C Pointers Segmentation Fault - c

Question

I'm having this strange segmentation fault. I'm trying to find if a patients id already exists in the list of patients using pointers. I think the code in question is:
#include <stdio.h>
#include <stdlib.h>
#include "health.h"
void addPatient(int patientID) {
printf("here1");
Chart* patients_chart;
// find the patient with id
patients_chart = patientList;
while(patients_chart == NULL || patients_chart->id == patientID) {
patients_chart = patients_chart->next;
printf("here2");
}
printf("here3");
// if patient wasn't found, add new patient
if (patients_chart == NULL) {
Chart *new_chart;
printf("here4");
// allocate and initialize new patient
new_chart = (Chart*)malloc(sizeof(Chart));
new_chart->id = patientID;
new_chart->buffer = NULL;
// insert new patient into list
new_chart->next = patientList;
patientList = new_chart;
printf("here5");
}
}
The included health.h is just method declarations and structs. I will list them below, but please note that my assignment restricts me from modifying any of the code in health.h. I will also post my code at the very end.
/*
* Patient's health chart: ID + linked list of health type readings
*/
typedef struct chartEntry* Chartptr; /* pointer to a Chart */
typedef struct chartEntry{
int id; /* patient ID */
CBuffptr buffer; /* pointer to first health type buffer */
Chartptr next; /* pointer to next patient */
}Chart;
extern Chartptr patientList;
I call the function in main with input like this one: 1,12:12:12,7,0
The 7 is the "command"
the 1 is the patient id in question
You can ignore the rest.
I understand how to find the patient, but I'm getting this annoying seg fault. Thank you for your time!

Answer 1

The following code is buggy:
while(patients_chart == NULL || patients_chart->id == patientID) {
patients_chart = patients_chart->next;
printf("here2");
}
You are advancing for as long as either the pointer is NULL or the pointer matches the patient ID. You're missing a negation there. Instead, use:
while(patients_chart != NULL && patients_chart->id != patientID) {
patients_chart = patients_chart->next;
printf("here2");
}

Answer 2

while(patients_chart == NULL || patients_chart->id == patientID) {
patients_chart = patients_chart->next;
}
Here if Condition 1 (patients_chart == NULL) is true, then you do this:
patients_chart = patients_chart->next;
which is Null Pointer Dereferencing, thus causing Seg Fault.