10-30-2001
dup in unix
hi ..,
You are right, its used to duplicate the file discriptor.
the application demands the situation for example,
you have a client to pull data from a server on a different m/c and
you want your clients to run as daemon processes,
you can not associate any terminal device associated for daemon processes.
In this case, you open a null file discriptor with
open("/dev/null",O_RDONLY)
and dup this discriptor for stdin , stdout and stderr.
this prevents your processes from directly writing or reading from stdio.
I hope this might help you a bit in understanding.
thanks
reddyb
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dup(2) System Calls Manual dup(2)
NAME
dup - duplicate an open file descriptor
SYNOPSIS
DESCRIPTION
fildes is a file descriptor obtained from a or system call. returns a new file descriptor having the following in common with the origi-
nal:
o Same open file (or pipe).
o Same file pointer (i.e., both file descriptors share one file pointer).
o Same access mode (read, write or read/write).
o Same file status flags (see fcntl(2),
The new file descriptor is set to remain open across system calls. See fcntl(2).
The file descriptor returned is the lowest one available.
RETURN VALUE
Upon successful completion, the file descriptor is returned as a non-negative integer. Otherwise, a value of -1 is returned and is set to
indicate the error.
ERRORS
fails if one or more of the following is true:
fildes is not a valid open file descriptor.
Request violates the maximum number of open file descriptors.
AUTHOR
was developed by AT&T and HP.
SEE ALSO
close(2), creat(2), dup2(2), exec(2), fcntl(2), open(2), pipe(2), privileges(5).
STANDARDS CONFORMANCE
dup(2)