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

NAME
     file -- an overview of file descriptor handling

SYNOPSIS
     #include <&lt;sys/file.h>&gt;
     #include <&lt;sys/filedesc.h>&gt;

     int
     falloc(struct proc *p, struct file **resultfp, int *resultfd);

     int
     fdrelease(struct proc *p, int fd);

     void
     FREF(struct file *fp);

     void
     FRELE(struct file *fp, struct proc *p);

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

     int
     getsock(struct filedesc *fdp, int fd, struct file **fpp);

     #include <&lt;sys/file.h>&gt;
     #include <&lt;sys/filedesc.h>&gt;
     #include <&lt;sys/vnode.h>&gt;

     int
     getvnode(struct filedesc *fdp, int fd, struct file **fpp);

DESCRIPTION
     These functions provide the interface for the UNIX file descriptors.
     File descriptors can be used to access vnodes (see vnode(9)), sockets
     (see socket(2)), pipes (see pipe(2)), kqueues (see kqueue(2)), and vari-
     ous special purpose communication endpoints.

     A new file descriptor is allocated with the function falloc() and freed
     with fdrelease().  falloc() and fdrelease() deal with allocating and
     freeing slots in the file descriptor table, expanding the table when nec-
     essary and initializing the descriptor.  It's possible to do those things
     in smaller steps, but it's not recommended to make complicated kernel
     APIs that require it.

     The files are extracted from the file descriptor table using the func-
     tions fd_getfile(), getvnode() and getsock().  fd_getfile() performs all
     necessary checks to see if the file descriptor number is within the range
     of file descriptor table, and if the descriptor is valid.  getsock() and
     getvnode() are special cases that besides doing fd_getfile() also check
     if the descriptor is a vnode or socket, return the proper errno on error
     and increase the use count with FREF().

CONCURRENT ACCESS
     Since multiple processes can share the same file descriptor table, it's
     important that the file is not freed in one process while some other
     process is still accessing it.  To solve that problem a special use count
     is kept with the functions FREF() and FRELE().  In most cases FREF()
     should be used on a file after it has been extracted from the file
     descriptor table and FRELE() should be called when the file won't be used
     anymore.  There are cases when this isn't necessary, but since FREF() and
     FRELE() are cheap to use, there is no reason to risk introducing bugs by
     not using them.

CODE REFERENCES
     The majority of those functions are implemented in
     sys/kern/kern_descrip.c.  The function prototypes and the macros are
     located in sys/sys/file.h and sys/sys/filedesc.h.

SEE ALSO
     vnode(9)

BSD                              June 4, 2013                              BSD