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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tcl_getopenfile

Tcl_GetOpenFile(3)					      Tcl Library Procedures						Tcl_GetOpenFile(3)

__________________________________________________________________________________________________________________________________________________

NAME

       Tcl_GetOpenFile - Return a FILE* for a channel registered in the given interpreter (Unix only)

SYNOPSIS

       #include <tcl.h>

       int
       Tcl_GetOpenFile(interp, chanID, write, checkUsage, filePtr)

ARGUMENTS

       Tcl_Interp *interp (in)		   Tcl interpreter from which file handle is to be obtained.

       const char *chanID (in)		   String identifying channel, such as stdin or file4.

       int write (in)			   Non-zero means the file will be used for writing, zero means it will be used for reading.

       int checkUsage (in)		   If  non-zero,  then	an  error will be generated if the file was not opened for the access indicated by
					   write.

       ClientData *filePtr (out)	   Points to word in which to store pointer to FILE structure for the file given by chanID.
_________________________________________________________________

DESCRIPTION

       Tcl_GetOpenFile takes as argument a file identifier of the form returned by the open command and returns at *filePtr a pointer to the  FILE
       structure for the file.	The write argument indicates whether the FILE pointer will be used for reading or writing.  In some cases, such as
       a channel that connects to a pipeline of subprocesses, different FILE pointers will be returned for reading and	writing.   Tcl_GetOpenFile
       normally  returns TCL_OK.  If an error occurs in Tcl_GetOpenFile (e.g. chanID did not make any sense or checkUsage was set and the file was
       not opened for the access specified by write) then TCL_ERROR is returned and the interpreter's result will contain an  error  message.	In
       the current implementation checkUsage is ignored and consistency checks are always performed.

       Note that this interface is only supported on the Unix platform.

KEYWORDS

       channel, file handle, permissions, pipeline, read, write

Tcl									8.0							Tcl_GetOpenFile(3)