scanf char* word in struct shows only the last input - c

Question

I am only allowed to use following headers
#include <stdio.h>
#include <stdlib.h>
and I defined my struct student as following:
struct dict
{
char* word;
struct dict* link;
};
There are many functions, but only one function that I have problem right now.
This function inserts a struct dict with certain name at the end of the link.
struct student *Linsert(struct dict *list, char *name)
{
struct student *pnew;
struct student *pn;
int exist = 1;
pnew = (struct dict *)malloc(sizeof(struct dict));
pnew -> next = NULL;
pnew -> name = name;
if (list != NULL)
{
for (pn = list; pn -> next != NULL; pn = pn -> next) ;
pn -> next = pnew;
}
else
list = pnew;
return list;
}
Using the following function,
//print all the values in the list
void printList(struct dict* list);
I did this:
int main(void)
{
struct dict *list = NULL;
char *name;
while (1) {
scanf("%s", name);
if (name == 'Q')
break;
list = Linsert(list, name);
printList(list);
}
return 0;
}
Lets say for input, I typed three
apple banana and orange, my result shows three of my last input.
What is the issue here?

Answer 1

I see two problems with your code:
You need to pass scanf a char array of size sufficient to store the input string, not simply a char pointer.
You need to copy strings passed into Linsert (use strdup).

Answer 2

Your main snippet alone has a whole bunch of problems:
name is an uninitialized pointer; it's pointing to some unknown location in memory that you haven't allocated and are allowed to use, therefore causing undefined behaviour. Perhaps you want char name[20] to allocate an array of 20 chars on the stack, and have scanf store the input in this buffer.
You're comparing a char* (a pointer to the start of a string) to a single char 'Q' - you're comparing a pointer and an integer value as your compiler warnings will tell you. You're not comparing the contents of the string for the value 'Q', you're comparing the memory address of name and the integer value for 'Q'. If you want to compare the string name with the string "Q", use strcmp and check for a return value of 0.
You also want to be making a copy of the variable passed to Linsert otherwise, as you've noticed, you'll be passing a pointer to the same location in memory every time, and a change to this block of memory will change each of your items.
If you turn your compiler warnings up you'll get even more warnings.

Answer 3

You are not allocating any memory for the name, so scanf is writing to some random location, and overwriting that every time through the loop.

Answer 4

One issue is that you've not allocated storage for the word member to point to. You have also not allocated space for name to point at. That is a principle cause of trouble.
You need to allocate space for name; the easiest way is:
char name[128];
You need to allocate space to store the word, and you need to copy the contents of name into the word so that when the next line overwrites name, it does not destroy the saved word.
Adapting your code, you might use:
struct student *Linsert(struct dict *list, char *name)
{
struct student *pnew;
struct student *pn;
pnew = (struct dict *)malloc(sizeof(struct dict));
if (pnew == 0)
...error...
pnew->next = NULL;
pnew->word = malloc(strlen(name) + 1);
if (pnew->word == 0)
...error...
strcpy(pnew->word, name);
if (list != NULL)
{
for (pn = list; pn->next != NULL; pn = pn->next)
;
pn->next = pnew;
}
else
list = pnew;
return list;
}
Do not omit the error checks on memory allocation - painful though it be. It will bite you when you forget.
Stylistically, do not use spaces around either -> or .; they are operators that bind very tightly, and they should not be spaced out like other binary operators.
There's a convenient function, strdup(), the duplicates a string, but it is not standard C (it is standard POSIX).

Answer 5

Since name is a char pointer, your assignment to the field of each dict struct will use the latest value it points to.