Function fdgrowtable

freebsd/kern/kern_descrip.c:1796–1890

* Grow the file table to accommodate (at least) nfd descriptors. */

 * Grow the file table to accommodate (at least) nfd descriptors.
 */
static void
fdgrowtable(struct filedesc *fdp, int nfd)
{
	struct filedesc0 *fdp0;
	struct freetable *ft;
	struct fdescenttbl *ntable;
	struct fdescenttbl *otable;
	int nnfiles, onfiles;
	NDSLOTTYPE *nmap, *omap;

	KASSERT(fdp->fd_nfiles > 0, ("zero-length file table"));

	/* save old values */
	onfiles = fdp->fd_nfiles;
	otable = fdp->fd_files;
	omap = fdp->fd_map;

	/* compute the size of the new table */
	nnfiles = NDSLOTS(nfd) * NDENTRIES; /* round up */
	if (nnfiles <= onfiles)
		/* the table is already large enough */
		return;

	/*
	 * Allocate a new table.  We need enough space for the number of
	 * entries, file entries themselves and the struct freetable we will use
	 * when we decommission the table and place it on the freelist.
	 * We place the struct freetable in the middle so we don't have
	 * to worry about padding.
	 */
	ntable = malloc(offsetof(struct fdescenttbl, fdt_ofiles) +
	    nnfiles * sizeof(ntable->fdt_ofiles[0]) +
	    sizeof(struct freetable),
	    M_FILEDESC, M_ZERO | M_WAITOK);
	/* copy the old data */
	ntable->fdt_nfiles = nnfiles;
	memcpy(ntable->fdt_ofiles, otable->fdt_ofiles,
	    onfiles * sizeof(ntable->fdt_ofiles[0]));

	/*
	 * Allocate a new map only if the old is not large enough.  It will
	 * grow at a slower rate than the table as it can map more
	 * entries than the table can hold.
	 */
	if (NDSLOTS(nnfiles) > NDSLOTS(onfiles)) {
		nmap = malloc(NDSLOTS(nnfiles) * NDSLOTSIZE, M_FILEDESC,
		    M_ZERO | M_WAITOK);
		/* copy over the old data and update the pointer */
		memcpy(nmap, omap, NDSLOTS(onfiles) * sizeof(*omap));
		fdp->fd_map = nmap;
	}

	/*
	 * Make sure that ntable is correctly initialized before we replace
	 * fd_files poiner. Otherwise fget_unlocked() may see inconsistent
	 * data.
	 */
	atomic_store_rel_ptr((volatile void *)&fdp->fd_files, (uintptr_t)ntable);