file descriptors

a25khan · January 26, 2004, 7:46pm

i m trying to learn processes in unix and i've been reading this but i don't quite get it. its regarding file descriptors. : each is a part of file pointers, they point to another area. indexes into an Operating system maintained table called "file descriptor table". one table per process. may contain 20 entries called file pointers which point to an area of memory containing info about the opening of the file.
can someone comment on that, please!

oombera · January 27, 2004, 12:14am

Here's a nice little (basic) tutorial: http://www.livefirelabs.com/unix\_tip\_trick\_shell\_script/june_2003/06092003.htm.

This explains what "info" about a file they hold ... as for the more technical side, you'll have to dig deeper.

The Standard UNIX File Descriptors - Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)

If you are familiar with UNIX I/O redirection, syntax similar to the following should not be new to you:

command > file 2>&1

Briefly, when command runs it sends "normal" output to file, and any error messages generated by command are also written to file. "2>&1" handles the latter.

Have you ever wondered where the numbers 2 and 1 come from? This may be common knowledge to our more experienced readers, buy may need some explaining to those who are relatively new to the UNIX operating system environment. The 2 and 1 are file descriptors. Okay, so what's a file descriptor?

When a UNIX program wants to use a file, it must first open that file. When it does so, UNIX will associate a number with the file. This number, which is used by the program when reading from and writing to the file, is the file descriptor.

A typical UNIX program will open three files when it starts. These files are:

standard input (also known as stdin)

standard output (also known as stdout)

standard error (also known as stderr)

Standard input has a file descriptor of 0, standard output uses 1, and the number 2 is used by standard error. Are you starting to see where this is headed?

Looking at our command again,

command > file 2>&1

you should now recognize that 2>&1 instructs the shell to send messages headed to stderr (2) to the same place messages to stdout (1) are sent. In our example, that place is file.

If you are wondering, > is equivalent to 1>, and < is short for <0.

a25khan · January 27, 2004, 5:01pm

user oombera,

thanks for your answer,
yeah i see where are we heading from here. when i read it in my text, didn't make any sense. but when you said that "unix reads the files by using their respective numbers(file descriptors)" then i really understood the idea behind this. i have one more question though, what does this have to do with processes!?
a25khan

fpmurphy · January 27, 2004, 6:46pm

Note that processes do not have a limit of 20 file descriptors
as suggested in your mail - rather the limit is OPEN_MAX.

Here is the formal POSIX definition of a file descriptor:

A per-process unique, non-negative integer used to identify an open file for the purpose of file access. The value of a file descriptor is from zero to OPEN_MAX. A process can have no more than OPEN_MAX file descriptors open simultaneously.

Note also the difference between file DESCRIPTOR and file DESCRIPTION. From SUSv3 ....

(Open) File Description

A record of how a process or group of processes is accessing a file. Each file descriptor refers to exactly one open file description, but an open file description can be referred to by more than one file descriptor. The file offset, file status, and file access modes are attributes of an open file description.

Finnbarr