how to pass list in functions in C - c

Question

I am new here and I need some help. I want to learn how can I pass a NULL list to a function fill it and then returning it to my main function.
FOR INSTANCE:
#include <cstdio>
typedef struct empNode {
char amka[12];
char first_name[30];
char last_name[30];
int year; //etos proslhpshs
float salary;
int num_of_children;
Child *children;
struct empNode *next;
} empNode;
void load_employees_from_file(empNode **employees);
//===========================================================
int main(void) {
empNode *employees = NULL;
load_employees_from_file(&employees);
while (employees != NULL) {
printf("%s ", employees->amka);
employees = employees->next;
}
}
//===========================================================
void load_employees_from_file(empNode **employees) {
FILE * fp;
int num_of_employees;
int i;
fp = fopen("employees.txt", "r");
if (fp == NULL) {
printf("Something went wrong, try again.\n");
return;
}
// here we read the first line of file to see how employee we have
fscanf(fp, "%d ", &num_of_employees);
while (num_of_employees) {
*employees = (empNode*) malloc(sizeof (empNode));
fscanf(fp, "%s ", (*employees)->amka);
fscanf(fp, "%s ", (*employees)->first_name);
fscanf(fp, "%s ", (*employees)->last_name);
fscanf(fp, "%d ", &(*employees)->year);
fscanf(fp, "%f ", &(*employees)->salary);
fscanf(fp, "%d\n", &(*employees)->num_of_children);
if ((*employees)->num_of_children > 0) {
(*employees)->children = (Child*) malloc(((*employees)->num_of_children) * sizeof (Child));
for (i = 0; i < (*employees)->num_of_children; i++) {
fscanf(fp, "%s ", (*employees)->children[i].fname);
strcpy((*employees)->children[i].lname, (*employees)->last_name);
fscanf(fp, "%d\n", &(*employees)->children[i].year_of_birth);
}
}
(*employees)->next = (empNode*) malloc(sizeof (empNode));
*employees = (*employees)->next;
num_of_employees--;
}
fclose(fp);
}
I get an error when I am running the while in my main function, more specifically my program crashes.
-------------------------------------------------------------------
ok let mu put it more clearly, after corrections i dont have problem with my list in the function the problem is that my list cant transfer to my main function. A clear example of this this:
#include<stdio.h>
#include<stdlib.h>
typedef struct node
{
int data;
struct node *next;
}node;
void read_int(node **nn);
int main(void)
{
node *nn=NULL;
read_int(&nn);
printf("%d", nn->data);
return 0;
}
void read_int(node **nn)
{
FILE *fp;
fp=fopen("test.txt", "r");
fscanf(fp, "%d", (*nn)->data);
fclose(fp);
}
here is exactly the same problem and more easier to understund, the test.txt files contains just the number 2. but i cant print it in my main.

Answer 1

In your function void load_employees_from_file(empNode **employees)
Change :
(*employees)->next = (empNode*) malloc(sizeof (empNode));
to
(*employees)->next = NULL;
as immediately after that *employees becomes next and the while loop starts from the beginning where memory is dynamically allocated:
*employees = (empNode*) malloc(sizeof (empNode));
and then populated.
Otherwise, if this was the last employee, the next element remains the NULL pointer.
In this way, your main's while loop termination condition will be true when the last next element (pointing to NULL) is reached.

Answer 2

Skimming through your code, there seems to be several problems. You should learn to use a debugger, which would let you step through the code line-by-line during execution and observe the actual behavior.
Here are some obvious issues I think you should look at:
What do you think the value of *employees will be when the while loop exits? Specifically, think about this statement: *employees = (*employees)->next;
I assume you want this list to be NULL-terminated (otherwise, the while (employees != NULL) in your main function wouldn't make sense). In that case, where do you set the NULL terminator value? You don't seem to ever set the next field to NULL anywhere...
Why do you call malloc twice in the body of your loop? You seem to be allocating two different blocks of memory to represent the same employee.
I think #1 above is the reason that your program is crashing right now. (But if you fix that issue, your program will probably still crash due to one of the other issues.)
Right now, on the last iteration of your loop, you do this:
(*employees)->next = (empNode*) malloc(sizeof (empNode));
*employees = (*employees)->next;
This means that *employees (which points to employees from main) will always end up pointing to an uninitialized empNode struct. Since the memory is uninitialized, the next pointer will be garbage. That's definitely going to cause some problems.