C: Variable is uninitialized for linked list - c

Question

I'm relatively new to C and I was creating a program that involves a linked-list. Here is a very abbreviated version of the code that's giving me trouble.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define STRLEN 100
struct Gene {
int num[4];
struct Gene *next;
};
typedef struct Gene item;
void build_list(item *current, item *head, FILE *in);
int main() {
FILE *input;
FILE *output;
input = fopen("test.data", "r");
output = fopen("test.out", "w+");
item *curr;
item *head;
head = NULL;
int i;
build_list(curr, head, input);
curr = head;
while(curr) {
for (i = 0; i < 4; ++i)
fprintf(output, "%d\n", curr->num[i]);
curr = curr->next;
}
fclose(input);
fclose(output);
free(curr);
}
void build_list(item *current, item *head, FILE *in) {
char gene[STRLEN];
char *tok;
char gene_name[STRLEN];
char *search = ",";
int j;
while (fgets(gene, sizeof(gene), in)) {
current = (item *)malloc(sizeof(item));
tok = strtok(gene, search);
strcpy(gene_name, tok);
for (j = 0; j < 4; ++j) {
tok = strtok(NULL, search);
current->num[j] = atoi(tok);
}
current->next = head;
head = current;
}
}
When I try to compile this, it says variable curr is uninitialized, but even when I initialize it with malloc it throws a segmentation fault, or it prints out nothing at all. Why could this be?

Answer 1

C uses pass by value for function argument passing. So, when you call build_list(curr, head, input);, curr and head themselves are passed by value and any changes made to those variables (corresponding parameters) will not reflect back to the caller.
So, in the caller,
while(curr)
is accessing unitialized variable (meeory) which invokes undefined behavior.
If you need to change curr and head themselves, you need to pass their address and make chages inside the funtion. Something like
build_list(&curr, &head, input);
and
void build_list(item **current, item **head, FILE *in)
and
*current = malloc(sizeof(item));
may get the job done for you.

Answer 2

#Sourav Ghosh have already explained what was wrong with your code and also suggested one way to solve it. Here is another way.
Instead of passing current and head as variables to be changed inside the function (i.e. as pointer-to-pointer), I would recommend that you use the function return value. In that way you don't have to use pointer-to-pointer.
Something like:
item* add_item(item* head)
{
// Place a new item in front
item* current = malloc(sizeof(item));
current->next = head;
return current;
}
item* build_list(item* head, FILE *in) {
char gene[STRLEN];
char *tok;
char gene_name[STRLEN];
char *search = ",";
int j;
while (fgets(gene, sizeof(gene), in))
{
// Get a new item
head = add_item(head);
// Fill data into the new item
tok = strtok(gene, search);
strcpy(gene_name, tok);
for (j = 0; j < 4; ++j)
{
tok = strtok(NULL, search);
head->num[j] = atoi(tok);
}
}
return head;
}
and from maincall it like:
head = NULL;
head = build_list(head, input);
Note: For readability, I skipped all check for failing malloc. In real code you should always check whether malloc returned NULL.