How to get number of elements in array of structs

Question

I have an array of sruct sockaddr_in:

struct sockaddr_in saddrlist[MAX] = {0};

the array is also being updated from external sources with memcpy, so thats the point where i dont know anymore about the count. and currNumOfelements.

Now at some point, i want to check how many elements are in the array. What I tried:

printf("before: %d",getsize(saddrlist));
memcpy(&myotherlist, saddrlist, sizeof(saddrlist) * MAX);
printf("after: %d",getsize(saddrlist));

Which results in:

before: 52448
after:  52448

int getsize(struct sockaddr_in saddrlist[10])
{
    uint8_t numOfElements = 0;
    for (size_t i = 0; i < MAX; i++)
        if (saddrlist[i] != 0)
            numOfElements++;
}

This obviously doesn't work because if (saddrlist[i] != 0) is an invalid comparison.. but how do i do it then? I get so confused using c..

If your array is updated from an external source, then you must ensure that this external source updates currNumOfElements too, or that it provides some other mechanism to determine whether it changed the number of valid elements. — Andreas Wenzel
– Andreas Wenzel, Commented Apr 24, 2022 at 20:43

Klompengard · Accepted Answer · 2022-04-25 00:48:47Z

1

You can use your variable currNumOfElements to determine how many elements are in the array. Because it's a static array, saddrlist will always have MAX elements in it. You get to decide what's a 'valid' configuration for your struct, and what's 'invalid'.

edited Apr 25, 2022 at 0:48

answered Apr 24, 2022 at 20:33

Klompengard

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2 Comments

codedor Over a year ago

check the edit, my bad

Klompengard Over a year ago

I checked your edit, and my answer should still suffice. Good luck!

Serge Ballesta · Accepted Answer · 2022-04-24 21:26:02Z

0

You could try to memcmp the addresses to a zero initialized one:

int getsize(struct sockaddr_in *saddrlist)
{
    int numOfElements = 0;
    struct sockaddr_in zero = { 0 };  // declare a local "null" sockaddr_in
    for (size_t i = 0; i < MAX; i++) {
        if (memcmp(saddrlist + i, &zero, sizeof(zero)) != 0)
            numOfElements++;
        else break;                   // stop at first "null" address
    }
    return numOfElements;
}

answered Apr 24, 2022 at 21:26

Serge Ballesta

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