creating a list of nodes with C - c

Question

I want to createe a list of nodes that links to each other, I use head to remember the address of the first node, prev to link, and newdata to get the input, but it turns out that my head prev and newdata are all in the same address, can someone help me with this plz
typedef struct node
{
void* stdPtr;
struct node* link;
}NODE;
NODE* createNode(void* std)
{
NODE* nodePtr;
nodePtr=(NODE*)malloc(sizeof(NODE));
nodePtr->stdPtr=std;
nodePtr->link=NULL;
return nodePtr;
}
typedef NODE* nodePtr;
int main(void)
{
FILE* fin;
fin=fopen("input.txt","r");
//define
int i=0;
intPtr ID,grade;
STD* stdinfo;
nodePtr head=NULL;
nodePtr prev=(nodePtr)malloc(sizeof(NODE));
//nodePtr newdata=(nodePtr)malloc(sizeof(NODE));
prev=head;
//malloc space to ptr
ID=(intPtr)malloc(sizeof(int));
grade=(intPtr)malloc(sizeof(int));
stdinfo=(STD*)malloc(sizeof(STD));
//read student data and compare
while(fscanf(fin,"%d%d",&(stdinfo->ID),&(stdinfo->grade))!=EOF)
{
nodePtr newdata=(nodePtr)malloc(sizeof(NODE));
newdata=createNode(stdinfo);
if (prev==NULL)
{
prev=newdata;
head=prev;//為什麼只有在這裡=prev後面會跑掉？
}
printf("%d %d\n",*(int*)head,*(int*)prev);
if(prev->link!=NULL)
{
prev=prev->link;
}
prev->link=newdata;
}
fclose(fin);
return 0;
}

Answer 1

Important parts are missing (types, input.txt) to run your program which makes it harder to help you.
You create an instance of STD prior to the loop, then overwrite whatever data you store in it on each iteration and store the address of that one instance in each node. You need to create an instance of both NODE and STD per loop iteration (or change the definition of NODE so it can hold the STD data instead of a pointer to STD).
You use this pattern a couple of times which allocate data, then immediately leak the data by overwriting the pointer:
nodePtr prev=(nodePtr)malloc(sizeof(NODE));
prev=head;
...
nodePtr newdata=(nodePtr)malloc(sizeof(NODE));
newdata=createNode(stdinfo);
Here is more straight forward version to get you going:
#include <stdio.h>
#include <stdlib.h>
typedef struct node {
void *stdPtr;
struct node* link;
} NODE;
NODE* createNode(void *std, NODE *prev) {
NODE *nodePtr = malloc(sizeof(NODE));
nodePtr->stdPtr=std;
nodePtr->link=prev;
return nodePtr;
}
void printNodes(NODE *tail) {
for(; tail; tail = tail->link) {
printf("%s\n", (const char *) tail->stdPtr);
}
}
int main(void) {
NODE *tail = NULL;
tail = createNode("hello", tail);
tail = createNode("world", tail);
printNodes(tail);
}