

100

60

80

io v_base

iov_ leniov_baseiov_ leniov_baseiov_ len

iovec{}

F igure 13.8 Data structures when recvmsg is called for a UDP socket.

msghdr{}



16

0

20

3

msg_name

msg_ flagsmsg_con tro llenmsg_con tro lm sg_ iov lenmsg_ iovmsg_name len

100

60

80

io v_base

iov_ leniov_baseiov_ leniov_baseiov_ len

iovec{} [ ]

F igure 13.9 Update o f F igure 13.8 when recvmsg return.

msghdr{}

cmsg_ typecmsg_ leve lcm sg_ len

sockaddr_ in{}16, AF_ INET, 2000

198.69.10.2

16IPPROTP_ IPIP_RECVDSTADDR206 .62 .226 .35


13.6 Ancillary Data13.6 Ancillary Data

• Ancillary data can be sent and received using the msg_control and msg_controllen members of the msghdr structure with sendmsg and recvmsg functions.

Protocol cmsg_level Cmsg_type DescriptionIPv4 IPPROTO_IP IP_RECVDSTADDR

IP_RECVIFreceive destination address with UDP datagramreceive interface index with UDP datagram

IPv6 IPPROTO_IPV6 IPV6_DSTOPTSIPV6_HOPLIMITIPV6_HOPOPTSIPV6_NEXTHOPIPV6_PKTINFOIPV6_RTHDR

specify / receive destination optionsspecify / receive hop limitspecify / receive hop-by-hop optionsspecify next-hop addressspecify / receive packet informationspecify / receive routing header

Unix domain SOL_SOCKET SCM_RIGHTSSCM_CREDS

send / receive descriptorssend / receive user credentials

Figure 13.11 summary of uses for ancillary data.


13.6 Ancillary Data 13.6 Ancillary Data (cont.)(cont.)

cmsg_len

cmsg_level

cmsg_type

pad

data

pad

cmsg_len

cmsg_level

cmsg_type

pad

data

c msghdr{}

c msghdr{}

ac c illarydata objec t

C MSG _ SPAC E ()

ac c illarydata objec t

C MSG _ SPAC E ()

msg_contro l

CM

SG_L

EN()

cmsg

_len

msg

_con

trol

len

cmsg

_len

CM

SG_L

EN()

F igure 13.12 Anc illary data containing two anc illary data objects .


13.6 Ancillary Data 13.6 Ancillary Data (cont.)(cont.)

cmsghdr{}

F igure 13.13 cmsghdr structure when used with Unix domain sockets.

cmsg_len

cmsg_level

cmsg_type

d iscr ip to r

16

SOL_SOCKET

SCM_RIGHTS

cmsghdr{}

cmsg_len

cmsg_level

cmsg_type

16

SOL_SOCKET

SCM_CREDS

fcred{}


13.7 How Much Data Is Queued?13.7 How Much Data Is Queued?

• Three techniques - page 365.


13.8 Sockets and Standard I/O13.8 Sockets and Standard I/O

• The standard I/O stream can be used with sockets, but there are a few items to consider.

– A standard I/O stream can be created from any desciptor by calling the fdopen function. Similarly, given a standard I/O stream, we can obtain the corresponding descriptor by calling fileno.

– fseek, fsetpos, rewind functions is that they all call lseek, which fails on a socket.

– The easiest way to handle this read-write problem is to open two standard I/O streams for a given socket: one for reading, and one for writing.


13.8 Sockets and Standard I/O13.8 Sockets and Standard I/O

• Example : str_echo Function using standard I/O

#include "unp.h"

voidstr_echo(int sockfd){

char line[MAXLINE];FILE *fpin, *fpout;

fpin = Fdopen(sockfd, "r");fpout = Fdopen(sockfd, "w");

for ( ; ; ) {if (Fgets(line, MAXLINE, fpin) == NULL) return; /* connection closed by other end */

Fputs(line, fpout);}

}


13.9 T/TCP: TCP for Transactions13.9 T/TCP: TCP for Transactions

• T/TCP is a slight modification to TCP that can avoid the three-way handshake between hosts that have communicated with each other recently.

• Benefit

– all the reliability of TCP is retained

– maintains TCP’s slow start and congestion avoidance, features that are often missing from UDP applications.


13.9 T/TCP: TCP for Transactions 13.9 T/TCP: TCP for Transactions (cont.)(cont.)

c lient server

socketsendto

read(blocks)

read returns

accept returnsread request<server proc esses reques t>

send replyc lose

Figure 13.15 Time line o f minimal T/ TC P transaction

socket, bind,listenaccept(blocks)

npc13

Documents

t recvmsg int sockfd

t sendmsg int sockfd

t recv int sockfd

int sec

peek msg

t servlen

t writev int filedes

t readv int filedes