A basic example of queue written in C (Arduino)

Basic_Queue_Example.c

// A basic example of queue written in C
// Useful for small microcontrollers
// Tags: Arduino | Atmega | AVR | STM32

#include <stdio.h>
#include <stdint.h>

// Maximum number of items in queue
#define QUEUE_MAX 5

// Add your custom properies (the prop "set" is required)
typedef struct QueueItem_s
{
    uint8_t value; // Custom property
    uint8_t set;
} QueueItem_t;

typedef struct Queue_s
{
    uint8_t last;
    uint8_t cons;
    QueueItem_t items[QUEUE_MAX];
} Queue_t;

// Returns 1 on success, otherwise if the queue is full returns 0
uint8_t queue_push(Queue_t *q, QueueItem_t i)
{
    if (q->last >= QUEUE_MAX)
    {
        q->last = 0;
    }

    if (q->items[q->last].set == 1)
    {
        return 0;
    }

    q->items[q->last++] = i;

    return 1;
}

// Returns next queue item, otherwire if the queue is empty then returns an (QueueItem_t){0, 0}
QueueItem_t queue_next(Queue_t *q)
{
    if (q->cons >= QUEUE_MAX)
    {
        q->cons = 0;
    }

    if (q->items[q->cons].set == 0)
    {
        return (QueueItem_t){0, 0};
    }

    QueueItem_t i = q->items[q->cons];

    q->items[q->cons].set = 0;
    q->cons++;

    return i;
}

void queue_clear(Queue_t *q)
{
    for (uint8_t i = 0; i < QUEUE_MAX; i++)
    {
        q->items[i].set = 0;
    }
    q->last = 0;
    q->cons = 0;
}

int main()
{
    // Declare queue instance
    Queue_t queue = {
        .last = 0,
        .cons = 0,
        .items = {}
    };

    // Queue testing
    uint8_t i;

    // Push 4 elements
    for (i = 0; i < 4; i++)
    {
        printf("push%d: %d\n", i, queue_push(&queue, (QueueItem_t){i * 10, 1}));
    }

    // Consume 4 elements
    for (i = 0; i < 4; i++)
    {
        printf("next%d: %d\n", i, queue_next(&queue).value);
    }

    // Push 6 elements (last element does not fit in queue)
    for (i = 0; i < 6; i++)
    {
        printf("push%d: %d\n", i, queue_push(&queue, (QueueItem_t){i * 10, 1}));
    }

    // Consume 6 elements (last element does not exists, on last element the queue will return empty element)
    for (i = 0; i < 6; i++)
    {
        printf("next%d: %d\n", i, queue_next(&queue).value);
    }

    // Output queue stats
    printf("\nstats\n");
    for (int si = 0; si < QUEUE_MAX; si++)
    {
        printf("%d: %d / %d\n", si, queue.items[si].value, queue.items[si].set);
    }
    // /Queue testing

    return 0;
}

Output.txt

push0: 1
push1: 1
push2: 1
push3: 1
next0: 0
next1: 10
next2: 20
next3: 30
push0: 1
push1: 1
push2: 1
push3: 1
push4: 1
push5: 0
next0: 0
next1: 10
next2: 20
next3: 30
next4: 40
next5: 0

stats
0: 10 / 0
1: 20 / 0
2: 30 / 0
3: 40 / 0
4: 0 / 0