source: branches/minix3-book/servers/fs/filedes.c

/* This file contains the procedures that manipulate file descriptors.
 *
 * The entry points into this file are
 *   get_fd:    look for free file descriptor and free filp slots
 *   get_filp:  look up the filp entry for a given file descriptor
 *   find_filp: find a filp slot that points to a given inode
 */

#include "fs.h"
#include "file.h"
#include "fproc.h"
#include "inode.h"

/*===========================================================================*
 *                              get_fd                                       *
 *===========================================================================*/
PUBLIC int get_fd(int start, mode_t bits, int *k, struct filp **fpt)
{
/* Look for a free file descriptor and a free filp slot.  Fill in the mode word
 * in the latter, but don't claim either one yet, since the open() or creat()
 * may yet fail.
 */

  register struct filp *f;
  register int i;

  *k = -1;                      /* we need a way to tell if file desc found */

  /* Search the fproc fp_filp table for a free file descriptor. */
  for (i = start; i < OPEN_MAX; i++) {
        if (fp->fp_filp[i] == NIL_FILP) {
                /* A file descriptor has been located. */
                *k = i;
                break;
        }
  }

  /* Check to see if a file descriptor has been found. */
  if (*k < 0) return(EMFILE);   /* this is why we initialized k to -1 */

  /* Now that a file descriptor has been found, look for a free filp slot. */
  for (f = &filp[0]; f < &filp[NR_FILPS]; f++) {
        if (f->filp_count == 0) {
                f->filp_mode = bits;
                f->filp_pos = 0L;
                f->filp_selectors = 0;
                f->filp_select_ops = 0;
                f->filp_pipe_select_ops = 0;
                f->filp_flags = 0;
                *fpt = f;
                return(OK);
        }
  }

  /* If control passes here, the filp table must be full.  Report that back. */
  return(ENFILE);
}

/*===========================================================================*
 *                              get_filp                                     *
 *===========================================================================*/
PUBLIC struct filp *get_filp(fild)
int fild;                       /* file descriptor */
{
/* See if 'fild' refers to a valid file descr.  If so, return its filp ptr. */

  err_code = EBADF;
  if (fild < 0 || fild >= OPEN_MAX ) return(NIL_FILP);
  return(fp->fp_filp[fild]);    /* may also be NIL_FILP */
}

/*===========================================================================*
 *                              find_filp                                    *
 *===========================================================================*/
PUBLIC struct filp *find_filp(register struct inode *rip, mode_t bits)
{
/* Find a filp slot that refers to the inode 'rip' in a way as described
 * by the mode bit 'bits'. Used for determining whether somebody is still
 * interested in either end of a pipe.  Also used when opening a FIFO to
 * find partners to share a filp field with (to shared the file position).
 * Like 'get_fd' it performs its job by linear search through the filp table.
 */

  register struct filp *f;

  for (f = &filp[0]; f < &filp[NR_FILPS]; f++) {
        if (f->filp_count != 0 && f->filp_ino == rip && (f->filp_mode & bits)){
                return(f);
        }
  }

  /* If control passes here, the filp wasn't there.  Report that back. */
  return(NIL_FILP);
}