When I am writing my own interpreter... Post: 302142009

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Top Forums Programming When I am writing my own interpreter... Post 302142009 by Legend986 on Wednesday 24th of October 2007 12:57:38 AM

10-24-2007

Registered User

Thanks to everyone here... I am slowly able to realize the shell... I'm actually stuck at piping... I am able to handle a single pipe but how do I solve the problem of multiple pipes? I know it can be solved using recursion but some pseudo algorithm will be excellent... I don't understand how to actually use recursion here... I'm currently doing something like a parent creates two children and the first one executes one command and pipes it onto the second child which displays the output...

And when I used valgrind, to my surprise I found 15 memory leaks from the piping function that I wrote and I don't understand what could've gone wrong... My pseudo code looks something like this:

Code:

	
	int fd[2]; /* provide file descriptor pointer array for pipe */
	pid_t pid1, pid2; /* process ids for each child */
	/* create pipe and check for an error */
	/* apply fork and check for error */
	    /* processing for child */
	    close (fd[1]); /* close output end, leaving input open */
	    /* set standard input to pipe */
	    if (fd[0] != STDIN_FILENO)
	    {
		if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO)
		{
		    perror("dup2 error for standard input");
		    exit(1);
		}
		close(fd[0]);
	    }
	   execlp the second function
	    //First child finished
	
	else
	{
	    /* processing for parent */
	    /* spawn second process */
	   /* apply fork again for second child*/
	  /* processing for child */
		
		close (fd[0]);
		/* set standard output to pipe */
		if (fd[1] != STDOUT_FILENO)
		{
		    if (dup2(fd[1], STDOUT_FILENO) != STDOUT_FILENO)
		    {
			perror("dup2 error for standard output");
			exit(1);
		    }
		    close(fd[1]); /* not needed after dup2 */
		}
		
		execlp the first function
		/* print to the pipe, now standard output */
	    }
	    else
	    {
		/* processing continues for parent */
		close(fd[0]);
		close(fd[1]);
		waitpid (pid1, NULL, 0); /* wait for first child to finish */
		waitpid (pid2, NULL, 0); /* wait for second child to finish */
	   }
	}

Am I doing something wrong?

Last edited by Legend986; 10-24-2007 at 02:45 AM..

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LEARN ABOUT MINIX

close

CLOSE(2)							System Calls Manual							  CLOSE(2)

NAME

       close - delete a descriptor

SYNOPSIS

       #include <unistd.h>

       int close(int d)

DESCRIPTION

       The  close  call deletes a descriptor from the per-process object reference table.  If this is the last reference to the underlying object,
       then it will be deactivated.  For example, on the last close of a file the current seek pointer associated with the file is  lost;  on  the
       last close of a TCP/IP descriptor associated naming information and queued data are discarded; on the last close of a file holding an advi-
       sory lock the lock is released (see further fcntl(2)).

       A close of all of a process's descriptors is automatic on exit, but since there is a limit on the number of active descriptors per process,
       close is necessary for programs that deal with many descriptors.

       When  a	process forks (see fork(2)), all descriptors for the new child process reference the same objects as they did in the parent before
       the fork.  If a new process is then to be run using execve(2), the process would normally inherit these descriptors.  Most of the  descrip-
       tors  can  be  rearranged with dup2(2) or deleted with close before the execve is attempted, but if some of these descriptors will still be
       needed if the execve fails, it is necessary to arrange for them to be closed if the execve succeeds.  For this reason, the call	``fcntl(d,
       F_SETFD, flags)'' is provided, that can be used to mark a descriptor "close on exec" by setting the FD_CLOEXEC flag:

	      fcntl(d, F_SETFD, fcntl(d, F_GETFD) | FD_CLOEXEC);

RETURN VALUE

       Upon successful completion, a value of 0 is returned.  Otherwise, a value of -1 is returned and the global integer variable errno is set to
       indicate the error.

ERRORS

       Close will fail if:

       [EBADF]	      D is not an active descriptor.

SEE ALSO

       open(2), pipe(2), execve(2), fcntl(2).

4th Berkeley Distribution					   May 22, 1986 							  CLOSE(2)