l and ttyS0

Gins · 08-06-2005, 12:19 PM

[root@c83-250-89-7 ka]# ls -l /dev/ttyS0
lr-xr-xr-x 1 root root 5 Aug 6 2005 /dev/ttyS0 -> tts/0
[root@c83-250-89-7 ka]#

I know 'r' means read only and 'x' means executable.

What is the meaning of 'l' ?

On top of this what is 'ttyS0' ?

harken · 08-06-2005, 12:50 PM

The 'l' in "lr-xr-xr-x" means that the file is actually a symbolic link, a pointer to another file. That's why it shows "/dev/ttyS0 -> tts/0", that's where it points to. See more on Linux file permissions at http://www.comptechdoc.org/os/linux/...ugfilesp.html.
/dev/ttyS0 is the first character device for serial terminal lines.

Gins · 08-06-2005, 01:00 PM

harken
Thanks for the reply.
The URL you mentioned is dead. I don't know why.

In Solaris systems, they talk about 'character special device files' and 'block special device files'.

Does this exist in Linux too? I have Mandrake linux 10.0 version.

If it exists, what is the purpose of them?

harken · 08-06-2005, 01:08 PM

There was an extra point character '.' in the link. Here it is: http://www.comptechdoc.org/os/linux/..._ugfilesp.html
As for the two types of files, yes they exist in Linux too. I'll quote from one of the man pages, for a brief explanation:

Quote:

A special file is a triple (boolean, integer, integer)
stored in the filesystem. The boolean chooses between
character special file and block special file. The two
integers are the major and minor device number.

Thus, a special file takes almost no place on disk, and is
used only for communication with the operating system, not
for data storage. Often special files refer to hardware
devices (disk, tape, tty, printer) or to operating system
services (/dev/null, /dev/random).

Block special files usually are disk-like devices (where
data can be accessed given a block number, and e.g. it is
meaningful to have a block cache). All other devices are
character special files. (Long ago the distinction was a
different one: I/O to a character special file would be
unbuffered, to a block special file buffered.)

harken · 08-06-2005, 01:13 PM

Also, a quotation from http://www.win.tue.nl/~aeb/linux/lk/lk-6.html, section 6.2, "Device special files" paragraph:

Quote:

The Unix philosophy is: "everything is a file". That makes life easy, the same system calls are used to read from any device. Special device nodes are inodes that refer to a device rather than to a file. They come in two kinds: block special devices are block-structured, allow random access, and, in case they contain a filesystem, can be mounted. The typical example is a disk. All (other) devices are character special devices. One has tapes, scanners, modems, etc. Also block special devices can be often accessed via a character special device node.

Device special files are very simple: the inode contains a pair of small integers (the major and minor device numbers), and these numbers are used by the system as index in some table to find the driver for the device.