Question

i use pointer for holding name and research lab property. But when i print the existing Vertex ,when i print the vertex, i cant see so -called attributes properly.
For example though real value of name is “lancelot” , i see it as wrong such as “asdasdasdasd”

struct vertex {
                int value;
                char*name;
                char* researchLab;
                struct vertex *next;
                struct edge *list;
};
    void GRAPHinsertV(Graph G, int value,char*name,char*researchLab) {
    //create new  Vertex.
        Vertex newV = malloc(sizeof newV);
        // set  value of new variable  to which belongs the person.
        newV->value = value;
        newV->name=name;
        newV->researchLab=researchLab;
        newV->next = G->head;
        newV->list = NULL;
        G->head = newV;
        G->V++;
    }

    /***
    The method   creates new person.
    **/
    void createNewPerson(Graph G) {
        int id;
        char name[30];
        char researchLab[30];
        // get requeired variables.
        printf("Enter id of the person to be added.\n");
        scanf("%d",&id);
        printf("Enter name of the person to be added.\n");
        scanf("%s",name);
        printf("Enter researc lab of the person to  be added\n");
        scanf("%s",researchLab);
        // insert the people to the social network.
        GRAPHinsertV(G,id,name,researchLab);
    }
    void ListAllPeople(Graph G)
    {
        Vertex tmp;
        Edge list;
        for(tmp = G->head;tmp!=NULL;tmp=tmp->next)
        {
            fprintf(stdout,"V:%d\t%s\t%s\n",tmp->value,tmp->name,tmp->researchLab);

        }
        system("pause");
    }

Answer 1

When you do this:

   newV->name=name;
   newV->researchLab=researchLab;

You are copying the pointer to the strings name and researchLab. You are not copying the strings themselves. In other words, after this, newV->name and name point to exactly the same location in memory where the name is stored; you have not created a duplicate copy of the data.

Since you then proceed to overwrite the name array in the createNewPerson function, at the end of this function, all of your vertex structs will have their name attribute pointing to the same memory location, which is only storing the last name entered.

Worse, when createNewPerson returns, its local name array goes out of scope, and is re-used for other things. Since your vertex structs are still pointing here for their name attributes, this is how you get garbage.

You need to duplicate the string. A simple way to do it is:

newV->name = strdup(name);

You will need to #include <string.h> to get the strdup library function.

And then you also need to make sure that you call free on the name attribute whenever you are disposing of a vertex structure.