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From: lemieux@ireq.hydro.qc.ca (Lemieux)
Newsgroups: comp.unix.questions
Subject: How do I unbuffer the standard output of a child process?
Keywords: fork, pipe, redirection, buffering
Message-ID: <6119@s3.ireq.hydro.qc.ca>
Date: 19 Mar 91 19:46:35 GMT
Sender: root@s3.ireq.hydro.qc.ca
Organization: Institut de Recherche d'Hydro-Quebec
Lines: 84


	My program forks a process and redirect the stdin and
	stdout of the child to a set of named pipes.

	Thus, I can send a command to the child process through
	one pipe and read the result from the other.

	One of the program's constraint is that I have to get the
	result immediately (I can't wait for the output buffer
	of the child process to be flushed).

	Unfortunately, I did not find a way to unbuffer the standard
	output of the child process (even with setbuf). How can I do that?
	Can someone help me, please?

	Thanks in advance.

	Here's what I tried (no validation is done for clarity!).

	#include <stdio.h>
	#include <sys/stat.h>
	
	#define	PIPE_IN	 "/tmp/pipe_in"
	#define		PIPE_OUT "/tmp/pipe_out"

	main(argc,argv)
	int	argc;
	char	*argv[];
	{
	int	pid;
	int	fd_in,fd_out;
	FILE	*fp,*fp_in,*fp_out;
	
		/* create pipes */
		mknod(PIPE_IN ,S_IFIFO | 0777,0);
		mknod(PIPE_OUT,S_IFIFO | 0777,0);

		/* create child process */
		if ((pid = fork()) == -1)
		{
			fprintf(stderr,"Can't fork child\n");
			exit(1);
		}

		/* server */
		if (pid > 0)
		{
			/* establish the communication medium */
			fd_in  = open(PIPE_IN ,O_WRONLY);
			fd_out = open(PIPE_OUT,O_RDONLY);
			fp_in  = fdopen(fd_in ,"w");
			fp_out = fdopen(fd_out,"r");
	
			/* unbuffer the streams */
			setbuf(fp_in,NULL);
			setbuf(fp_out,NULL);
	
			/* lots of stuff deleted for clarity */
			...
		}
		/* client */
		else
		{
			/* redirect stdin and stdout */
			freopen(PIPE_IN,"r",stdin);
			fp = freopen(PIPE_OUT,"w",stdout);
	
			/*
			 * THIS COMMAND IS SUPPOSE TO UNBUFFER
			 * THE CHILD OUTPUT
			 */
			setbuf(fp,NULL);

			/* execute the client program */
			execl(argv[1],argv[2],0);
		}
	}

-----------------------------------------------------------------------------
Eric LEMIEUX				| Internet: lemieux@ireq.hydro.qc.ca
Institut de Recherche d'Hydro-Quebec	|
1800 Montee Sainte-Julie		| TEL: (514) 652-8139 
Varennes, Quebec, Canada		| FAX: (514) 652-8309
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