Unix Network Programming The socket struct and data handling System - - PDF document

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Unix Network Programming The socket struct and data handling System - - PDF document

Mar 14, 2023 104 likes •295 views

Introduction to Computer Networks Polly Huang EE NTU http://cc.ee.ntu.edu.tw/~phuang phuang@cc.ee.ntu.edu.tw Unix Network Programming The socket struct and data handling System calls Based on Beej's Guide to Network Programming 1 The Unix

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Introduction to Computer Networks

Polly Huang EE NTU http://cc.ee.ntu.edu.tw/~phuang phuang@cc.ee.ntu.edu.tw

Unix Network Programming

The socket

struct and data handling

System calls

Based on Beej's Guide to Network Programming

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The Unix Socket

  • A file descriptor really
  • A file descriptor really
  • The Unix fact

– When Unix programs do any sort of I/O, they do it by reading or writing to a file descriptor – A file descriptor is simply an integer associated

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with an open file

A File Descriptor

  • A file in Unix can be
  • A file in Unix can be

– A network connection – A FIFO queue – A pipe – A terminal

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– A real on-the-disk file – Or just about anything else

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Jeez, everything in Unix is a file!

Well, we know how to handle files!

  • In theory
  • In theory

– The read() and write() calls allows to communicate through a socket

  • In practice

– The send() and recv() offer much greater

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control over your data transmission

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The structs

  • int

int

– For the file descriptor

  • struct sockaddr

– Space holder for “types” of addresses

  • struct sockaddr_in

– Specific for the “Internet” type

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– _in for Internet

  • struct in_addr

– 4 byte IP address

struct sockaddr

struct sockaddr { struct sockaddr { unsigned short sa_family; // address family, AF_xxx char sa_data[14]; // 14 bytes of protocol address };

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}

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struct sockaddr_in

struct sockaddr in { struct sockaddr_in { short int sin_family; // Address family unsigned, AF_INET short int sin_port; // Port number, in network byte order struct in_addr sin_addr; // Internet address in network byte order

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// Internet address, in network byte order unsigned char sin_zero[8]; // Same size as struct sockaddr };

Struct in_addr

struct in addr { struct in_addr { // Internet address (a structure for historical reasons) unsigned long s_addr; // that's a 32-bit long, or 4 bytes };

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Reference

  • Let ina be of type struct sockaddr in
  • Let ina be of type struct sockaddr_in
  • ina.sin_addr.s_addr references the 4-byte

IP address in network byte order

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Types of Byte Ordering

  • Network Byte Order
  • Network Byte Order

– Most significant byte first – Need conversion from the app program to the network

  • Host Byte Order

– Least significant byte first – Usually no need in app program

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– Usually no need in app program – But need conversion if data coming from the network

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Functions to Convert

  • htons()

htons()

– Host to Network Short

  • htonl()

– Host to Network Long

  • ntohs()

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– Network to Host Short

  • ntohl()

– Network to Host Long

Storing the IP address

ina sin addr s addr = inet addr("10 12 110 57”); ina.sin_addr.s_addr inet_addr( 10.12.110.57 );

  • Returns “–1” on error
  • For unsigned short it’s 255.255.255.255

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  • A broadcast address
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A Cleaner Interface

  • #include <sys/socket h>

#include <sys/socket.h>

  • #include <netinet/in.h>
  • #include <arpa/inet.h>
  • int inet_aton(const char *cp, struct in_addr *inp);

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An Example

struct sockaddr in my addr; struct sockaddr_in my_addr; my_addr.sin_family = AF_INET;

// host byte order

my_addr.sin_port = htons(MYPORT);

// short, network byte order

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inet_aton("10.12.110.57", &(my_addr.sin_addr)); memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct

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Things to Note

  • inet addr() and inet aton() both convert
  • inet_addr() and inet_aton() both convert

IP addresses into the network byte order

  • Not all platforms implement inet_aton()

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Get the IP Address Back

  • printf("%s" inet ntoa(ina sin addr));
  • printf( %s , inet_ntoa(ina.sin_addr));
  • inet_ntoa() returns a pointer to a char*
  • And…

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Use strcpy()

char *a1, *a2; . . a1 = inet_ntoa(ina1.sin_addr); // this is 192.168.4.14 a2 = inet_ntoa(ina2.sin_addr); // this is 10.12.110.57 printf("address 1: %s\n",a1); printf("address 2: %s\n" a2);

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printf( address 2: %s\n ,a2); This program will print: address 1: 10.12.110.57 address 2: 10.12.110.57

System Calls

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socket() Creating the File Descriptor

#include <sys/types h> #include <sys/types.h> #include <sys/socket.h> int socket(int domain, int type, int protocol); domain: AF_INET

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_ type: SOCK_STREAM or SOCK_DGRAM protocol: 0 or getprotobyname()

bind() Associating Port with the FD

  • #include <sys/types h>

#include <sys/types.h>

  • #include <sys/socket.h>
  • int bind(int sockfd, struct sockaddr *my_addr, int

addrlen);

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Example (Typical Server)

#include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define MYPORT 3490 main() { int sockfd; struct sockaddr_in my_addr; sockfd = socket(AF_INET, SOCK_STREAM, 0); // do some error checking! my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order my_addr.sin_addr.s_addr = inet_addr("10.12.110.57");

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memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct // don't forget your error checking for bind(): bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); . . .

connect() Making a Connection

  • #include <sys/types h>

#include <sys/types.h>

  • #include <sys/socket.h>
  • int connect(int sockfd, struct sockaddr *serv_addr, int

addrlen);

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Example (Typical Client)

#include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define DEST_IP "10.12.110.57" #define DEST_PORT 23 main() { int sockfd; struct sockaddr_in dest_addr; // will hold the destination addr sockfd = socket(AF_INET, SOCK_STREAM, 0); // do some error checking! dest_addr.sin_family = AF_INET; // host byte order dest_addr.sin_port = htons(DEST_PORT); // short, network byte order d t dd i dd dd i t dd (DEST IP)

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dest_addr.sin_addr.s_addr = inet_addr(DEST_IP); memset(&(dest_addr.sin_zero), '\0', 8); // zero the rest of the struct // don't forget to error check the connect()! connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr)); . . .

listen() Waiting for Connection

#include <sys/socket h> #include <sys/socket.h> int listen(int sockfd, int backlog); On the server side, you see typically this:

socket();

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bind(); listen(); /* accept() goes here */

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accept() Getting a Connection

#include <sys/socket h> #include <sys/socket.h> int accept(int sockfd, void *addr, int *addrlen);

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The Server Example

#include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define MYPORT 3490 // the port users will be connecting to #define BACKLOG 10 // how many pending connections queue will hold main() { int sockfd, new_fd; // listen on sock_fd, new connection on new_fd struct sockaddr_in my_addr; // my address information struct sockaddr_in their_addr; // connector's address information int sin_size; sockfd = socket(AF_INET, SOCK_STREAM, 0); // do some error checking! my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order my addr.sin addr.s addr = INADDR ANY; // auto-fill with my IP Polly Huang, NTU EE 28 y_ _ _ _ ; y memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct // don't forget your error checking for these calls: bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); listen(sockfd, BACKLOG); sin_size = sizeof(struct sockaddr_in); new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size); . . .

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send() and recv() Data Transmission

int send(int sockfd, const void *msg, int len, int flags); int recv(int sockfd, void *buf, int len, unsigned int flags);

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Example

char *msg = “Hello World!"; char msg Hello World! ; int len, bytes_sent; . . len = strlen(msg); bytes_sent = send(sockfd, msg, len, 0); . . .

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sendto() and recvfrom() Transmission the Datagram Style

int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen); int recvfrom(int sockfd, void *buf, int len, unsigned int flags, struct sockaddr *from, int *fromlen);

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Or if transmitting over TCP socket,

  • ne can simply use send() and recv().

close() and shutdown() Closing the Communication

close(sockfd); int shutdown(int sockfd, int how);

  • 0 -- Further receives are disallowed
  • 1 -- Further sends are disallowed

2 F th d d i di ll d (lik l ())

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  • 2 -- Further sends and receives are disallowed (like close())
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Reference

  • Beej's Guide to Network Programming
  • Beej s Guide to Network Programming

– http://www.ecst.csuchico.edu/~beej/guide/net/

  • Additional system calls
  • TCP stream client, server example
  • UDP datagram example

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  • UDP datagram example