Pointer Assignment Issue in C code

Question

int rtadd(IPaddr net, IPaddr mask, IPaddr gw, unsigned metric)
{
    struct route *prt, *srt, *prev;
    srt= malloc(sizeof(struct route));
    bool isdup;
    int hv = 0, i = 0;

    if (!Route.ri_valid)
        rtinit();

    prt = rtnew(net, mask, gw, metric);

    prt->rt_key = 0;
    for (i - 0; i < 32; ++i)
        prt->rt_key += (mask >> i) & 1;

    prev = NULL;
    hv = rthash(net);
    isdup = false;
    for (srt = rttable[hv]; srt; srt = srt->rt_next) //trouble point (issue with srt = rttable[hv])?
    {
        if (prt->rt_key > srt->rt_key) 
        {
            break;
        }
        if (srt->rt_net == prt->rt_net && srt->rt_mask == prt->rt_mask)
        {
            isdup = true;
            break;
        }
        prev = srt;
    }

    return 0;
}

In the above code I created a pointer srt and used a malloc command to initialize it. When I assign it to the array rttable which is declared as struct route *rttable[RT_TSIZE];, the srt pointer becomes NULL. What could be causing this?

The code which is being done for a project in my networking class is based off of Douglas Comer's TCP/IP volume II book:

https://www.amazon.com/Internetworking-TCP-Vol-Implementation-Internals/dp/0139738436

srt->rt_next (which is assigned to srt) eventually becomes null, probably. — rom1v
– rom1v, Commented Jan 31, 2017 at 21:40
See I thought this as well, except after testing, the srt pointer becomes NULL after directly assigning it to rttable[hv] — Wakester
– Wakester, Commented Jan 31, 2017 at 21:45
I don't think so as I initialize rttable here void rtinit(void) { int i; for (i = 0; i < RT_TSIZE; ++i) rttable[i] = 0; Route.ri_bpool = malloc(sizeof(struct route)); Route.ri_valid = true; Route.ri_default = NULL; } — Wakester
– Wakester, Commented Jan 31, 2017 at 21:47
You initialize rttable, but you don't initialize rttable[hv] (now you posted the code, in fact, you explicitly initialize all items to 0). — rom1v
– rom1v, Commented Jan 31, 2017 at 21:48

J. Laderoute · Accepted Answer · 2017-01-31 21:51:43Z

1

The malloc is now a memory leak since you are not actually using that allocated memory address. In your for loop, you are re-assigning your src pointer to a different location in memory by pointing it to rttable[hv]. So, if you examined rttable[hv] you will most likely find it's value set to 0 or null.

answered Jan 31, 2017 at 21:51

J. Laderoute

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