Message passing systems are popular because they support client-server interactions, where:
Note: a process can be a client and a server at the same time.
Need to locate a process. Use a port (like a mailbox).
There are two types of ports:
Sockets are a generalization of file input/output that supports interprocess communication.
Solution: applications use sockets and bind port names to them. Processes then send messages to the bound (port) name.
Finger Client:
Describe for a server that forks off a process to handle the connection. Use an alternate port for the server.
Look at picture!
Client send user name and receives back a message.
< wpi 1 >make sockclient cc -g sockclient.c -o sockclient < wpi 2 >make sockserver cc -g sockserver.c -o sockserver < wpi 3 >./sockserver 4242& [1] 28721 < wpi 4 >./sockclient wpi 4242 Hello cew, you are visitor number 1 to this server < wpi 5 >./sockclient wpi 4242 Hello cew, you are visitor number 2 to this server < wpi 6 >kill %1
/* adapted from Sample 25_1 from "Computer Networks and Internets, 2nd ed by Comer */
/* sockclient.c - code for example client program that uses TCP */
#ifndef unix
#define WIN32
#include <windows.h>
#include <winsock.h>
#else
#define closesocket close
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#endif
#include <stdio.h>
#include <string.h>
#include <pwd.h>
#define PROTOPORT 5193 /* default protocol port number */
extern int errno;
char localhost[] = "localhost"; /* default host name */
/*------------------------------------------------------------------------
* Program: client
*
* Purpose: allocate a socket, connect to a server, and print all output
*
* Syntax: client [ host [port] ]
*
* host - name of a computer on which server is executing
* port - protocol port number server is using
*
* Note: Both arguments are optional. If no host name is specified,
* the client uses "localhost"; if no protocol port is
* specified, the client uses the default given by PROTOPORT.
*
*------------------------------------------------------------------------
*/
main(argc, argv)
int argc;
char *argv[];
{
struct hostent *ptrh; /* pointer to a host table entry */
struct protoent *ptrp; /* pointer to a protocol table entry */
struct sockaddr_in sad; /* structure to hold an IP address */
int sd; /* socket descriptor */
int port; /* protocol port number */
char *host; /* pointer to host name */
int n; /* number of characters read */
char buf[1000]; /* buffer for data from the server */
#ifdef WIN32
WSADATA wsaData;
WSAStartup(0x0101, &wsaData);
#endif
struct passwd *ppwd;
memset((char *)&sad,0,sizeof(sad)); /* clear sockaddr structure */
sad.sin_family = AF_INET; /* set family to Internet */
/* Check command-line argument for protocol port and extract */
/* port number if one is specified. Otherwise, use the default */
/* port value given by constant PROTOPORT */
if (argc > 2) { /* if protocol port specified */
port = atoi(argv[2]); /* convert to binary */
} else {
port = PROTOPORT; /* use default port number */
}
if (port > 0) /* test for legal value */
sad.sin_port = htons((u_short)port);
else { /* print error message and exit */
fprintf(stderr,"bad port number %s\n",argv[2]);
exit(1);
}
/* Check host argument and assign host name. */
if (argc > 1) {
host = argv[1]; /* if host argument specified */
} else {
host = localhost;
}
/* Convert host name to equivalent IP address and copy to sad. */
ptrh = gethostbyname(host);
if ( ((char *)ptrh) == NULL ) {
fprintf(stderr,"invalid host: %s\n", host);
exit(1);
}
memcpy(&sad.sin_addr, ptrh->h_addr, ptrh->h_length);
/* Map TCP transport protocol name to protocol number. */
if ( ((int)(ptrp = getprotobyname("tcp"))) == 0) {
fprintf(stderr, "cannot map \"tcp\" to protocol number");
exit(1);
}
/* Create a socket. */
sd = socket(PF_INET, SOCK_STREAM, ptrp->p_proto);
if (sd < 0) {
fprintf(stderr, "socket creation failed\n");
exit(1);
}
/* Connect the socket to the specified server. */
if (connect(sd, (struct sockaddr *)&sad, sizeof(sad)) < 0) {
fprintf(stderr,"connect failed\n");
exit(1);
}
/* Repeatedly read data from socket and write to user's screen. */
if ((ppwd = getpwuid(getuid())) == NULL) {
fprintf(stderr,"could not get user name\n");
exit(1);
}
/* send our user name */
send(sd, ppwd->pw_name, strlen(ppwd->pw_name)+1, 0);
n = recv(sd, buf, sizeof(buf), 0);
while (n > 0) {
write(1,buf,n);
n = recv(sd, buf, sizeof(buf), 0);
}
/* Close the socket. */
closesocket(sd);
/* Terminate the client program gracefully. */
exit(0);
}
/* adapted from Sample 25_2 from "Computer Networks and Internets, 2nd ed by Comer */
/* sockserver.c - code for example server program that uses TCP */
#ifndef unix
#define WIN32
#include <windows.h>
#include <winsock.h>
#else
#define closesocket close
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#endif
#include <stdio.h>
#include <string.h>
#define PROTOPORT 5193 /* default protocol port number */
#define QLEN 6 /* size of request queue */
int visits = 0; /* counts client connections */
/*------------------------------------------------------------------------
* Program: server
*
* Purpose: allocate a socket and then repeatedly execute the following:
* (1) wait for the next connection from a client
* (2) send a short message to the client
* (3) close the connection
* (4) go back to step (1)
*
* Syntax: server [ port ]
*
* port - protocol port number to use
*
* Note: The port argument is optional. If no port is specified,
* the server uses the default given by PROTOPORT.
*
*------------------------------------------------------------------------
*/
main(argc, argv)
int argc;
char *argv[];
{
struct hostent *ptrh; /* pointer to a host table entry */
struct protoent *ptrp; /* pointer to a protocol table entry */
struct sockaddr_in sad; /* structure to hold server's address */
struct sockaddr_in cad; /* structure to hold client's address */
int sd, sd2; /* socket descriptors */
int port; /* protocol port number */
int alen; /* length of address */
char buf[1000]; /* buffer for string the server sends */
char ubuf[100]; /* buffer for user string */
#ifdef WIN32
WSADATA wsaData;
WSAStartup(0x0101, &wsaData);
#endif
memset((char *)&sad,0,sizeof(sad)); /* clear sockaddr structure */
sad.sin_family = AF_INET; /* set family to Internet */
sad.sin_addr.s_addr = INADDR_ANY; /* set the local IP address */
/* Check command-line argument for protocol port and extract */
/* port number if one is specified. Otherwise, use the default */
/* port value given by constant PROTOPORT */
if (argc > 1) { /* if argument specified */
port = atoi(argv[1]); /* convert argument to binary */
} else {
port = PROTOPORT; /* use default port number */
}
if (port > 0) /* test for illegal value */
sad.sin_port = htons((u_short)port);
else { /* print error message and exit */
fprintf(stderr,"bad port number %s\n",argv[1]);
exit(1);
}
/* Map TCP transport protocol name to protocol number */
if ( ((int)(ptrp = getprotobyname("tcp"))) == 0) {
fprintf(stderr, "cannot map \"tcp\" to protocol number");
exit(1);
}
/* Create a socket */
sd = socket(PF_INET, SOCK_STREAM, ptrp->p_proto);
if (sd < 0) {
fprintf(stderr, "socket creation failed\n");
exit(1);
}
/* Bind a local address to the socket */
if (bind(sd, (struct sockaddr *)&sad, sizeof(sad)) < 0) {
fprintf(stderr,"bind failed\n");
exit(1);
}
/* Specify size of request queue */
if (listen(sd, QLEN) < 0) {
fprintf(stderr,"listen failed\n");
exit(1);
}
/* Main server loop - accept and handle requests */
while (1) {
alen = sizeof(cad);
if ( (sd2=accept(sd, (struct sockaddr *)&cad, &alen)) < 0) {
fprintf(stderr, "accept failed\n");
exit(1);
}
if (recv(sd2, ubuf, sizeof(ubuf), 0) > 0) {
visits++;
sprintf(buf, "Hello %s, you are visitor number %d to this server\n",
ubuf, visits);
send(sd2,buf,strlen(buf),0);
}
closesocket(sd2);
}
}