problem with adding to a linked list

tommycahir

there prob is that everything is grand when i enter the first node but when i go to add a second the contents of the node that i had just added become the same as the one that i just created
this all happens when i use the line

strcpy(newnode->SiteName,site);

shown below is the code so far and the structure of the node any help would be graetly appreciated
tanx

struct vertexnode{

char* SiteName;
int XCoord;
int YCoord;
struct vertexnode *nextvertex;
struct arcnode *nextarc;
};


vertexnode* InsertNewVertex(vertexnode* graph)
{
int xcord,ycord;
char site[10];

clrscr();
vertexnode* newnode;
newnode = GetvertexNode(); // allocates memory
newnode->nextvertex = NULL;
newnode->nextarc = NULL;
printf("please enter the name of the site you wish to enter\n");
fflush(stdin);
gets(site);
strcpy(newnode->SiteName,site); // problem line
printf("please enter the x coord of the site you wish to enter\n");
scanf("%d",&xcord);
newnode->XCoord = xcord;
printf("please enter the y coord of the site you wish to enter\n");
scanf("%d",&ycord);
newnode->YCoord = ycord;
newnode->nextvertex = graph;
graph = newnode;
graph->nextvertex->SiteName;
graph->nextvertex->nextvertex->SiteName;
return(graph);

hussey

I edited your code and mine ran fine

see my code (in zip file)

compiled with borland 4.5

tommycahir

tanx hussey
that helped alot
could you tell me what the prob was though?

hussey

not really sure

could have been something simple like the way you called it or mem allocation etc..


you code looks ok to me

tommycahir

got the code from a lecturer and it had worked in my other projects as well so i don't think ythat it was that but here it is, this prob really bugging me cos i dont like when bugs happen and can't be explained


/* This function allocates memory for the node structure
and returns a pointer to this block of memory. This memory
is available until it is released using free() */

vertexnode *GetvertexNode(void)
{
vertexnode *pt ;
pt = (vertexnode *) malloc(sizeof(vertexnode) ) ;
if (pt == (vertexnode *) NULL )
{
printf("Failed to allocate memory\n") ;
exit(1) ;
}
return pt ;
}

called usin
head = InsertNewVertex(head);

with the following prototype
vertexnode* InsertNewVertex(vertexnode*);

and head is

vertexnode* head;

please some one help cos bugs really bug me...

i nearly sure it something to do with the strcpy ..

Zab

Where do you allocate memory for the string? SiteName is just a pointer, so when you use malloc all you are making is enough room for the pointer. You then have to point the pointer somewhere with some space for the string, or you could change it to an array of chars.

Also, your last few lines:
---
graph = newnode;
graph->nextvertex->SiteName;
graph->nextvertex->nextvertex->SiteName;
return(graph);
---
Don't do anything useful... why don't you just return newnode?

Zab.

tommycahir

graph->nextvertex->SiteName;
graph->nextvertex->nextvertex->SiteName;

were just put in so that i could put a watch on the sitename part of the node

i see now that i was trying to place an array of chars where there was only room for a pointer;

tanx for the tip!!

hussey

i see now that i was trying to place an array of chars where there was only room for a pointer;

an array of chars is a pointer!!!!!!!!!!!!!!!!!

so thats not your problem

have a look at my other code .. its the uses the same code except I put a main in it

deadl0ck

IN the original post, where you do the strcpy, where do you actually malloc the memory for newnode->SiteName ?

You're trying to copy into unallocated memory...or didi you just leave this bit out in the code you pasted in ?

tommycahir

deadl0ck - the malloc part of the code is shown in the third post and also all the other needeed code to figure this out..

HUSSEY - using the code that you gave me now will let you no if any probs but i still f**ked if i no what gone wrong.

deadl0ck

Are you talking about this:

vertexnode *GetvertexNode(void)
{
vertexnode *pt ;
pt = (vertexnode *) malloc(sizeof(vertexnode) ) ;
if (pt == (vertexnode *) NULL )
{
printf("Failed to allocate memory\n") ;
exit(1) ;
}
return pt ;
}

This only allocates memory for the Vertex node structure structure, you need to allocate

Each pointer in this need to be alloced as well:

char* SiteName;
struct vertexnode *nextvertex;
struct arcnode *nextarc;

So - in this you also need:

SiteName = (char *)malloc(SIZE * sizeof(char));

where SIZE is however big you want it (Perhaps 11 in you case as you're copying in a string of Size 10 + 1 for the '\0')

etc...

tommycahir

got it working perfectly by changing the char * in the struct to an array of 11 and there no probs now tanx guys for all the help was greatly appreciated

deadl0ck

So it was the memory issue then ?

Talliesin

Originally posted by hussey
an array of chars is a pointer!!!!!!!!!!!!!!!!!

Not quite. Most operations on an array of chars is really an operation on a pointer, and when you pass an array you really pass a pointer.
However when you are allocating you are allocating with the actual bunch of chars, rather than your way of addressing them, so in that context there is quite a difference.

tommycahir

yea totally messed up with my pointer allocation.. damn pointers are wrecking my head at the mo for the three projects that i doing
aghhh.