CODEWITHSUNDEEP
c++linked-listruntime-errordynamic-memory-allocation
Matteo
Matteo

Reputation: 47

getting a run time error in dynamic memory allocation

The program is supposed to allocate memory as an operating system would do.

This is the main function

int main (int argc, char* argv[]){
  string command;

 if(argc==2)
      command = argv[1];

 else cout<<"Not enough commands"<<endl;

 if (command.compare("best"))
     cout<<"Using best fit algorithm"<<endl;

 if (command.compare("worst"))
     cout<<"Using worst fit algorithm"<<endl;

 cout<<"\t1.Add Program\n";
 cout<<"\t2.Kill Program\n";
 cout<<"\t3.Fragmentation\n";
 cout<<"\t4.Print Memory\n";
 cout<<"\t5.Exit\n";

 LinkedList Memory;
 Memory.createMemory();

 int choice;
 cin>>choice;
 cout<<"choice - "<<choice<<endl;

 if  (choice==1){
     string programName;
     cin>>programName;
     cout<<"Program name - "<<programName<<endl;
     int si;
     cin>>si;
     cout<<"Program size (KB) - "<<si<<endl;
     Memory.addProgram(si, programName);
 }
 if  (choice==2){
     string programName;
     cin>>programName;
     cout<<"Program name - "<<programName<<endl;
     Memory.killProgram(programName);
 }
 if  (choice==4){
     Memory.print();
 }
 if  (choice==5){
     return 1;
}
return 0;
}

And this is the linkedlist class with its functions

 class LinkedList{
 private:
    struct node{
      string name;
      node *next;
    };
    typedef struct node * nodePointer;
    nodePointer head;
 public:
    void createMemory();
    void addProgram(int val, string s);
    void killProgram(string s1);
    void print();
    void fragmentation();
 LinkedList(){head=NULL;}
 };
 void LinkedList::createMemory(){
 int i=0;
 node* temp=head;
 while(i<32){
    temp->name="Free";
    temp=temp->next;
    i++;
 }
 };

 void LinkedList::addProgram(int val, string s){
 int i=0;
 node* temp=head;
 while(temp->name!="Free")
    temp=temp->next;
 while(temp->name=="Free"){
    while (i<val){
      temp->name=s;
      temp=temp->next;
      i++;
 }
 }
 };

 void LinkedList::killProgram(string s){
    node* temp=head;
 while(temp->name!=s)
    temp=temp->next;
 while(temp->name==s)
    temp->name="Free";
 };

 void LinkedList::print(){
 node*temp=head;
 int i=0;
 while(i<32){
  cout<<temp->name<<"\t";
  temp=temp->next;
    if ((i+1)%8==0){
       cout<<endl;
 }
 i++;
 }
 };

Whenever I call one of the class functions I get a run time error and i don't understand why

Upvotes: 1

Views: 436

Answers (3)

Jiahao Cai
Jiahao Cai

Reputation: 1250

When you invoke Memory.createMemory();, this program will crash. Because you didn't allocate memory for temp in the function createMemory(), temp points to NULL, so temp->name="Free";will crash your program. You should use new to allocate memory for temp. You should try this site to help you.

BTW, the way you treat the command parameter is not appropriate, what if the user input more than 2 parameters, and you tell the user there is Not enough commands.

if(argc==2)
  command = argv[1];
else cout<<"Not enough commands"<<endl;

Upvotes: 0

E.D.
E.D.

Reputation: 667

LinkedList memory;
memory.createMemory();

This code is probably the source of your error. memory doesn't have any value yet, so when you try to access the function "createMemory" inside memory, you get a null pointer error.
In general, try to give more information when you ask a question. This question would be much clearer if you added what error you were getting, what line it occurs at, the error message, or similar things.

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409266

You have the member LinkedList::head which you initialize to a null pointer.

Then in LinkedList::createMemory you do node* temp=head which makes temp a null pointer.

Lastly in the loop in createMemory you do temp->name="Free" which dereferences the null pointer and lead to undefined behavior and a very likely crash.

If you want 32 preallocated nodes in the list, then you should actually allocate memory for those nodes.

Upvotes: 1

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