FILEDESC(9)            NetBSD Kernel Developer's Manual            FILEDESC(9)

     filedesc, dupfdopen, falloc, fd_getfile, fdalloc, fdcheckstd, fdclear,
     fdclone, fdcloseexec, fdcopy, fdexpand, fdfree, fdinit, fdrelease,
     fdremove, fdshare, fdunshare -- file descriptor tables and operations

     #include <sys/file.h>
     #include <sys/filedesc.h>

     falloc(struct lwp *l, struct file **resultfp, int *resultfd);

     struct file *
     fd_getfile(struct filedesc *fdp, int fd);

     dupfdopen(struct lwp *l, int indx, int dfd, int mode, int error);

     fdalloc(struct proc *p, int want, int *result);

     fdcheckstd(struct lwp *l);

     fdclear(struct lwp *l);

     fdclone(struct lwp *l, struct file *fp, int fd, int flag,
         const struct fileops *fops, void *data);

     fdcloseexec(struct lwp *l);

     struct filedesc *
     fdcopy(struct proc *p);

     fdexpand(struct proc *p);

     fdfree(struct lwp *l);

     struct filedesc *
     fdinit(struct proc *p);

     fdrelease(struct lwp *l, int fd);

     fdremove(struct filedesc *fdp, int fd);

     fdshare(struct proc *p1, struct proc *p2);

     fdunshare(struct lwp *l);

     For user processes, all I/O is done through file descriptors.  These file
     descriptors represent underlying objects supported by the kernel and are
     created by system calls specific to the type of object.  In NetBSD, six
     types of objects can be represented by file descriptors: data files,
     pipes, sockets, event queues, crypto, and miscellaneous.

     The kernel maintains a descriptor table for each process which is used to
     translate the external representation of a file descriptor into an inter-
     nal representation.  The file descriptor is merely an index into this ta-
     ble.  The file descriptor table maintains the following information:

        the number of descriptors allocated in the file descriptor table;
        approximate next free descriptor;
        a reference count on the file descriptor table; and
        an array of open file entries.

     On creation of the file descriptor table, a fixed number of file entries
     are created.  It is the responsibility of the file descriptor operations
     to expand the available number of entries if more are required.  Each
     file entry in the descriptor table contains the information necessary to
     access the underlying object and to maintain common information.  See
     file(9) for details of operations on the file entries.

     New file descriptors are generally allocated by falloc() and freed by
     fdrelease().  File entries are extracted from the file descriptor table
     by fd_getfile().  Most of the remaining functions in the interface are
     purpose specific and perform lower-level file descriptor operations.

     The following functions are high-level interface routines to access the
     file descriptor table for a process and its file entries.

     falloc(p, *resultfp, *resultfd)
              Create a new open file entry and allocate a file descriptor for
              process p.  This operation is performed by invoking fdalloc() to
              allocate the new file descriptor.  The credential on the file
              entry are inherited from process p.  The falloc() function is
              responsible for expanding the file descriptor table when neces-

              A pointer to the file entry is returned in *resultfp and the
              file descriptor is returned in *resultfd.  The falloc() function
              returns zero on success, otherwise an appropriate error is

     fd_getfile(fdp, fd)
              Get the file entry for file descriptor fd in the file descriptor
              table fdp.  The file entry is returned if it is valid and use-
              able, otherwise NULL is returned.

     dupfdopen(l, indx, dfd, mode, error)
              Duplicate file descriptor dfd for lwp l.

     The following functions operate on the file descriptor table for a

     fdalloc(p, want, *result)
              Allocate a file descriptor want for process p.  The resultant
              file descriptor is returned in *result.  The fdalloc() function
              returns zero on success, otherwise an appropriate error is

              Check the standard file descriptors 0, 1, and 2 and ensure they
              are referencing valid file descriptors.  If they are not, create
              references to /dev/null.  This operation is necessary as these
              file descriptors are given implicit significance in the Standard
              C Library and it is unsafe for setuid(2) and setgid(2) processes
              to be started with these file descriptors closed.

              Clear the descriptor table for lwp l.  This operation is per-
              formed by invoking fdinit() to initialise a new file descriptor
              table to replace the old file descriptor table and invoking
              fdfree() to release the old one.

     fdclone(l, fp, fd, flag, fops, data)
              This function is meant to be used by devices which allocate a
              file entry upon open.  fdclone() fills fp with the given parame-
              ters.  It always returns the in-kernel errno value EMOVEFD,
              which is meant to be returned from the device open routine.
              This special return value is interpreted by the caller of the
              device open routine.

              Close any files for process p that are marked ``close on exec''.
              This operation is performed by invoking fdunshare() for the
              process and invoking fdrelease() on the appropriate file

              Copy the file descriptor table from process p and return a
              pointer to the copy.  The returned file descriptor is guaranteed
              to have a reference count of one.  All file descriptor state is
              maintained.  The reference counts on each file entry referenced
              by the file descriptor table is incremented accordingly.

              Expand the file descriptor table for process p by allocating
              memory for additional file descriptors.

              Decrement the reference count on the file descriptor table for
              lwp l and release the file descriptor table if the reference
              count drops to zero.

              Create a file descriptor table using the same current and root
              directories of process p.  The returned file descriptor table is
              guaranteed to have a reference count of one.

     fdrelease(l, fd)
              Remove file descriptor fd from the file descriptor table of lwp
              l.  The operation is performed by invoking closef().

     fdremove(fdp, fd)
              Unconditionally remove the file descriptor fd from file descrip-
              tor table fdp.

     fdshare(p1, p2)
              Share the file descriptor table belonging to process p1 with
              process p2.  Process p2 is assumed not to have a file descriptor
              table already allocated.  The reference count on the file
              descriptor table is incremented.  This function is used by

              Ensure that lwp l does not share its file descriptor table.  If
              its file descriptor table has more than one reference, the file
              descriptor table is copied by invoking fdcopy().  The reference
              count on the original file descriptor table is decremented.

     Successful operations return zero.  A failed operation will return a non-
     zero return value.  Possible values include:

     [EBADF]            Bad file descriptor specified.

     [EMFILE]           Cannot exceed file descriptor limit.

     [ENOSPC]           No space left in file descriptor table.

     The framework for file descriptor handling is implemented within the file


NetBSD 8.1                       July 24, 2006                      NetBSD 8.1

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