/*  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	1main(){	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);}