A file by any other name

By Hal Stern, Unix Insider |  Operating Systems

Despite all of the work that has been done with
object-oriented systems and user interfaces, the Unix filesystem is
the primary mechanism we use to locate and interact with our data. The
hierarchical namespace is simple and relatively easy to use -- until
you begin adding symbolic links, NFS mounts, removable media, and
other wormholes that take you from one disk to another with little
warning. Large Unix environments stress the filesystem in new and
creative ways: Processes can go file-crazy and run into system-resource
limitations. Users who have yet to discover the wonder of directories
complain of terrible system performance, or just when you think your list
of headaches is complete, you try to unmount a CD-ROM but continually
get "filesystem busy" error messages, or you promise to remove the
mailbox of a user who is hanging you up -- as soon as you determine
his or her identity.

This month, we'll sort through some filesystem navigation issues.
We'll look at open() to see how processes find files, and
examine some performance issues. From there, it's off to the links --
hard and symbolic -- to see how they alter paths through the
filesystem. Finally, we'll look at the tools available to find the
user associated with an open file or directory. While we may not offer
any solutions to the deep-versus-wide directory layout argument, we'll
try to make sure that no matter what you call your files, you'll get the
right bits.

Open duplicity

We see the filesystem as a tree of directories and filenames. These
physical names aren't used inside of a process. Logical
names
known as file descriptors, or file handles, are used to
identify a file for reading or writing. Unix file descriptors are
integers returned by the open() or dup()
system calls. Pass a filesystem pathname to open() along
with the read or write permissions you want, and open()
returns a file descriptor or an error:

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