Creating a FIFO queue in c

Question

I am trying to code a FIFO queue which takes data in and once full releases the oldest piece of data to make room for the new data.

I am new at programming but have managed to come up with the following code:

int Q[size], f=0, r=-1;

int Qfull()
{
    if (r==size) return 1;
    return 0;
}

int Qinsert()
{
    if(Qfull())
    {
        elem=Q[f];
        f=f+1;
        return elem;
        r++;
        Q[r]=SPI1BUF;

    }
    else
    {
        r++;
        Q[r]=SPI1BUF;
    }
}

The problem i am having is that this does not shift data and will fail once the array is full due to r increasing past the array size. Is there any way to solve this?

Since you are learning so a direct code may not be the best answer for you. I suggest you look at the link above and/or ask more direct question on specific problems with your code :) Your QFull size check is wrong and therefore it will crash as soon as it fills up. — Imran Saeed
– Imran Saeed, Commented Nov 4, 2013 at 16:08
If your queue is of a fixed size, you might want to consider using a ring buffer. Have two pointers, one to the first element, and one to where the next should go. When you remove an element, just increment the "last element" pointer; if it's then equal to Q+size, set it to Q. — cHao
– cHao, Commented Nov 4, 2013 at 16:10
(You could do it with array indexes too, and that could be easier. Then you could just say next = (next + 1) % size;.) — cHao
– cHao, Commented Nov 4, 2013 at 16:13

Community · Accepted Answer · 2017-05-23 12:21:33Z

1

What Dan said, and you really can't put statements after a return; they don't get executed.

edited May 23, 2017 at 12:21

CommunityBot

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answered Nov 4, 2013 at 16:08

ThorMJ

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Comments

Dillon Welch · Accepted Answer · 2013-11-04 16:40:59Z

0

You are trying to make a cyclic queue without doing the extra steps to make that work. You either need to do that (as mentioned by cHao) or look into using a linked list. I would recommend working towards the cyclic queue as it should not require much modification.

Also, you have the same two lines in both of your if/else clauses. You should be able to move those two lines out and save yourself that if clause. I'm not 100% on your logic, so make sure that it can go before the if statement.

int Qinsert()
{
    r++;
    Q[r]=SPI1BUF;
    if(Qfull())
    {
    //...
    }
}

answered Nov 4, 2013 at 16:40

Dillon Welch

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