Pipes & Forks

[Crispin Goswell]

In article <21729 at adm.BRL.MIL> jp27+ at andrew.cmu.edu (Jacques Parker) writes:
>
>I made the following program.  When compiled & run, it will accept
>input, but not act on it.

As listed here it did not compile at all on the systems that I have to hand.

>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"};

I had to replace this with:

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

the [] to get it to compile, and NULL to get execvp to work when it ran.

>
>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 */

I added:

     char    cmd[BUFSIZ];

here to get it to compile. This is optimistic: the gets() does not check for
buffer overflow.

>    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);

I had to add the " here:

	printf("%s\n",cmd);

>	fflush(NULL);

I had to replace this with:

	fflush (stdout);

to get the program to work.
>    }

This article probably belongs in comp.lang.c.

Name:   Crispin Goswell                   |-------|__   Informatics Department
UUCP: {... | mcvax}!ukc!rlinf!caag        |  Tea  |  | Rutherford Appleton Lab
JANET: caag at uk.ac.rl.inf                  \  Mug  /_/          Chilton, Didcot
ARPA: caag%inf.rl.ac.uk at nsfnet-relay.ac.uk \_____/           Oxon OX11 0QX, UK

"Where are you when the Sun goes down? You're so far away from me."
        - Dire Straits