File Descriptors


 
Thread Tools Search this Thread
Top Forums Programming File Descriptors
# 1  
Old 12-24-2001
Question File Descriptors

Hi,
I have written a daemon process, to perform certain operations in the background.
For this I have to close, the open file descriptors,

Does anybody know how to find out the number of open file descriptors ?


Thanks in Advance,
Sheetal
# 2  
Old 12-24-2001
I don't know any portable way for a process to find out the number of files it has open.

But consider this case...I open 16 files and allocate fd 0 through fd 15. Then I close fd 0 through fd 14. This leaves only fd 15 open. If you could magically find out that only one file is open, how would that help you? You still don't know which fd to close.

The shell will allocate fd 0, 1, and 2 and pass these to you. Your program should keep track of which files it opens.

If you have lost track of want files are open, the only thing I can suggest to do a getrlimit() to obtain the highest possible fd that could ever be allocated. Then close all possible fd's. The close call will fail if the file is not open, so you just ignore that error.
# 3  
Old 12-24-2001
In general when you write a daemon process you...

Become a session leader - fork()
Become a process-group leader - setsid()
Dissociate from controlling terminal - setsid() normally does this too.
chdir to '/' - chdir()
Set file creation mask to 0 - umask(0)

You still need to close unneeded descriptors. Assuming you
haven't opened anything, you can simply do...

for (i = 0; i < 10; i++)
close(i);

As Perderabo says, "The close call will fail if the file is not open,
so you just ignore that error."

At this point, you have initalized your daemon process.
A great reference book with examples of this is...
Advanced Programming in the UNIX Environment
by W. Richard Stevens
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Questions about file descriptors

Hi, I'm playing with KSH I entered following command in terminal { echo "stdout" >&1; echo "stderr" >&2; } > out And I get only stoud in a new file out. My question is: Where did my stderr vanish ? (5 Replies)
Discussion started by: solaris_user
5 Replies

2. UNIX for Dummies Questions & Answers

Semaphores and File Descriptors

What is the difference between a file descriptor and a semaphore? My basic understanding is: - a file descriptor is a small positive integer that the system uses instead of the file name to identify an open file or socket. - a semaphore is a variable with a value that indicates the... (1 Reply)
Discussion started by: Mr_Webster
1 Replies

3. HP-UX

exec and file descriptors

Hi, I speak and write english more or less, so I hope my asking be clear. :) In the company I am working, they are using control-m software to lunch shell scripts. So i put this command in all shell scripts: export LOGFILE_tmp=$PRODUC_DATA/tmp/${SCRIPT}_${PAIS}_`date... (0 Replies)
Discussion started by: anamcara
0 Replies

4. UNIX for Dummies Questions & Answers

Write/read to file descriptors

Is it possible to write to file descriptor 0 and read from 1 or 2? How could this be implemented? (3 Replies)
Discussion started by: machshev
3 Replies

5. UNIX for Dummies Questions & Answers

how to list the files using File Descriptors

hello, I have written a script named listall.sh with the following codes init. #!/bin/bash PATH="/proj/cmon/$1" echo $PATH if ; then echo "Usage: $0 ***" exit 1 else ls -l $PATH/*.sc fi Here there are 3 subdirectories (namely - src, data and jobs)under /proj/cmon, so... (2 Replies)
Discussion started by: shyjuezy
2 Replies

6. UNIX for Advanced & Expert Users

File Descriptors + cron

Hi All, This thread is going to be a discussion basically bringing out more information from the experts on cron jobs and the associated file handles. So, here is the question. There is definitely a constant ' n ' as the maximum number of file handles alloted to a process ' p '. Will... (7 Replies)
Discussion started by: matrixmadhan
7 Replies

7. Programming

Sockets and File descriptors

I am in a Systems programming class this semester, and our current project is to write a program utilizing sockets and fork. For the project, I decided to make my own instant messaging program. I have the code completed, but I have a problem that keeps old clients from communicating with new... (3 Replies)
Discussion started by: gstlouis
3 Replies

8. Shell Programming and Scripting

Multiple co-processor file descriptors

I have a script that creates a KSH co-process for Oracle sqlplus and I am presently interacting with it via print -p and read -p. I also need to interact with another Oracle database what isn't permitted to have any direct connection to the first. Presently, I simply disconnect from the first... (10 Replies)
Discussion started by: tmarikle
10 Replies

9. UNIX for Dummies Questions & Answers

file descriptors

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... (3 Replies)
Discussion started by: a25khan
3 Replies

10. UNIX for Advanced & Expert Users

File Descriptors

Hello all, A few questions on file descriptors ... scenario : Sun Ultra 30 with Sun OS 5.5.1 , E250 with Solaris 2.6 In one of my servers, the file descriptor status from the soft limit and hard limits are 64 and 1024 respectively for root user. Is the soft limit (64) represents the... (3 Replies)
Discussion started by: shibz
3 Replies
Login or Register to Ask a Question