Shell Implementation not working correctly

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# 1  
Old 11-04-2007
Shell Implementation not working correctly

	//save in/out
	int tmpin = dup(0);
	int tmpout = dup(1);
	//set initial input
	int fdin;
	   fdin = open(_inputFile, O_RDONLY | O_CREAT, S_IREAD | S_IWRITE);
	   //use default input
	   fdin = dup(tmpin);

	int ret;
	int fdout;
	for(int i = 0; i < _numberOfSimpleCommands; i++)
	   //redirect input
	   dup2(fdin, 0);
	   //setup output
	   if(i == _numberOfSimpleCommands-1) 
	   {// last command must be set so output redirection is set

	         fdout = open(_outFile, O_WRONLY | O_CREAT, S_IREAD | S_IWRITE);
	         fdout = dup(tmpout);
                   //simple command
	      //create pipe
	      int fdpipe[2];
                  fdout = fdpipe[0];
                  fdin = fdpipe[1];

	    //redirect output
	    dup2(fdout, 1);

	    //create child process
	    ret = fork();
	    if(ret == 0)
	     }//if ret

	//restore in/out defaults
	dup2(tmpin, 0);
	dup2(tmpout, 1);
		//wait for last command
		waitpid(ret, 0, 0);

This is the code i have for the implementation of a shell that supports pipes and i/o redirection

So far this code has worked for one input and one outout redirection.
But will not work for pipeing, and i cannot for the life of me figure out why.
Ex. cat | grep malloc

A simple command in my implementation is just a command with its arguments.

Any help would be great. Also please tell me if i need to clarify anything.

Last edited by AirBronto; 11-05-2007 at 04:12 PM..
# 2  
Old 11-04-2007
Just on code inspection....

I would close tmpin and tmpout in the child process.

Remember a child inherits every file descriptor, not just 0,1 and 2.

So the 2nd child will be holding the write end to it's own stdin open.
# 3  
Old 11-05-2007
This is how i see my code working with the file descriptors and pipes and well every thing. Take a look at the picture to see what i am talking about, it is very descriptive and took a long time to do in paint so please take a look.
# 4  
Old 11-06-2007
Originally Posted by porter
I would close tmpin and tmpout in the child process.
Do you understand what I mean here?

After the fork, in the child's leg, close these descriptors because the child does not want/need/expect them.
# 5  
Old 11-07-2007
At the point when fork is called, the parent process is not pointing to the standard input and output, tmpin and tmpout. So wouldnt the child not have anything pointing to those after fork?
# 6  
Old 11-07-2007
As far as I can see both tmpin and tmpout are both open immediately prior to the fork and hence will be both be open for both parent and child.

A suggestion, whenever you close a file descriptor, put -1 in the variable holding it. Then put in some debugging code immediately after the fork as follows...

if (ret==0)

# 7  
Old 11-07-2007
See when i do something like this:
tmpin = dup(0);
I am just saving the fact that my parent process's 0th file descriptor was pointing to stdin in tmpin.

By the time my program gets to fork(), the 0th file descriptor is eather pointing to a new input file or stdin if there was not one. And in both cases the child will need were ever the 0th is pointing to. It may be tmpin or not but if it is it will need that open so it can use it.
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