Why am I getting a segmentation fault when using scanf("%lf", GPA) if I am using a pointer for GPA, and scanf("%lf", &GPA) if it is not a pointer? - c

Question

void fillQueueWithStudents(Queue *q)
{
int counter = 1;
char answer = 'Y';
//this signifies the current variables that
//are being dealt with
char *currName;
double GPA;
student stud;
while((answer != 'N') || (answer != 'n'))
{
printf("Please enter the info for student %i \n",counter);
//Gets the name of the current student being entered
printf("Name: ");
scanf("%s", currName);
printf("\n");
//Gets the gpa of the current student being entered
printf("GPA: ");
scanf("%lf", &GPA);
//somewhere here is the problem
printf("\n");
//makes our student have the current variables
stud.name = currName;
stud.gpa = GPA;
//enqueues this student
enQueue(q,stud);
//increments to the next student
counter++;
//This sees if the user wants to enter another student
printf("Do you want to enter another student. Enter 'N' or 'n' for
no. \n");
scanf("%c", &answer);
printf("\n");
}
}
I printed hello before and after the scanf and they both printed. It does not let me enter a value for GPA. I am wondering if it is scanning the entire newline or something like that. I do not understand why it will not let me put in a value. I am guessing it is trying to put too much into the GPA variable.
Below is the complete program.
#include <stdlib.h>
#include <stdio.h>
typedef struct
{
char *name;
float gpa;
}student;
//This struct will represent a node in the queue
typedef struct Node
{
student *stud;
struct Node *next;
}QNode;
//This struct will identify the queue
//It will store the front and rear values to make
//it easier to enqueue and dequeue
typedef struct
{
QNode *front, *rear;
}Queue;
/*
This creates a new node with some data k.
It then sets its next value to NULL.
Then it returns the new node
Note: This node is not yet known to the queue. I will
use a separate function to do that
*/
QNode* newNode (student *stud)
{
QNode *temp = (QNode*)malloc(sizeof(QNode*));
temp->stud->gpa = stud->gpa;
temp->stud->name = stud->name;
temp->next = NULL;
return temp;
}
//Creats a empty queue
Queue* createQueue()
{
Queue *q = (Queue*)malloc(sizeof(Queue*));
q->front = q->rear = NULL;
return q;
}
/*
Uses the newNode function to make a new node in our linked list.
Then it puts that node in the correct place in the queue. This
is at the back
*/
void enQueue(Queue *q, student stud)
{
QNode *temp = newNode(&stud);//this creats a new node
/*
Checks if queue is empty, and if it is sets
our new node temp to rear and front. Then it
ends the function
*/
if(q->rear == NULL)
{
q->front = q->rear = temp;
return;
}
//sets the current rears next to our new node temp
q->rear->next = temp;
//this makes temp the new rear
q->rear = temp;
}
/*
This function serves/dequeues a node.
As always nodes are dequeued from the front of the
queue
*/
QNode *deQueue(Queue *q)
{
//if the queue is empty tell the user by
//returning NULL
if(q->rear == NULL)
{
return NULL;
}
/*
I will return front to the user in case they want
to use the data included in it. I have to save
front first to do that. So I save it hear
*/
QNode *temp = q->front;
//Now I make front equal to its next
q->front = q->front->next;
/*
This makes rear NULL if front is NULL
This means the list is empty and we have dequeued the last
node.
*/
if(q->front == NULL)
{
q->rear == NULL;
}
printf("The element dequeued was %s \n",temp->stud->name);
//This returns the node we have dequeued
return temp;
}
void printQueue(Queue *q)
{
//Check is the queue is empty and if it is stop the
//function
if(q->rear == NULL)
{
return;
}
/*
I will now print out each node out one by one.
To do this I create a QNode pointer and make it equal to the
front. I will then print all contents of front
and make temp equal to the next print and repeat this
till the end.
*/
QNode *temp = q->front;
while(temp != NULL)
{
int counter = 1;
printf("Student %i: Name: %s GPA: %f \n",
counter, temp->stud->name, temp->stud->gpa);
//increment the counter to display next student
counter++;
//Get the next node
temp = temp->next;
}
}
/*
This function keeps adding students to the queue until the
user enters 'N' that signifies they are done
*/
void fillQueueWithStudents(Queue *q)
{
int counter = 1;
char answer = 'Y';
//this signifies the current variables that
//are being dealt with
char *currName;
float GPA;
student stud;
while((answer != 'N') || (answer != 'n'))
{
printf("Please enter the info for student %i \n",counter);
//Gets the name of the current student being entered
printf("Name: ");
scanf("%s", currName);
printf("\n");
//Gets the gpa of the current student being entered
printf("GPA: ");
scanf("%f", &GPA);
printf("\n");
//makes our student have the current variables
stud.name = currName;
stud.gpa = GPA;
//enqueues this student
enQueue(q,stud);
//increments to the next student
counter++;
//This sees if the user wants to enter another student
printf("Do you want to enter another student. Enter 'N' or 'n' for no. \n");
scanf("%c", &answer);
printf("\n");
}
}
int main()
{
Queue *q = createQueue();
fillQueueWithStudents(q);
QNode *temp;
do {
temp = deQueue(q);
free(temp);
printQueue(q);
} while(temp != NULL);
return 0;
}

Answer 1

This:
scanf("%s", currName);
is undefined behavior, since currName is not initialized. Using scanf() with plain %s to read in a name is not a good idea. Use a buffer (!) and then tell scanf() the buffer size:
char currName[128];
if(scanf("%127s", currName) == 1)
{
}
... and check the return value, as shown.
Note that %s stops at the first whitespace character, so you won't be able to read a name containing spaces.