hw1.c

this gives details of the network programming

//
// This code can be used to test your layer 4 implementation
//   (which in turn should use your layer 3 implementation, etc.)
//
// Included is a set of functions that can be used as a layer 1
//   implementation, and a main program.
//
// Question, suggestions, etc. should be sent to [e-mail]

#include <stdio.h>

//------------------------------------------------------------------
// Sample layer 1 implementation - this can be used to
// provide half-duplex communication by using the shell to
// create a pipe between a sender process and a receiver process.

// sample l1_read just calls read on stdin
int l1_read( char *b) {
  return(read(0,b,1));
}

// sample l1_write just calls write to stdout
int l1_write(char b) {
  return(write(1,&b,1));
}

//------------------------------------------------------------------
// layer 4 prototypes

int l4_write(char *, int , char *, int );
int l4_read(char *, int *, char *, int *);


//------------------------------------------------------------------
// Main program. This program can be a sender or a receiver,
//  it depends on how many command line arguments are supplied
//  when the program is run. 

int main(int argc,char **argv) {

  char namebuf[101];
  char valuebuf[101];
  int namelen;
  int valuelen;

  // If there are 2 command line arguments then this program will send
  //   the first (argv[1]) as the username and the second as the password
  //   It uses l4_write to send each as a named value...
  // If there are not 2 command line arguments, the program assumes it should
  //   be a reader, so it calls l4_read to get the named values.

  if (argc!=3) {

    // I'm a reader - read the first named value

    // IMPORTANT: l4_read requires that namelen and valuelen have values that
    //    indicate how large the buffers namebuf and valuebuf are. They must
    //    be initialized before calling l4_read.

    namelen = 100;
    valuelen = 100;

    if (l4_read(namebuf,&namelen,valuebuf,&valuelen)==-1) {
      fprintf(stderr,"Reading error\n");
      exit(1);
    } 

    // we expect C strings (with null termination), but make sure each is null terminated
    // (what if a bad guy sent the message?) before using as
    // a C string

    namebuf[namelen]='\0';
    valuebuf[valuelen]='\0';

    // Our application level protocol expects the first value to
    // be the username - make sure the name sent matches this.

    if (strcmp(namebuf,"username")!=0) {
      fprintf(stderr,"Error - first value received was not named \"username\"\n");
      exit(1);
    }

    // We got something named "username", print it out...

    printf("Username: %s\n",valuebuf);

    // now get the password

    // namelen and valuelen need to be re-initialized, otherwise they
    // will have the values that reflect what happened the last time we called
    // l4_read
    namelen = 100;
    valuelen = 100;

    if (l4_read(namebuf,&namelen,valuebuf,&valuelen)==-1) {
      fprintf(stderr,"Reading error\n");
      exit(1);
    } 

    // we expect a C strings - make sure each is null terminated
    // (what if a bad guy sent the message?) before using as
    // a C string

    namebuf[namelen]='\0';
    valuebuf[valuelen]='\0';

    // Our application level protocol expects this value to
    // be the password - make sure the name sent matches this.

    if (strcmp(namebuf,"password")!=0) {
      fprintf(stderr,"Error - secondt value received was not named \"password\"\n");
      exit(1);
    }


    // OK - We got something named "password", print it out...

    printf("Password: %s\n",valuebuf);

  } else {


    // I'm a writer (argv == 3)
    //  the first command line argument is the username, so send it
    // using l4_write. Make sure the username is not more than 100 bytes
    // (this is part of the application level protocol - usernames can't
    // be more than 100 bytes).

    if (strlen(argv[1])>100) {
      fprintf(stderr,"Error - username is too long\n");
      exit(1);
    }

    // length is OK - send it
    // NOTE: why add one to the lengths? This way the receiver is sent the
    // terminating null.
    if (l4_write("username",strlen("username")+1,argv[1],strlen(argv[1])+1)==-1) {
      // something went wrong when sending
      fprintf(stderr,"Error sending username\n");
      exit(1);
    }

    //  the second command line argument is the password, so send it
    // using l4_write. Make sure the password is not more than 100 bytes
    // (this is part of the application level protocol - passwords can't
    // be more than 100 bytes).

    if (strlen(argv[2])>100) {
      fprintf(stderr,"Error - password is too long\n");
      exit(1);
    }

    // length is OK - send it
    if (l4_write("password",strlen("password")+1,argv[2],strlen(argv[2])+1)==-1) {
      // something went wrong when sending
      fprintf(stderr,"Error sending password\n");
      exit(1);
    }
  }
  return(0);
}