在分配链表并在 c 中为其赋值时出现分段错误

Question

I am trying to create a singly linked list and initialize it with the first given n integers.But i am getting segmentation fault whenever i run it.This is my code.

 typedef struct floatList{float fval;struct floatList * fnext;}node_f;


 node_f* FL_firstInts(int n){

        node_f *res=(node_f*)malloc(sizeof(node_f));
        res=res->fnext;

        for(int i=1;i<=n;i++){
            res->fval=i;
            res=res->fnext;
        }
        return res;
    }

void FL_show(struct floatList *list, char *label){
    int i=0;
    while(list->fnext!=NULL){
        printf("%d: %f\n",i,f->fval);           
        list=list->fnext;
        i++;
    }
}

And in order to test in the main function, i write the following

node_f *ten = FL_firstInts(10);
FL_show(ten,"10 first integers");

But as i run the program, i get segmentation fault,how do i fix it?

Answer 1

node_f *res=(node_f*)malloc(sizeof(node_f));
res=res->fnext;

The reason for your crash is that you're never initialising the res->fnext pointer.
So before accessing it set it to the actual next element in your list.

In general your code is a bit fuzzy.
You're allocating memory for one node_f , but you're actually trying to put n elements into it.

To allocate memory for the n elements just multiply the size of one element by n .

node_f *res= (node_f*) malloc(sizeof(node_f) * n);

Afterwards initialise the fnext pointers.

for(size_t index{0}; index < n - 1; index++)
  res[index].fnext = &res[index + 1];
res[n - 1].fnext = nullptr;

Answer 2

In the function FL_firstInts you allocated uninitialized object of the type node_f

node_f *res=(node_f*)malloc(sizeof(node_f));

So the following statement

res=res->fnext;

already invokes undefined behavior.

The function can be defined at least the following way

node_f * FL_firstInts( int n )
{
    node_f *head = NULL;
    node_f **current = &head;
`
    for ( int i = 0; i < n; i++ )
    {
        *current = malloc( sizeof( node_f ) );

        ( *current )->fval  = i;
        ( *current )->fnext = NULL;

        current = &( *current )->fnext;
    }

    return head;
}

Th function FL_show has the same bug and moreover the parameter label is not used.

The function can be defined like

void FL_show( const node_f *head, const char *label )
{
    if ( label ) puts( label );

    for ( int i = 0; list != NULL; list = list->fnext )
    {
        printf( "%d: %f\n", i, f->fval );           
        i++;
    }
}