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Full Discussion: When I am writing my own interpreter...
Top Forums Programming When I am writing my own interpreter... Post 302142009 by Legend986 on Wednesday 24th of October 2007 12:57:38 AM
Old 10-24-2007
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 HPUX

sh

sh(1)							      General Commands Manual							     sh(1)

NAME

       sh - overview of various system shells

SYNOPSIS

   POSIX Shell
       option] ...  string] [arg ...]

       option] ...  string] [arg ...]

   Korn Shell
       option] ...  string] [arg ...]

       option] ...  string] [arg ...]

   C Shell
       [command_file] [argument_list ...]

   Key Shell
DESCRIPTION

   Remarks
       The POSIX .2 standard requires that, on a POSIX-compliant system, executing the command activates the POSIX shell (located in file on HP-UX
       systems), and executing the command produces an on-line manual entry that displays the syntax of the POSIX shell command-line.

       However, the command has historically been associated with the conventional Bourne shell, which could confuse some users.   To  meet  stan-
       dards  requirements  and also clarify the relationships of the various shells and where they reside on the system, this entry provides com-
       mand-line syntax and a brief description of each shell, and lists the names of the manual entries where each shell is described in  greater
       detail.

       The Bourne shell is removed from the system starting with HP-UX 11i Version 1.5.  Please use the POSIX shell as an alternative.

   Shell Descriptions
       The HP-UX operating system supports the following shells:

	      POSIX-conforming command programming language and command interpreter
			residing  in  file Can execute commands read from a terminal or a file.  This shell conforms to current POSIX standards in
			effect at the time the HP-UX system release was introduced, and is similar to the Korn shell in many respects.	Similar in
			many  respects to the Korn shell, the POSIX shell contains a history mechanism, supports job control, and provides various
			other useful features.

	      Korn-shell command programming language and commands interpreter
			residing in file Can execute commands read from a terminal or a file.  This shell, like the POSIX shell, contains  a  his-
			tory mechanism, supports job control, and provides various other useful features.

	      A command language interpreter
			that incorporates a command history buffer, C-language-like syntax, and job control facilities.

	      Restricted version of the POSIX shell command interpreter.
			Sets  up  a  login  name  and  execution  environment whose capabilities are more controlled (restricted) than normal user
			shells.

	      restricted version of the Korn-shell command interpreter
			Sets up a login name and execution environment whose capabilities  are	more  controlled  (restricted)	than  normal  user
			shells.

	      An extension of the standard Korn Shell
			that uses hierarchical softkey menus and context-sensitive help.

							  +--------------+--------------------+
							  | To obtain:	 |  Use the command:  |
							  +--------------+--------------------+
							  | POSIX Shell  |  /usr/bin/sh ...   |
							  | Korn Shell	 |  /usr/bin/ksh ...  |
							  | C Shell	 |  /usr/bin/csh ...  |
							  | Key Shell	 |  /usr/bin/keysh    |
							  +--------------+--------------------+
       These shells can also be the default invocation, depending on the entry in the file.  See also chsh(1).

WARNINGS

       Many manual entries contain descriptions of shell behavior or describe program or application behavior similar to ``the shell'' with a ref-
       erence to ``see sh(1)''.

SEE ALSO

       For more information on the various individual shells, see:

       keysh(1)       Key Shell description.

       ksh(1)	      Korn Shell description.

       sh-posix(1)    POSIX Shell description.

       csh(1)	      C Shell description.

																	     sh(1)
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