file(9) - NetBSD Manual Pages

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


NAME

     file, closef, ffree, FILE_IS_USABLE, FILE_USE, FILE_UNUSE,
     FILE_SET_MATURE -- operations on file entries


SYNOPSIS

     #include <sys/file.h>

     int
     closef(struct file *fp, struct lwp *l);

     void
     ffree(struct file *fp);

     int
     FILE_IS_USABLE(struct file *fp);

     void
     FILE_USE(struct file *fp);

     void
     FILE_UNUSE(struct file *fp, struct lwp *l);

     void
     FILE_SET_MATURE(struct file *fp);


DESCRIPTION

     The file descriptor table of a process references a file entry for each
     file used by the kernel.  See filedesc(9) for details of the file
     descriptor table.  Each file entry is given by:

     struct file {
             LIST_ENTRY(file) f_list;        /* list of active files */
             int             f_flag;
             int             f_iflags;       /* internal flags */
             int             f_type;         /* descriptor type */
             u_int           f_count;        /* reference count */
             u_int           f_msgcount;     /* message queue references */
             int             f_usecount;     /* number active users */
             kauth_cred_t    f_cred;         /* creds associated with descriptor */
             struct fileops {
                     int (*fo_read)(struct file *fp, off_t *offset,
                             struct uio *uio, kauth_cred_t cred, int flags);
                     int (*fo_write)(struct file *fp, off_t *offset,
                             struct uio *uio, kauth_cred_t cred, int flags);
                     int (*fo_ioctl)(struct file *fp, u_long com, void *data,
                             struct lwp *l);
                     int (*fo_fcntl)(struct file *fp, u_int com, void *data,
                             struct lwp *l);
                     int (*fo_poll)(struct file *fp, int events,
                             struct lwp *l);
                     int (*fo_stat)(struct file *fp, struct stat *sp,
                             struct lwp *l);
                     int (*fo_close)(struct file *fp, struct lwp *l);
             } *f_ops;
             off_t           f_offset;
             void         *f_data;         /* descriptor data */
     };

     NetBSD treats file entries in an object-oriented fashion after they are
     created.  Each entry specifies the object type, f_type, which can have
     the values DTYPE_VNODE, DTYPE_SOCKET, DTYPE_PIPE and DTYPE_MISC.  The
     file entry also has a pointer to a data structure, f_data, that contains
     information specific to the instance of the underlying object.  The data
     structure is opaque to the routines that manipulate the file entry.  Each
     entry also contains an array of function pointers, f_ops, that translate
     the generic operations on a file descriptor into the specific action
     associated with its type.  A reference to the data structure is passed as
     the first parameter to a function that implements a file operation.  The
     operations that must be implemented for each descriptor type are read,
     write, ioctl, fcntl, poll, stat, and close.  See vnfileops(9) for an
     overview of the vnode file operations.  All state associated with an
     instance of an object must be stored in that instance's data structure;
     the underlying objects are not permitted to manipulate the file entry
     themselves.

     For data files, the file entry points to a vnode(9) structure.  Pipes and
     sockets do not have data blocks allocated on the disk and are handled by
     the special-device filesystem that calls appropriate drivers to handle
     I/O for them.  For pipes, the file entry points to a system block that is
     used during data transfer.  For sockets, the file entry points to a sys-
     tem block that is used in doing interprocess communications.

     The descriptor table of a process (and thus access to the objects to
     which the descriptors refer) is inherited from its parent, so several
     different processes may reference the same file entry.  Thus, each file
     entry has a reference count, f_count.  Each time a new reference is cre-
     ated, the reference count is incremented.  When a descriptor is closed,
     the reference count is decremented.  When the reference count drops to
     zero, the file entry is freed.

     Some file descriptor semantics can be altered through the flags argument
     to the open(2) system call.  These flags are recorded in f_flags member
     of the file entry.  For example, the flags record whether the descriptor
     is open for reading, writing, or both reading and writing.  The following
     flags and their corresponding open(2) flags are:

           FAPPEND    O_APPEND
           FASYNC     O_ASYNC
           O_FSYNC    O_SYNC
           FNDELAY    O_NONBLOCK
           O_NDELAY   O_NONBLOCK
           FNONBLOCK  O_NONBLOCK
           FFSYNC     O_SYNC
           FDSYNC     O_DSYNC
           FRSYNC     O_RSYNC
           FALTIO     O_ALT_IO

     Some additional state-specific flags are recorded in the f_iflags member.
     Valid values include:

           FIF_WANTCLOSE  If set, then the reference count on the file is
                          zero, but there were multiple users of the file.
                          This can happen if a file descriptor table is shared
                          by multiple processes.  This flag notifies potential
                          users that the file is closing and will prevent them
                          from adding additional uses to the file.
           FIF_LARVAL     The file entry is not fully constructed (mature) and
                          should not be used.

     The read(2) and write(2) system calls do not take an offset in the file
     as an argument.  Instead, each read or write updates the current file
     offset, f_offset in the file according to the number of bytes trans-
     ferred.  Since more than one process may open the same file and each
     needs its own offset in the file, the offset cannot be stored in the per-
     object data structure.


FUNCTIONS

     closef(fp, l)
              The internal form of close(2) which decrements the reference
              count on file entry fp.  The closef() function  release all
              locks on the file owned by lwp l, decrements the reference count
              on the file entry, and invokes ffree() to free the file entry.

     ffree(struct file *fp)
              Free file entry fp.  The file entry was created in falloc(9).

     FILE_IS_USABLE(fp)
              Ensure that the file entry is usable by ensuring that neither
              the FIF_WANTCLOSE flag nor the FIF_LARVAL flag is set in
              f_iflags.

     FILE_USE(fp)
              Increment the reference count on file entry fp.

     FILE_UNUSE(fp, l)
              Decrement the reference count on file entry fp.  If the
              FIF_WANTCLOSE flag is set in f_iflags, the file entry is freed.

     FILE_SET_MATURE(fp)
              Mark the file entry as being fully constructed (mature) by
              clearing the FIF_LARVAL flag in f_iflags.


CODE REFERENCES

     The framework for file entry handling is implemented within the file
     sys/kern/kern_descrip.c.


SEE ALSO

     dofileread(9), filedesc(9), vnfileops(9), vnode(9)

NetBSD 10.99                     May 17, 2009                     NetBSD 10.99