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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
pgrep, pkill -- find or signal processes by name SYNOPSIS
pgrep [-LSafilnoqvx] [-F pidfile] [-G gid] [-M core] [-N system] [-P ppid] [-U uid] [-c class] [-d delim] [-g pgrp] [-j jid] [-s sid] [-t tty] [-u euid] pattern ... pkill [-signal] [-ILafilnovx] [-F pidfile] [-G gid] [-M core] [-N system] [-P ppid] [-U uid] [-c class] [-g pgrp] [-j jid] [-s sid] [-t tty] [-u euid] 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 following options are available: -F pidfile Restrict matches to a process whose PID is stored in the pidfile file. -G gid Restrict matches to processes with a real group ID in the comma-separated list gid. -I Request confirmation before attempting to signal each process. -L The pidfile file given for the -F option must be locked with the flock(2) syscall or created with pidfile(3). -M core Extract values associated with the name list from the specified core instead of the currently running system. -N system Extract the name list from the specified system instead of the default, which is the kernel image the system has booted from. -P ppid Restrict matches to processes with a parent process ID in the comma-separated list ppid. -S Search also in system processes (kernel threads). -U uid Restrict matches to processes with a real user ID in the comma-separated list uid. -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. -a Include process ancestors in the match list. By default, the current pgrep or pkill process and all of its ancestors are excluded (unless -v is used). -c class Restrict matches to processes running with specified login class class. -f Match against full argument lists. The default is to match against process names. -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. -j jid Restrict matches to processes inside jails with a jail ID in the comma-separated list jid. The value ``any'' matches processes in any jail. The value ``none'' matches processes not in jail. -l Long output. For pgrep, 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. For pkill, display the kill command used for each process killed. -n Select only the newest (most recently started) of the matching processes. -o Select only the oldest (least recently started) of the matching processes. -q Do not write anything to standard output. -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 of the form ttyxx or the shortened form xx. A single dash ('-') matches processes not associated with a terminal. -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. If any pattern operands are specified, they are used as regular expressions to match the command name or full argument list of each process. If the -f option is not specified, then the pattern will attempt to match the command name. However, presently FreeBSD will only keep track of the first 19 characters of the command name for each process. Attempts to match any characters after the first 19 of a command name will quietly fail. Note that a running pgrep or pkill process will never consider itself nor system processes (kernel threads) as a potential match. EXIT STATUS
The pgrep and pkill utilities 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. COMPATIBILITY
Historically the option ``-j 0'' means any jail, although in other utilities such as ps(1) jail ID 0 has the opposite meaning, not in jail. Therefore ``-j 0'' is deprecated, and its use is discouraged in favor of ``-j any''. SEE ALSO
kill(1), killall(1), ps(1), flock(2), kill(2), sigaction(2), pidfile(3), re_format(7) HISTORY
The pkill and pgrep utilities first appeared in NetBSD 1.6. They are modelled after utilities of the same name that appeared in Sun Solaris 7. They made their first appearance in FreeBSD 5.3. AUTHORS
Andrew Doran <ad@NetBSD.org> BSD
August 9, 2013 BSD
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