FILE descriptor <-> FILE pointers

[Dave Decot]

> fdopen can be used to convert file descriptor to a FILE pointer
> 
> fileno can be used to convert a FILE pointer to file descriptor
> 
> A file descriptor is returned by open and used by dup, write, read etc
> 
> A FILE pointer is returned by fopen and used fprintf, fscanf , fget*
> and printf and scanf (implicitly).

There is also some even more unfortunate terminology floating around,
regarding the following three things:

1  The thing in the kernel for each group of shared file descriptors
   that describe the state of the same open file, containing Thing 2
   mentioned below, the (O_RDWR ...) mode passed to open(), and the
   (O_APPEND | O_SYNC | ... ) flags manipulated by open() and fcntl().

2  The current lseek() position for the file descriptor.

3  The current fseek() position for the standard I/O stream.

All three of the above have commonly been referred to as a "file pointer"
(not to be confused with "FILE pointer"!).

POSIX straightens this all out by using the following terminology:

    file descriptor		Same as always, containing the:
      file descriptor flags	    FD_CLOEXEC, etc.

    open file description	The shared thing in the kernel containing the:
      file access mode		    O_RDWR, etc.
      file status flags		    O_APPEND, etc.
      file offset		    Current lseek() position

    stream			Thing referenced by a FILE *, contaning:
      file position indicator	    Current fseek() position

Whew!

Dave