FILE(9)

FILE(9) | 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 system 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 created, 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 transferred. 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)

May 17, 2009 | NetBSD 8.3