Alloc fd

450 /*
451  * allocate a file descriptor, mark it busy.
452  */
453 int __alloc_fd(struct files_struct *files,  //struct files_struct : open file table structure
454                unsigned start, unsigned end, unsigned flags)
455 {
456         unsigned int fd;
457         int error;
458         struct fdtable *fdt;
459 
460         spin_lock(&files->file_lock);
461 repeat:
462         fdt = files_fdtable(files);
463         fd = start;
464         if (fd < files->next_fd)
465                 fd = files->next_fd;
466 
467         if (fd < fdt->max_fds)
468                 fd = find_next_zero_bit(fdt->open_fds, fdt->max_fds, fd);
469 
470         /*
471          * N.B. For clone tasks sharing a files structure, this test
472          * will limit the total number of files that can be opened.
473          */
474         error = -EMFILE;
475         if (fd >= end)
476                 goto out;
477 
478         error = expand_files(files, fd);
479         if (error < 0)
480                 goto out;
481 
482         /*
483          * If we needed to expand the fs array we
484          * might have blocked - try again.
485          */
486         if (error)
487                 goto repeat;
488 
489         if (start <= files->next_fd)
490                 files->next_fd = fd + 1;
491 
492         __set_open_fd(fd, fdt);
493         if (flags & O_CLOEXEC)
494                 __set_close_on_exec(fd, fdt);
495         else
496                 __clear_close_on_exec(fd, fdt);
497         error = fd;
498 #if 1
499         /* Sanity check */
500         if (rcu_dereference_raw(fdt->fd[fd]) != NULL) {
501                 printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
502                 rcu_assign_pointer(fdt->fd[fd], NULL);
503         }
504 #endif
505 
506 out:
507         spin_unlock(&files->file_lock);
508         return error;
509 }