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Full Discussion: File Descriptor Table
Top Forums UNIX for Advanced & Expert Users File Descriptor Table Post 302148521 by porter on Sunday 2nd of December 2007 08:49:25 PM
Old 12-02-2007
Basically a file descriptor table is a kernel mantained array of pointers to kernel objects representing open files of some kind.

Imagine for a moment that these were C++ object...

Code:
int read(int fd,char *buf,size_t len)
{
    return fds[fd]->read(buf,len);
}

int write(int fd,char *buf,size_t len)
{
    return fds[fd]->write(buf,len);
}

int close(int fd,char *buf,size_t len)
{
    fds[fd]->close();
    fds[fd]=NULL;
}

But as kernels are generally written in C, other mechanisms, (pointers to jump tables etc) are used to provide the polymorphism.

Also reference counting is heavily used, so "close" for instance only really drops a reference count, if it gets to zero then the true close occurs.

What you need to consider in the model is where flags such as whether a file descriptor is blocking or not, and where the file-offset should live. Ask yourself, if I use "dup()" do both file descriptors have the same file offset?

If you do look inside the Linux kernel for instance you will notice that the BSD sockets API is handled quite differently to normal ioctl/read/write.
Last edited by porter; 12-02-2007 at 09:56 PM..
 

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LEARN ABOUT BSD

dup

DUP(2)								System Calls Manual							    DUP(2)

NAME

       dup, dup2 - duplicate a descriptor

SYNOPSIS

       newd = dup(oldd)
       int newd, oldd;

       dup2(oldd, newd)
       int oldd, newd;

DESCRIPTION

       Dup duplicates an existing object descriptor.  The argument oldd is a small non-negative integer index in the per-process descriptor table.
       The value must be less than the size of the table, which is returned by getdtablesize(2).  The new descriptor returned by the  call,  newd,
       is the lowest numbered descriptor that is not currently in use by the process.

       The  object referenced by the descriptor does not distinguish between references using oldd and newd in any way.  Thus if newd and oldd are
       duplicate references to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append  mode,  non-
       blocking  I/O  and asynchronous I/O options are shared between the references.  If a separate pointer into the file is desired, a different
       object reference to the file must be obtained by issuing an additional open(2) call.  The close-on-exec flag on the new file descriptor	is
       unset.

       In  the	second	form  of  the call, the value of newd desired is specified.  If this descriptor is already in use, the descriptor is first
       deallocated as if a close(2) call had been done first.

RETURN VALUE

       The value -1 is returned if an error occurs in either call.  The external variable errno indicates the cause of the error.

ERRORS

       Dup and dup2 fail if:

       [EBADF]	      Oldd or newd is not a valid active descriptor

       [EMFILE]       Too many descriptors are active.

SEE ALSO

       accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2), socketpair(2), getdtablesize(2)

4th Berkeley Distribution					   May 13, 1986 							    DUP(2)
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