Socket programming – well commented examples

"It will overflow to filler[3]"

Not with any correct C compiler, it wont. Here's a simple test program I ran on Linux:

    #include <stdio.h>

    int main()
    {
      char filler[16] = {0};
      int i;
      unsigned short s1 = 0xFFFF;
      unsigned long  l1 = 0xFFFFFFFF;

      filler[0] = (unsigned short)s1;
      filler[2] = (unsigned short)s1;
      filler[4] = (unsigned long)l1;

      for (i = 0; i < 8; i++)
        printf("filler[%d] = %d\n", i, filler[i]);
    }

And when you run it, all the elements of filler that haven't been explicitly assigned to are still 0, even though the unsigned shorts and longs that were assigned to them have '1' in all their bits:

    $ gcc filler.c
    $ ./a.out
    filler[0] = -1
    filler[1] = 0
    filler[2] = -1
    filler[3] = 0
    filler[4] = -1
    filler[5] = 0
    filler[6] = 0
    filler[7] = 0

Since htons() swaps the bytes in its result, you should just be able to assign the value it returns directly:

    struct sockaddr_in serv_addr;
    ...
    serv_addr.sin_port = htons(port);

(See, for example: https://en.wikibooks.org/wiki/C_Programming/Networking_in_UN...)