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Full Discussion: how to access files via their descriptors?
Top Forums UNIX for Dummies Questions & Answers how to access files via their descriptors? Post 302282523 by Corona688 on Saturday 31st of January 2009 01:57:28 PM
Old 01-31-2009
To get a (new) file descriptor to do anything with you must first call open()*, which takes a filename. So yes, to open a file, you need a filename; the act of opening a file creates a file descriptor to represent that, the same way that running a program creates a new process ID to represent that.

Files are rather different from processes though, in that the PID is fundamental, and systemwide. The filename a process was ran with really isn't relevant since
a) If you have 4 xterms doing different things, you can't tell them apart by what file you ran
b) Lots of processes clone themselves, instead of loading new programs

So the PID is really the only way to identify processes. Commands like killall just trawl through the system's big list of PID's for you.

It's the files themselves that're unusual, and the ways you can use them, not the ways they're opened. There's several kinds of files:
  • Ordinary files, just blocks of data like you'd expect.
  • Symbolic links, a kind of special file that leads to a different file or directory sort of like a hyperlink leads to a website.
  • Hard links, where the same file is literally in two entirely seperate places. They have to be on the same partition, and share the same inode.
  • Named pipes aka FIFOs, where data written into the file by one process just gets pumped into whatever process is reading it.
  • Domain sockets, a kind of beefed-up version of named pipes that adds things like the ability for one server to talk to many clients over it.
  • Device files, which literally represent some hardware device or system-provided data source. 'dd if=/dev/random of=/dev/dsp' will read random data from the kernel and hurl it into your sound device on Linux systems with OSS compatibility...

You can also use FD's that have already been opened for you. The FD's 0, 1, and 2 are usually provided for you, where 0 represents the input stream(keyboard, for a console), 1 represents the output stream(the console display, for a console), and 2 represents error message output(also the console display). You can rearrange them how you please, having input coming from a file instead of console and having output going into a file or into another process etc etc etc.

* Okay, socket() and pipe() make FDs too, but have nothing to do with files on disk.
Last edited by Corona688; 01-31-2009 at 03:17 PM..
 

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LEARN ABOUT OSF1

rc0

rc0(8)							      System Manager's Manual							    rc0(8)

NAME

       rc0 - Runs command script executed when stopping the system

SYNOPSIS

       rc0

DESCRIPTION

       The  rc0  script  contains  run commands that enable a smooth shutdown and bring the system to a single-user state; run levels 0 and s.	In
       addition to commands listed in within the script itself, rc0 contains instructions to run commands found in the /sbin/rc0.d directory.  The
       script  defines the conditions under which the commands execute; some commands run if the system is being shut down while others run if the
       system is being shut down and rebooted to single user.

       By convention, files in the /sbin/rc0.d directory begin with either the letter "K" or the letter "S" and are followed by a two-digit number
       and a filename, for example: K00enlogin	   K05lpd     K60cron	  K30nfs

       In  general, the system starts commands that begin with the letter "S" and stops commands that begin with the letter "K."  The numbering of
       commands in the /sbin/rc0.d directory is important since the numbers are sorted and the commands are run in ascending order.  Files in  the
       /sbin/rc0.d directory are normally links to files in the /etc/init.d directory.

       An  entry  in  the  inittab  file  causes the system to execute the rc0 script, for example: ss:Ss:wait:/sbin/rc0 shutdown < /dev/console >
       /dev/console 2>&1 s0:0:wait:/sbin/rc0 off < /dev/console > /dev/console 2>&1

       The following operations are typical of those that result from executing the rc0 script and the commands located in the /sbin/rc0.d  direc-
       tory: Notify users that the system is shutting down.  Sync the disks Stop system services and daemons Stop processes Kill processes Unmount
       file systems Invoke init if the system is being shut down to single user

       The killall command sends a SIGTERM signal to stop running processes; SIGKILL follows to kill all processes except the process which initi-
       ated the call.  The umount -a command unmounts all file systems except the root file system.

FILES

       Specifies the command path Specifies the directory of commands that corresponds to the run level

RELATED INFORMATION

       Commands: init(8), killall(8), rc2(8), rc3(8), shutdown(8) delim off

																	    rc0(8)
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