terminal problem Post: 302102696

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Top Forums Programming terminal problem Post 302102696 by heck on Friday 12th of January 2007 07:48:02 AM

01-12-2007

Registered User

inherited file descriptors

i am not sure but, i could be the following reason:

Open descriptors are inherited to child process, this may cause the use of resources unneccessarily. Unneccesarry descriptors should be closed before fork() system call (so that they are not inherited) or close all open descriptors as soon as the child process starts running.

Code:

	for (i=getdtablesize();i>=0;--i) close(i); /* close all descriptors */

There are three standart I/O descriptors: standart input 'stdin' (0), standart output 'stdout' (1), standart error 'stderr' (2). A standard library routine may read or write to standart I/O and it may occur to a terminal or file. For safety, these descriptors should be opened and connectedthem to a harmless I/O device (such as /dev/null).

Code:

i=open("/dev/null",O_RDWR); /* open stdin */
	dup(i); /* stdout */
	dup(i); /* stderr */

As Unix assigns descriptors sequentially, fopen call will open stdin and dup calls will provide a copy for stdout and stderr.

Be aware that system() from <cstdlib> or <stdlib.h> is sth. like a wrapper for fork() and exec(). popen() too as far as is know.

But, try this.

I would be helpfull if we knew what you are executing and what output you have on the screen.
cheeers

heck

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dup

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)

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