"Program received signal SIGSEGV, Segmentation fault." error message from gdb debugger

Question

Something in the while loop is giving me this error. I can't figure out what to look up because this error seems way to common to find out what to do with my specific example

#include <stdlib.h>
#include <stdio.h>
/*Structure declarations*/
struct Data {
    int id_number;
    float ratings;
};
typedef struct Node{
    struct Data player;
    struct Node *next;
}Node;

/*function declaration*/
void push(struct Node *list_head, int unique_id, float grade);
int main(){
    /* Initialize list_head to Null since list is empty at first */
    Node *list_head = NULL;     
    Node *traversalPtr;

    push(list_head, 1, 4.0);
    push(list_head, 2, 3.87);
    push(list_head, 3, 3.60);

    traversalPtr = list_head;
    while(traversalPtr -> next != NULL){
        printf("%d\n",traversalPtr -> player.id_number);
        traversalPtr = traversalPtr -> next;
    }   
}

...function declarations

Welcome! Please include the definition of push in your question. As @VladfromMoscow says, you have a problem with your push function declaration/definition. — John
– John, Commented Sep 30, 2019 at 14:51
Just to point out, push is not necessarily calling itself because the definition of push has not been provided, just the declaration. — Rob Streeting
– Rob Streeting, Commented Sep 30, 2019 at 14:52
We really need the definition of push() to conclusively answer this. @CodakBlack, would it be possible to edit this into your question? — Rob Streeting
– Rob Streeting, Commented Sep 30, 2019 at 14:55
If you already run your program in a debugger and get this error, you should see exactly where you got that error. There you can inspect your variables and check for expected vs real values. — Gerhardh
– Gerhardh, Commented Sep 30, 2019 at 16:00

Vlad from Moscow · Accepted Answer · 2019-09-30 14:56:57Z

The problem is that the function

void push(struct Node *list_head, int unique_id, float grade);

deals with copies of original pointers defined in main because the pointers are passed by values.

You should declare the function like

void push(struct Node **list_head, int unique_id, float grade);

and call it like

push( &list_head, 1, 4.0 );

Here is an example of how the function can be defined (I assume that the function appends nodes to its tail).

int push(struct Node **list_head, int unique_id, float grade)
{
    struct Node *node = malloc( sizeof( struct Node ) );
    int success = node != NULL;

    if ( success )
    {
        node->player.id_number = unique_id;
        node->player.ratings   = grade; 
        node->next = NULL;

        while ( *list_head ) list_head = &( *list_head )->next;

        *list_head = node;
    }

    return success; 
}

Also this loop

traversalPtr = list_head;
while(traversalPtr -> next != NULL){
    printf("%d\n",traversalPtr -> player.id_number);
    traversalPtr = traversalPtr -> next;
}

is incorrect. It should look like

traversalPtr = list_head;
while(traversalPtr != NULL){
    printf("%d\n",traversalPtr -> player.id_number);
    traversalPtr = traversalPtr -> next;
}

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