_archive_write_header(struct archive *_a, struct archive_entry *entry)
{
	struct archive_write *a = (struct archive_write *)_a;
	int ret, r2;

	archive_check_magic(&a->archive, ARCHIVE_WRITE_MAGIC,
	    ARCHIVE_STATE_DATA | ARCHIVE_STATE_HEADER, "archive_write_header");
	archive_clear_error(&a->archive);

	if (a->format_write_header == NULL) {
		archive_set_error(&(a->archive), -1,
		    "Format must be set before you can write to an archive.");
		a->archive.state = ARCHIVE_STATE_FATAL;
		return (ARCHIVE_FATAL);
	}

	/* In particular, "retry" and "fatal" get returned immediately. */
	ret = archive_write_finish_entry(&a->archive);
	if (ret == ARCHIVE_FATAL) {
		a->archive.state = ARCHIVE_STATE_FATAL;
		return (ARCHIVE_FATAL);
	}
	if (ret < ARCHIVE_OK && ret != ARCHIVE_WARN)
		return (ret);

	if (a->skip_file_set &&
	    archive_entry_dev_is_set(entry) &&
	    archive_entry_ino_is_set(entry) &&
	    archive_entry_dev(entry) == (dev_t)a->skip_file_dev &&
	    archive_entry_ino64(entry) == a->skip_file_ino) {
		archive_set_error(&a->archive, 0,
		    "Can't add archive to itself");
		return (ARCHIVE_FAILED);
	}

	/* Format and write header. */
	r2 = ((a->format_write_header)(a, entry));
	if (r2 == ARCHIVE_FATAL) {
		a->archive.state = ARCHIVE_STATE_FATAL;
		return (ARCHIVE_FATAL);
	}
	if (r2 < ret)
		ret = r2;

	a->archive.state = ARCHIVE_STATE_DATA;
	return (ret);
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.