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Top Forums Programming How to capturs system() output Post 302312743 by Panos1962 on Sunday 3rd of May 2009 05:47:30 AM
Old 05-03-2009
It's not so easy

Hi!
You called system("...") in your C program. First of all, the returned value has to do with the exit status of the program run under system, not the output of the command!

Second: Every process (process is a running program) has it's standard output to send the output and a standard error to send error messages. In your case the messages possibly are error messages of "pgrep" process and are sent to your terminal (the default standard error chanel). To check thsi fact run your program as:

Code:
your_program >panos1 2>panos2

where your_program is the name of the executable, panos1 is a file to use as standard output and panos2 is a file to use as standard error. After running the above command, you probably have the messages in panos2 as this file is used as standard error for your executable and inherited to child processes too (e.g. pgrep run under system as a chile process).

Third: To capture the standard output (or the the standard error) of a process run as child of another process you have to use the pipe system calls or functions; see the manual pages in popen, pipe etc. It's not easy and some experience in the C language is needed.

Bye...
 

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PKILL(1)						    BSD General Commands Manual 						  PKILL(1)

NAME
pkill -- find or signal processes by name SYNOPSIS
pgrep [-filnvx] [-d delim] [-G gid] [-g pgrp] [-P ppid] [-s sid] [-t tty] [-U uid] [-u euid] pattern ... pkill [-signal] [-filnvx] [-G gid] [-g pgrp] [-P ppid] [-s sid] [-t tty] [-U uid] [-u euid] pattern ... prenice [-l] priority pattern ... DESCRIPTION
The pgrep command searches the process table on the running system and prints the process IDs of all processes that match the criteria given on the command line. The pkill command searches the process table on the running system and signals all processes that match the criteria given on the command line. The prenice command searches the process table on the running system and sets the priority of all processes that match the criteria given on the command line. The following options are available for pkill and pgrep: -d delim Specify a delimiter to be printed between each process ID. The default is a newline. This option can only be used with the pgrep command. -f Match against full argument lists. The default is to match against process names. -G gid Restrict matches to processes with a real group ID in the comma-separated list gid. -g pgrp Restrict matches to processes with a process group ID in the comma-separated list pgrp. The value zero is taken to mean the process group ID of the running pgrep or pkill command. -i Ignore case distinctions in both the process table and the supplied pattern. -l Long output. Print the process name in addition to the process ID for each matching process. If used in conjunction with -f, print the process ID and the full argument list for each matching process. -n Match only the most recently created process, if any. -P ppid Restrict matches to processes with a parent process ID in the comma-separated list ppid. -s sid Restrict matches to processes with a session ID in the comma-separated list sid. The value zero is taken to mean the session ID of the running pgrep or pkill command. -t tty Restrict matches to processes associated with a terminal in the comma-separated list tty. Terminal names may be specified as a fully qualified path, in the form 'ttyXX', or 'pts/N', (where XX is any pair of letters, and N is a number), or the shortened forms 'XX' or 'N'. A single dash ('-') matches processes not associated with a terminal. -U uid Restrict matches to processes with a real user ID in the comma-separated list uid. -u euid Restrict matches to processes with an effective user ID in the comma-separated list euid. -v Reverse the sense of the matching; display processes that do not match the given criteria. -x Require an exact match of the process name, or argument list if -f is given. The default is to match any substring. -signal A non-negative decimal number or symbolic signal name specifying the signal to be sent instead of the default TERM. This option is valid only when given as the first argument to pkill. The -l flag is also availale for prenice. Note that a running pgrep or pkill process will never consider itself or system processes (kernel threads) as a potential match. EXIT STATUS
pgrep, pkill, and prenice return one of the following values upon exit: 0 One or more processes were matched. 1 No processes were matched. 2 Invalid options were specified on the command line. 3 An internal error occurred. SEE ALSO
grep(1), kill(1), ps(1), kill(2), sigaction(2), re_format(7), signal(7), renice(8) HISTORY
pkill and pgrep first appeared in NetBSD 1.6. They are modelled after utilities of the same name that appeared in Sun Solaris 7. prenice was introduced in NetBSD 6.0. BSD
December 7, 2010 BSD
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