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From: jp27+@andrew.cmu.edu (Jacques Parker)
Newsgroups: comp.unix.wizards
Subject: Pipes & Forks
Message-ID: <21729@adm.BRL.MIL>
Date: 12 Dec 89 19:08:51 GMT
Sender: news@adm.BRL.MIL
Lines: 59


I made the following program.  When compiled & run, it will accept
input, but not act on it.  It should accept commands, and the child
process should receive and execute them.  This
does not happen until the program is terminated (Enter ^D).  Any ideas? 
How do I insure that the child receives the parents output immediately?

Thank you for any help.

					Jacques Parker


#include <stdio.h>

#define MAXLEN  100
#define ST_INP	0
#define ST_OUT	1

extern char **environ;

char **arg = { "/bin/csh","-x"};

main(argc,argv,envp)
int argc;
char **argv, **envp;
{
    
    int     pd[2];
    int     pid;
    int     bak_inp = dup (ST_INP);/* Backup the input FD */
    int     bak_out = dup (ST_OUT);/* Backup the output FD */

    if (pipe(pd) < 0)
	printf("%s: Can't create pipe", cmd);

    dup2(pd[1], ST_OUT);	/* ST_OUT --> pipe WRITE */
    dup2(pd[0], ST_INP);	/* ST_INP --> pipe READ  */
    
    if ((pid = fork()) < 0)
	printf("%s: Can't spawn a process.", cmd);
    if (pid == 0) {
	dup2(bak_out, ST_OUT); 	/* child out --> ST_OUT */
				/* child in --> Pipe READ */
	execvp(arg[0],arg,environ);
	printf("Shell not started. '%s' failed.",cmd);
    }

    dup2(bak_inp,ST_INP);	/* parent out --> Pipe WRITE */
				/* parent in --> ST_INP */

    while (gets(cmd) != NULL) { /* Enter ^D to end. */
	printf("%s\n,cmd);
	fflush(NULL);
    }

}
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