Segmentation fault: Implementing Priority Queues in C - c

Question

I'm trying to implement priority. A higher value of variable prior implies a lower priority in my code. Here it is:
#include <stdio.h>
#include <stdlib.h>
struct MinMax_PriorityQueue{
int ele, prior;
struct MinMax_PriorityQueue *next;
};
int isEmpty(struct MinMax_PriorityQueue **pq){
return ((*pq)==NULL);
}
int checkPriority(int p){
return(p>0);
}
void enqueue(struct MinMax_PriorityQueue **pq, int x, int p){
struct MinMax_PriorityQueue *temp=malloc(sizeof(*temp));
//struct MinMax_PriorityQueue *temp1=*pq;
if(!checkPriority(p)){
printf("Priority should be greater than 0");
return;
}
temp->ele=x;
temp->prior=p;
temp->next=NULL;
/*if(isEmpty(pq)){
*pq=temp;
}
while(temp1->next!=NULL){
temp1=temp1->next;
}*/
(*pq)->next=temp;
printf("The item %d with priority %d has been enqueued into the priority queue\n", x, p);
}
int maxPriority(struct MinMax_PriorityQueue **pq){
struct MinMax_PriorityQueue *temp=malloc(sizeof(*temp));
temp=*pq;
int maxp=temp->prior;
while(temp!=NULL){
if(temp->prior<=maxp)
maxp=temp->prior;
temp=temp->next;
}
return maxp;
}
void dequeue(struct MinMax_PriorityQueue **pq){
if(isEmpty(pq)){
printf("The priority queue is empty. No more elements can be removed!\n");
}
int maxp=maxPriority(pq);
struct MinMax_PriorityQueue *temp=*pq;
while(temp!=NULL){
if(temp->prior==maxp){
printf("The item %d with priority %d has been dequeued from the priority queue\n", temp->ele, temp->prior);
free(temp);
break;
}
temp=temp->next;
}
}
void minSearch(struct MinMax_PriorityQueue **pq){
struct MinMax_PriorityQueue *temp=malloc(sizeof(*temp));
temp=*pq;
int minp=0;
while(temp!=NULL){
if(temp->prior>=minp)
minp=temp->prior;
temp=temp->next;
}
temp=*pq;
while(temp!=NULL){
if(temp->prior==minp){
printf("The element %d has minimum priority\n", temp->ele);
}
temp=temp->next;
}
}
void maxSearch(struct MinMax_PriorityQueue **pq){
int maxp=maxPriority(pq);
struct MinMax_PriorityQueue *temp=*pq;
while(temp!=NULL){
if(temp->prior==maxp){
printf("The element %d has maximum priority\n", temp->ele);
}
temp=temp->next;
}
}
void display(struct MinMax_PriorityQueue *pq){
struct MinMax_PriorityQueue *temp=pq;
printf("The contents of the priority queue are:\n");
if(isEmpty(&temp)){
printf("Nothing to be shown, the priority queue is empty.\n");
return;
}
for(int i=0;temp!=NULL;temp=temp->next){
if(i){
printf(" ------ \n");
}
printf("| %d |\n", temp->ele);
i=1;
}
}
int main()
{
int choice, element, priority;
printf("LET'S START WITH AN EMPTY QUEUE\n\n");
struct MinMax_PriorityQueue *pq=malloc(sizeof(*pq));
pq=NULL;
while(1){
printf("\nMENU\n");
printf("----\n");
printf("\t1. Enqueue\n");
printf("\t2. Dequeue\n");
printf("\t3. Display queue\n");
printf("\t4. Search minimum priority\n");
printf("\t5. Search maximum priority\n");
printf("\t6. Exit\n");
printf("Enter your choice: ");
scanf("%d", &choice);
switch(choice){
case 1: printf("Enter the element to be enqueued: ");
scanf("%d", &element);
printf("Enter its priority: ");
scanf("%d", &priority);
enqueue(&pq, element, priority);
break;
case 2: dequeue(&pq);
break;
case 3: display(pq);
break;
case 4: minSearch(&pq);
break;
case 5: maxSearch(&pq);
break;
case 6: printf("Program terminated successfully!\n");
return 0;
default: printf("Invalid input");
}
}
}
Upon enqueuing, I find a segmentation fault in the line: (*pq)->next
in the enqueue() function. I can't wrap my head around why that is happening. Is it because I took an argument of type struct MinMax_PriorityQueue **?
Any help is appreciated.

Answer 1

(*pq)->next=temp; in the enqueue function causes segfault as *pq is NULL on the first call.
You shouldn't have commented out the check for NULL. You need it.. but like
if(isEmpty(pq)){
*pq=temp;
return; // Add this
}
BTW:
The reason the *pq is NULL on the first call of enqueue is this code in main
struct MinMax_PriorityQueue *pq=malloc(sizeof(*pq));
pq=NULL; // You set pq back to NULL (and leaks memory)
But you shouldn't do the malloc in main to start with. Simply do:
struct MinMax_PriorityQueue *pq=NULL; // Empty queue