int initialize_descr(int newfd, struct descr *p)
{
    struct descr *d;
    d = descr_list;
    if ( d != NULL ) 
    {
        // Okay, you're trying to find the end of the list.
        while ( d->next != NULL)
        {
            d = d->next;
        }

        // Since the while loop ended, we know d->next == NULL.
        // So you're setting d=NULL, making the whole while loop pointless.
        d = d->next;
    }    
 

    // except it was pointless anyway, because the first thing you do
    // with 'd' once you've found the end of the list is trash it,
    // making it point to whatever malloc returns instead.
    if(!(d = malloc( sizeof(struct descr))))
    {
        nonfatal("malloc failure");
        return 0;
    }
    d->newfd = newfd;
    d->state = 1;
    d->next = NULL;

    // This does not do what you think it does.  You are setting the local variable p
    // to the address stored in d.  This does absolutely nothing to the memory
    // at that address.  It also does absolutely nothing to the variable that was
    // fed into this function.
    p = d;
    // No default return value, so you have no idea what it actually returns here
}

struct descr *initialize_descr(int newfd)
{
    struct descr *d=descr_list;

    if(d == NULL)
    {
      d=malloc(sizeof(struct descr));       // Empty list, so allocate new space
      descr_list=d; // list is no longer empty
    }
    else
    {    // Find the end of the list
        while(d->next != NULL) d=d->next;
        // Allocate new memory and add it to ->next, NOT d
        d->next=malloc(sizeof(struct descr));
        if(d->next == NULL) return(NULL); // alloc error
        // It is now in the list.
        // Set 'd' to the new memory so we can set it below.
        d=d->next;
    }

    if(d == NULL) return(NULL); // alloc error

    // Either way the first 'if' goes, 'd' is now memory we want to initialize.
    d->newfd = newfd;
    d->state = 1;
    d->next = NULL; // It's the end of the list
    return(d); // return a pointer to the new memory
}

...

struct descr *d=initialize_descr(fd);
...