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From: bruno@skipper.dfrf.nasa.gov (Bruno Chenet)
Newsgroups: comp.unix.programmer
Subject: LOST in a PIPE
Message-ID: <782@skipper.dfrf.nasa.gov>
Date: 25 Feb 91 16:24:00 GMT
Distribution: usa
Organization: NASA Ames-Dryden FRF, Edwards, CA
Lines: 68


        To anybody that understands  fork(), system() or popen() 



        I am sending the command "tar tvf /dev/rst0" to list the content
 of a tape drive. I want to be able to grap each file description that shows
up on my screen as soon as possible since I want to display it in my own
X-window.
I tried to redirect the output to a file but then I cannot read this file
as long as information is redirected to it. (And this can take 5 or more
minutes when I am listing a full tape or CD). I cannot afford to let the
user waits that long.

Originally I was sending the command using a "popen()" as follow:

**********************
          sprintf(cmdline," %s ",  "tar tvf /dev/rfd0");
          if ((fp = popen(cmdline, "r")) == NULL)
                printf("popen error");

try #1:
/*
        while ((fgets(line, MAXLINE, fp)) != NULL) {
            n = strlen(line);
            if (write(1, line, n) != n)
            printf("data write error|n");
*/        


try #2: 
/*
        while ((ch = fgetc(fp)) != EOF) {
            if (ch == '\n') {
                 buf[i++] = ch;
                 fprintf(stderr," %s",buf);
                 i=0;
             }
             else   buf[i++] = ch;
        }
*/



**********************
 I should be able to read the stderr stream but I don't know how to
do it. I think that I need to use fork processing and read the stderr
until a EOL is reached. At this point I should be able to grab this line
and put it in some kind of buffer to be able to display it in my window 
and then give the control to the tar command again and so on...


I have also another problem of the same type: What happen if I send a
command that is not right. The system will answer with some kind of 
error message send to stderr. I need to grab this message and display
it in in my "report window" (I am trying to build a kind of front end
to the UNIX system using the X-Window environment so that UNIX becomes
more "user friendly"). 

Should I use "popen()", "system()", "shell scripts" or just "fork()" ?

        Please, I would take any ideas that would help me solve these
problems.


                        Thank you for your time,

                                        Bruno


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