/*  popen.c -  a version of the Unix popen() library function
 *	FILE *popen( char *command, char *mode )
 *		command is a regular shell command
 *		mode is "r" or "w" 
 *		returns a stream attached to the command, or NULL
 *		execls "sh" "-c" command
 *    todo: what about signal handling for child process?
 */
#include	<stdio.h>
#include	<signal.h>

#define	READ	0
#define	WRITE	1

main()
{
	FILE	*fp;
	char	buf[BUFSIZ];

	fp = popen("ls","r");
	while( fgets(buf, BUFSIZ,fp) != NULL)
		fputs(buf, stdout);
}


FILE *popen(const char *command, const char *mode)
{
	int	pfp[2], pid;		/* the pipe and the process	*/
	FILE	*fdopen(), *fp;		/* fdopen makes a fd a stream	*/
	int	parent_end, child_end;	/* of pipe 			*/

	if ( *mode == 'r' ){		/* figure out direction		*/
		parent_end = READ;
		child_end = WRITE ;
	} else if ( *mode == 'w' ){
		parent_end = WRITE;
	       	child_end = READ ;
	} else return NULL ;

	if ( pipe(pfp) == -1 ) 			/* get a pipe		*/
		return NULL;
	if ( (pid = fork()) == -1 ){		/* and a process	*/
		close(pfp[0]);			/* or dispose of pipe	*/
		close(pfp[1]);
		return NULL;
	}

	/* --------------- parent code here ------------------- */
	/*   need to close one end and fdopen other end		*/

	if ( pid > 0 ){	
		if (close( pfp[child_end] ) == -1 )
			return NULL;
		return fdopen( pfp[parent_end] , mode);	/* same mode */
	}

	/* --------------- child code here --------------------- */
	/*   need to redirect stdin or stdout then exec the cmd	 */

	if ( close(pfp[parent_end]) == -1 )	/* close the other end	*/
		exit(1);			/* do NOT return	*/

	if ( dup2(pfp[child_end], child_end) == -1 )
		exit(1);

	if ( close(pfp[child_end]) == -1 )	/* done with this one	*/
		exit(1);
						/* all set to run cmd	*/
	execl( "/bin/sh", "sh", "-c", command, NULL );
	exit(1);
}