simple patient managing program using queue - c

Question

I'm making simple patient managing program using circular queue but q.rear always have "0" value while executing exit_hos()
I thought that addq() makes variable "rear" different, but It doesn't work.
is_empty() always return front and rear is same.
I think I'm misunderstanding some codes and memory concepts.
how can I fix these functions?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_SIZE 50
#define MAX_QUEUE_SIZE 6
typedef struct {
char** value;
int front;
int rear;
} Queue;
void init_queue(Queue* q) {
q->value = (char**)malloc(sizeof(char*) * MAX_QUEUE_SIZE);
q->front = 0;
q->rear = 0;
}
int is_full(Queue* q) {
if (((q->rear +1) % MAX_QUEUE_SIZE) == q->front)
return 1;
else
return 0;
}
int is_empty(Queue* q) {
if (q->front == q->rear)
return 1;
else
return 0;
}
void addq(Queue* q, char* value) {
q->rear = (q->rear+1) % MAX_QUEUE_SIZE;
q->value[q->rear] = value;
printf("addq: %s", value);
return;
}
char* deleteq(Queue* q) {
q->front = (q->front + 1) % MAX_QUEUE_SIZE;
return q->value[q->front];
}
void arrive(Queue q) {
int input;
char name[MAX_SIZE];
printf("\n");
printf("1. submit\n");
printf("2. cancel\n");
scanf("%d", &input);
if (input == 1) {
if (is_full(&q) == 1) {
printf("Service is not available\n");
}
else {
printf("name: ");
scanf("%s", name);
addq(&q, name);
}
}
else if (input == 2) {
return;
}
else {
printf("input error\n");
return;
}
return;
}
void exit_hos(Queue q) {
char patient[MAX_SIZE];
if (is_empty(&q) == 1)
{
printf("There is no patient waiting\n");
}
else {
strcpy(patient, deleteq(&q));
printf("patient: %s", patient);
}
return;
}
int main() {
int input;
Queue q;
init_queue(&q);
while (1)
{
printf("\nINPUT\n");
printf("1. Arrive hostpital\n");
printf("2. Exit hospital\n");
printf("3. service exit\n");
scanf("%d", &input);
if (input == 1)
arrive(q);
else if (input == 2) {
exit_hos(q);
}
else if (input == 3) {
printf("exit\n");
return 0;
}
else {
printf("input error\n");
}
}
free(q.value);
return 0;
}

Answer 1

I think that this line is wrong:
q->value = (char**)malloc(sizeof(char*) * MAX_QUEUE_SIZE);
I think that it should be:
char * _value = (char*)malloc(sizeof(char*) * MAX_QUEUE_SIZE);
q->value = &_value;
malloc is going to return a pointer to a char array. q->value is a pointer to a pointer to a char array. So you want to set it to the address of the char array that malloc is created for you.
Change you init_queue code to this and it will work:
void init_queue(Queue* q) {
char * _value = (char*)malloc(sizeof(char*) * MAX_QUEUE_SIZE);
q->value = &_value;
q->front = 0;
q->rear = 0;
}
Output:
Chris#DESKTOP-BCMC1RF ~
$ ./main.exe
INPUT
1. Arrive hostpital
2. Exit hospital
3. service exit
1
1. submit
2. cancel
1
name: fred
addq: fred
INPUT
1. Arrive hostpital
2. Exit hospital
3. service exit
2

Answer 2

If you already have a max queue size and a max size, you are better off pre-allocating the whole thing as an array, reducing memory headaches. As a general rule, avoid headaches unless they provide a feature you want.
Note: This method of keeping track of and re-using memory is called a circular buffer (not to be confused with the linked list types that are more commonly called queues).
#define MAX_SIZE 50
#define MAX_QUEUE_SIZE 6
typedef struct {
char value [MAX_QUEUE_SIZE][MAX_SIZE + 1]; //+1 to hold extra null termination
unsigned int front;
unsigned int size; //size is a clearer than rear, which could have meant end item or end+1 and needed special empty queue handling
} Queue;
void init_queue(Queue* q) {
memset(q,0,sizeof(Queue)); //just zero it all
//more info on this and some situation-dependent alternatives https://stackoverflow.com/questions/11152160/initializing-a-struct-to-0
}
int is_full(const Queue* q) {
return q->size >= MAX_QUEUE_SIZE;
}
int is_empty(const Queue* q) {
return q->size == 0;
}
//sometimes called a push operation
//return 0 if failed
int addq(Queue* q, const char* value) {
//error check, abort, error handling section:
//full queue -> abort
if(is_full(q)) return 0;
//long value -> truncate handled via strncpy
//actual operation
const unsigned int destination = (q->front + q->size) % MAX_QUEUE_SIZE;
strncpy(q->value[destination],value,MAX_SIZE);
q->size = q->size + 1;
printf("addq: %s", q->value[destination]);
return q->size;
}
//sometimes called a pop operation
//return value may not persist if addq is called, but fine for your use of copying on call
const char* deleteq(Queue* q) {
if(is_empty(q)) return 0;
const char * retval = q->value[q->front];
q->front = (q->front + 1) % MAX_QUEUE_SIZE;
q->size = q->size - 1;
return retval;
}
also remember to use either MAX_SIZE + 1 or strncpy with MAX_SIZE - 1 since "No null-character is implicitly appended at the end of destination if source is longer than num."
(and strcpy and scanf as you sling them onto arrays is unsafe)