Perform other functions while untarring file using "system" command in a C program

[Jerry Feldman]

The system(2) function by defnition waits until the command completes. 
As John mentioned, you could use the popen() function or you could do a 
fork()/exec() yourself. Also note that the GNU tar command with the z 
option forks a gzip(or gunzip) which then pipes the output file to tar. 

As a quick example:
/* signal handler */
void sighand(int sig)
{
	if (sig == SIGCHLD) {
		finished = 1;
	}
}
pid_t chld = fork();
finished = 0;
set up signal to catch the SIGCHLD signal;

if (chld == 0) {
	exec(tar command); 
	return 0; /* or exit() */
} else if (chld == -1) {
	perror("fork");
	exit(-1);
} else { /* parent */
	do {
		/* calculate percent */		
 		printf("%d %% Completed", percent);
	} while(! finished);
}
/* done */



On 31 Jan 2001, at 13:04, John Abreau wrote:

> On Wed, 31 Jan 2001, zoqix wrote:
> 
> > Hi all, 
> > I'm using the "system" command to do an untarring of a file.tar.gz file
> > in my C program. However, I would like to display some status
> > information like "10 % Completed". I've written some code to calculate
> > the percentage. However, the code could only be runned after the
> > "system" command finished untarring my zipped file. e.g.   
> > 
> > system("tar zxf file.tar.gz");
> > do
> > {
> > printf("%d %% Completed", percent);
> > }while (!finished) 
> > 
> > Therefore, the printf will always print 100 % Completed. Is there any
> > way to let the unzipping continue at background and return to print the
> > completion status?
> > 
> 
> A look through the info files for GNU tar shows the following:
> 
> 	`--checkpoint'
> 		This option directs `tar' to print periodic checkpoint
> 		messages as it reads through the archive.  Its intended
> 		for when you want a visual indication that `tar' is still
> 		running, but don't want to see `--verbose' output.
> 
> It doesn't give a percentage, and it's not immediately clear what the
> number it does give actually represents, but it's probably a good starting
> point.
> 
> To use its output, you'd have to replace the "system" call with a call to
> popen that reads from the stdout of the "tar" process. You'd also want to
> change the buffering of data in the pipe so you can immediately read the
> output from tar at the newline, rather than waiting for a complete block
> to be buffered.
> 
> --
> John Abreau / Executive Director, Boston Linux & Unix 
> ICQ#28611923 / AIM abreauj / Email jabr at blu.org
> 
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Jerry Feldman <gaf at blu.org>
Associate Director
Boston Linux and Unix user group
http://www.blu.org
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