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Shell Implementation not working correctly

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# 8  
Old 11-07-2007
Yes, tmpin will be 3, and tmpout will be 4. These will both be open file descriptors that the child processes will inherit.
# 9  
Old 11-07-2007
Ok yes i see. I had a different idea of what dup did, i have only been doing this Unix stuff for about a week. But in reallity it wont really change my program to close those because they will just close when the child process closes, its just messy. The real question is why my stupid pipes are not working. It Makes No Sense! Smilie. But thanks man.
# 10  
Old 11-07-2007
I'll give it a compile and have a look.
# 11  
Old 11-07-2007
Oh, and another peeve of mine, don't use C++ comments in C code. Smilie

EDIT: Oops, this is actually C++ code as the for statement defines the variable.

Last edited by porter; 11-07-2007 at 04:39 PM..
# 12  
Old 11-07-2007
man pipe


the first element is the read end, the second element is the write end of the pipe.

On some platforms(Solaris) pipes are bidirectional, but this is classical UNIX.

Also, you need to reap (with some form of wait) all the pids you get back from fork, not just the last one, else you end up with zombies, so I would expect you to capture all those pids.

Last edited by porter; 11-07-2007 at 04:45 PM..
# 13  
Old 11-08-2007
Yes my good man. But if you look at the drawing and think about it the output of the command that is going to run will need to be written to the input of next command. So making the output write to that file will pipe over to the input file and the 0th descriptor will be set to that at the top of the loop. Just look at the pics and think about it and you will get it.
# 14  
Old 11-08-2007
Originally Posted by AirBronto
Just look at the pics and think about it and you will get it.
Perhaps, but the difference between you and me is I have a working pipeline from your code with that small change.

$ uname -a
SunOS foobar 5.7 Generic_106541-08 sun4c sparc SUNW,Sun_4_50
$ /usr/local/bin/gcc ab.c -o ab
$ echo "fred" | ./ab
0+1 records in
0+1 records out
0+1 records in
0+1 records out
0+1 records in
0+1 records out
0+1 records in
0+1 records out

and the complete source should look very familiar....

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <fcntl.h>

struct cmd
  char **_arguments;

char *args[]={

struct cmd cmdList[]={

struct cmd *_simpleCommands[]={

char *_inputFile;
char *_outFile;

int main(int argc,char **argv)
int _background=0;
int _numberOfSimpleCommands=(sizeof(cmdList)
	int tmpin=dup(0);
	int tmpout=dup(1);
	int ret=-1;
	int fdout=-1;
	int fdin=-1;
	int i;
	int rc=0;

	   fdin = open(_inputFile, O_RDONLY | O_CREAT, S_IREAD | S_IWRITE);

	for (i = 0; i < _numberOfSimpleCommands; i++)
	   rc=dup2(fdin, 0); if (rc==-1) perror("dup2(fdin)");
	   close(fdin); fdin=-1;

	   if(i == _numberOfSimpleCommands-1) 
	         fdout=open(_outFile, O_WRONLY | O_CREAT, S_IREAD | S_IWRITE);
	      int fdpipe[2];


	    rc=dup2(fdout,1); if (rc==-1) perror("dup2(fdout)");

	    close(fdout); fdout=-1;

	    ret = fork();
			rc=close(tmpin); if (rc) perror("close(tmpin)");
			rc=close(tmpout); if (rc) perror("close(tmpout)");

			execvp( (*_simpleCommands)[i]._arguments[0],


	rc=dup2(tmpin, 0); if (rc==-1) perror("dup2(tmpin,0)");
	rc=dup2(tmpout, 1); if (rc==-1) perror("dup2(tmpin,0)");
		waitpid(ret, 0, 0);

    return 0;

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