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From: rearl@gnu.ai.mit.edu (Robert Earl)
Newsgroups: comp.unix.questions,comp.lang.c
Subject: Re: UNIX commands in C
Message-ID: <REARL.91May1113957@nutrimat.gnu.ai.mit.edu>
Date: 1 May 91 15:39:57 GMT
References: <1991Apr28.153127.24926@jack.sns.com> <24527@well.sf.ca.us>
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In-reply-to: ron@well.sf.ca.us's message of 1 May 91 05:07:42 GMT

In article <24527@well.sf.ca.us> ron@well.sf.ca.us (Ronald Hayden) writes:
|
|   There are actually several ways to do this (check out the book C
|   Programming In a UNIX Environment for a complete discussion), but for
|   a simple command such as "who", you can simply:
|
|
|   #include <stdio.h>
|
|   main ()
|   {
|    printf("\nTesting the UNIX 'who' command --\n");
|    system("who");
|    printf("\nDone.\n");
|    exit(1);
|   }

Since system() [and popen()] does an implicit fork, it's good practice
to explicitly flush output buffers before you call this routine;
otherwise you end up with possibly duplicated or misleading output.
For instance, here's what I get when I redirect the output of this
program to a file (block buffered):

	guest    ttyp1    May  1 09:59 (192.35.86.25)
	rearl    ttyp4    May  1 11:05 (dialin.ucsd.edu)
	ggray    ttyp6    May  1 10:43 (geech)
	guest    ttyp8    May  1 11:20 (192.35.86.26)
	
	Testing the UNIX 'who' command --
	
	Done.
	
Probably not what you intended.  Add "fflush(stdout)" or
"fflush(NULL)" (to flush all buffers) before calling system().  I
think this is a FAQ in one group or another...

By the way, any reason why this program was made to return failure?

--robert
rearl@gnu.ai.mit.edu
rearl@watnxt3.ucr.edu