fscanf to a linked list - c

Question

I'm in trouble with my code again.
I want to fscanf result.txt to structures with linked list, but it don't work;
I think the simply linked list must be enought;
The problem is: the program just write the first line, but nothing else.
result.txt format:
point name (for examples)
623 john
457 peter
312 chuck
etc.
The code:
#include <stdio.h>
#include <stdlib.h>
typedef struct ranklist {
int point;
char* name;
struct ranklist *next;
} ranklist;
int how_many_records(FILE *fp){
char ch;
int line=0;
int status;
rewind(fp);
while((status=fscanf(fp, "%*d %*[^\n]%c", &ch))==1)
++line;
if(status != EOF){
++line;
}
rewind(fp);
return line;
}
int how_many_letter(FILE *fp){
int letter = 0;
long pos = ftell(fp);
//fscanf(fp, " %*[^\n]%n", &letter);
fscanf(fp, " %*s%n", &letter);
fseek(fp, pos, SEEK_SET);
return letter;
}
int main(void){
FILE *fp = fopen("result.txt","r");
int name_length;
int lines = how_many_records(fp);
ranklist *r = malloc(lines * sizeof(*r));
ranklist *first = r;
for ( r=first ;r != NULL; r = r->next){
fscanf(fp, "%d", &(r->point));
name_length = how_many_letter(fp);
r->name = malloc(name_length + 1);
fscanf(fp,"%s", r->name);
}
fclose(fp);
for ( r=first ;r != NULL; r = r->next){
printf("%d %s\n", r->point, r->name);
}
free(r);
return 0;
}

Answer 1

fscanf(fp, "%d", &r[y].point);
Here, y is uninitialized.
You need to put y = 0, or, IMO, better to use &(r->point) [and the same goes for r[y].name also]
Suggestion: for reading and parsing whole lines better to use fgets()

Answer 2

You have multiple problems with the creation of the list.
Lets start with the loop:
for ( r=first ;r != NULL; r = r->next){
Nowhere in the loop do you initialize r->next, so after the first iteration you will make y point to totally random memory, leading to undefined behavior.
As mentioned in another answer you don't initialize the variable y, which is another cause of undefined behavior.
You also change r, so r will not point to the original memory you allocated so doing r[y] is a third cause of undefined behavior.
And after the loop you call free with the modified pointer y, which will also lead to undefined behavior. And you don't free the names you allocate in the loop, so have multiple memory leaks as well.
There is no need to pre-allocate the nodes at all, just allocate when needed.
Something like
ranklist *head = NULL;
FILE *fp = fopen(...);
// Read the file and create the list
char buffer[256];
while (fgets(buffer, sizeof(buffer), fp) != NULL)
{
ranklist *node = malloc(sizeof(ranklist));
node->name = malloc(strlen(buffer)); // Will allocate a little more than needed
sscanf(buffer, "%d %s", &node->point, node->name);
node->next = head;
head = node;
}
// Print the list
for (ranklist *n = head; n != NULL; n = n->next)
{
printf("%d %s\n", n->point, n->name);
}
// Free the list
while (head != NULL)
{
// Unlink the first node in the list
ranklist *n = head;
head = n->next;
// And free it
free(n->name);
free(n);
}
The code above doesn't have any error checking, it also have some overhead when allocating space for the name, but it will be safe, it will not handle unrealistically long names, and the list will actually be a stack (last item read will be first in the list).
Function to get the length of the name from the buffer read with fgets:
size_t get_name_length(char *buffer)
{
// Since the buffer was read with `fgets` we need to get rid
// of the newline at the end, if it's there
size_t length = strlen(buffer);
if (buffer[length - 1] == '\n')
buffer[length - 1] = '\0'; // "Remove" by terminating the string
// Find the space dividing the point and the name
char *space = strchr(buffer, ' ');
// Just in case there are multiple whitespace characters in the string
while (*space != '\0' && isspace(*space))
++space;
// Now `space` points to the first non-space letter in the name
// (or to the string terminator, if there's no name
// Then length of the name is the remainder of the string
return strlen(space);
}

Answer 3

FILE *fp = fopen("result.txt","r");
int name_length;
//Just create a link if you use as an array
//int lines = how_many_records(fp);
//ranklist *r = malloc(lines * sizeof(*r));
ranklist *first=NULL, *curr, *r;
int point;
while(1==fscanf(fp, "%d", &point)){
r = malloc(sizeof(*r));//allocate one record
r->next = NULL;
r->point = point;
name_length = how_many_letter(fp);
r->name = malloc(name_length + 1);
fscanf(fp, "%s", r->name);
//make link
if(first == NULL)
first = curr = r;
else
curr = curr->next = r;
}
fclose(fp);
for (r = first; r != NULL; r = r->next){
printf("%d %s\n", r->point, r->name);
}
for (r = first; r != NULL; ){
ranklist *tmp = r->next;//Save next for free(r)
free(r->name);
free(r);
r = tmp;
}
//output the list
void print(ranklist *first){
while(first != NULL){
printf("%d %s\n", first->point, first->name);
first = first->next;
}
}