When I am writing my own interpreter... Post: 302142174

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Top Forums Programming When I am writing my own interpreter... Post 302142174 by Legend986 on Wednesday 24th of October 2007 11:01:06 PM

10-25-2007

Registered User

Hmm... I seem to be doing something wrong. Here's what I've been doing...

Code:

I'm creating a filedescriptor fd[2]
fork child1
if(pid1 == 0) {
        close(fd[1]); 
        setting fd[0] to STDIN_FILENO using dup2
         close(fd[0]);
        }
        execlp("wc","wc",(char *) 0);
    }
    else {
        fork child 2
        if(pid2 == 0) {
               close(fd[1]);  
               set fd[0] to STDIN_FILENO using dup2
               close(fd[0]);
               set fd[1] to STDOUT_FILENO using dup2
               close(fd[1]);
               execlp("wc","wc", (char *) 0);
        }
        else {
            //parent continues
            fork child 3
            if(pid3 == 0) {
                    close(fd[0]);
                    set fd[1] to STDOUT_FILENO using dup2
                    close(fd[1])
                    execlp("ls","-la", (char *) 0);
            }
        }

What I'm trying to do here is creating a two level pipe for the command: ls -la | wc | wc

It works upto one level but after that I'm getting an "dup2: Bad File Descriptor" error maybe because of the child 2... I'm not sure what mistake I'm doing... Any suggestions on how to get around this one?

Last edited by Legend986; 10-25-2007 at 12:08 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)