Question

Question from Operating Systems

Proc,Forks,Exec

Examine the code given and do changes as mentioned in the steps below:

#include "HALmod.h"

int GetCommand (string tokens [])
{
string commandLine;
bool commandEntered;
int tokenCount;

do
{
cout << "HALshell> ";
while (1)
{
getline (cin, commandLine);
commandEntered = CheckForCommand ();
if (commandEntered)
{
break;
}

}
} while (commandLine.length () == 0);

tokenCount = TokenizeCommandLine (tokens, commandLine);

return tokenCount;
}

int TokenizeCommandLine (string tokens [], string commandLine)
{
char *token [MAX_COMMAND_LINE_ARGUMENTS];
char *workCommandLine = new char [commandLine.length () + 1];
int i;
int tokenCount;

for (i = 0; i < MAX_COMMAND_LINE_ARGUMENTS; i ++)
{
tokens [i] = "";
}
strcpy (workCommandLine, commandLine.c_str ());
i = 0;
if ((token [i] = strtok (workCommandLine, " ")) != NULL)
{
i ++;
while ((token [i] = strtok (NULL, " ")) != NULL)
{
i ++;
}
}
tokenCount = i;

for (i = 0; i < tokenCount; i ++)
{
tokens [i] = token [i];
}

delete [] workCommandLine;

return tokenCount;
}

//Do not touch the below function
bool CheckForCommand ()
{
if (cullProcess)
{
cullProcess = 0;
cin.clear ();
cout << "\b\b \b\b";
return false;
}

return true;
}

  

int ProcessCommand (string tokens [], int tokenCount)
{
if (tokens [0] == "shutdown" || tokens [0] == "restart" || tokens [0] == "lo")
{
   if (tokenCount > 1)
   {
        cout << "HALshell: " << tokens [0] << " does not require any arguments" << endl;
   return 1;
   }
   cout << endl;
   cout << "HALshell: terminating ..." << endl;

return 0;
  
}
else
   return 1;
}
char ** ConvertToC (string tokens [], int tokenCount)
{
char ** words;
words = (char **) malloc (sizeof (char*) * tokenCount);

for (int i=0; i<tokenCount; i++)
{
words[i]=strdup(tokens[i].c_str());
}
return words;
}

Step 1- Modify the "ConvertToC" function ( last function) as mentioned:

1. allocate an extra "word" for NULL (Hint: tokenCount+1)
2. store the value of NULL into the extra "word"

Step 2- Modify the "ProcessCommand" function ( second last function) as mentioned:

You will be emulating a shell. First, the shell checks for specific commands ("shutdown", "lo", etc). If it does not recognize those commands then, a fork occurs and the child will execute the command as typed.

The algorithm to implement is as follows (Hint: add code inside the else):

1. fork to create a child and parent

2. the child will do the following:
Call the ConverToC function and capture the return into a char** variable
Call execvp sending it two arguments: the first element (or word) of the char** variable, and the entire array
Print an error message
exit

3. the parent will do the following:
wait for the child
return 1

4. don't forget to handle the error case of the fork

//Dont try to run the program, just do the following steps.

Answer 1

#include "HALmod.h"

int GetCommand (string tokens [])
{
string commandLine;
bool commandEntered;
int tokenCount;

do
{
cout << "HALshell> ";
while (1)
{
getline (cin, commandLine);
commandEntered = CheckForCommand ();
if (commandEntered)
{
break;
}

}
} while (commandLine.length () == 0);

tokenCount = TokenizeCommandLine (tokens, commandLine);

return tokenCount;
}

int TokenizeCommandLine (string tokens [], string commandLine)
{
char *token [MAX_COMMAND_LINE_ARGUMENTS];
char *workCommandLine = new char [commandLine.length () + 1];
int i;
int tokenCount;

for (i = 0; i < MAX_COMMAND_LINE_ARGUMENTS; i ++)
{
tokens [i] = "";
}
strcpy (workCommandLine, commandLine.c_str ());
i = 0;
if ((token [i] = strtok (workCommandLine, " ")) != NULL)
{
i ++;
while ((token [i] = strtok (NULL, " ")) != NULL)
{
i ++;
}
}
tokenCount = i;

for (i = 0; i < tokenCount; i ++)
{
tokens [i] = token [i];
}

delete [] workCommandLine;

return tokenCount;
}

//Do not touch the below function
bool CheckForCommand ()
{
if (cullProcess)
{
cullProcess = 0;
cin.clear ();
cout << "\b\b \b\b";
return false;
}

return true;
}

  

int ProcessCommand (string tokens [], int tokenCount)
{

if (tokens [0] == "shutdown" || tokens [0] == "restart" || tokens [0] == "lo")
{
if (tokenCount > 1)
{
cout << "HALshell: " << tokens [0] << " does not require any arguments" << endl;
return 1;
}
cout << endl;
cout << "HALshell: terminating ..." << endl;

return 0;
  
}
else{
char ** va;
pid_t pid;
pid = fork();
if (pid == 0) {
va=ConvertToC();
execvp(*word, words);
printf("Error\n");
}

  
else{
wait(NULL);
}
return 1;
}
}
char ** ConvertToC (string tokens [], int tokenCount)
{
char ** words;
char ** word
words = (char **) malloc (sizeof (char*) * tokenCount);
word = (char **) malloc (sizeof (char*) * tokenCount+1);
word=NULL;
for (int i=0; i<tokenCount; i++)
{
words[i]=strdup(tokens[i].c_str());
}
return words;
}