/**	tinybc.c	* a tiny calculator that uses dc to do its work **			* demonstrates bidirectional pipes **			* input looks like number op number which **			  tinybc converts into number \n number \n op \n p **			  and passes result back to stdout ** **		+-----------+                +----------+ **	stdin  >0           >== pipetodc ====>          | **	        |  tinybc   |                |   dc -   | **	stdout <1           <== pipefromdc ==<          | **		+-----------+                +----------+ ** **			* program outline **				a. get two pipes **				b. fork (get another process) **				c. in the dc-to-be process, **					 connect stdin and out to pipes **					 then execl dc **				d. in the tinybc-process, no plumbing to do **					 just talk to human via normal i/o **					 and send stuff via pipe **				e. then close pipe and dc dies **                     * note: does not handle multiline answers **/#include	<stdio.h>#define	oops(m,x)	{ perror(m); exit(x); }main(){	int	pid, todc[2], fromdc[2];	/* equipment	*/	/* make two pipes */	if ( pipe(todc) == -1 || pipe(fromdc) == -1 )		oops("pipe failed", 1);	/* get a process for user interface */	if ( (pid = fork()) == -1 )		oops("cannot fork", 2);	if ( pid == 0 )				/* child is dc	*/		be_dc(todc, fromdc);	else {		be_bc(todc, fromdc);		/* parent is ui	*/		wait(NULL);			/* wait for child */	}}be_dc(int in[2], int out[2])/* *	set up stdin and stdout, then execl dc */{    /* setup stdin from pipein	*/	if ( dup2(in[0],0) == -1 )	/* copy read end to 0   */		oops("dc: cannot redirect stdin",3);	close(in[0]);			/* moved to fd 0	*/	close(in[1]);			/* won't write here	*/    /* setup stdout to pipeout	*/	if ( dup2(out[1], 1) == -1 )	/* dupe write end to 1	*/		oops("dc: cannot redirect stdout",4);	close(out[1]);			/* moved to fd 1	*/	close(out[0]);			/* won't read from here	*/    /* now execl dc with the - option */	execlp("dc", "dc", "-", NULL );	oops("Cannot run dc", 5);}be_bc(int todc[2], int fromdc[2])/* *	read from stdin and convert into to RPN, send down pipe *	then read from other pipe and print to user *	Uses fdopen() to convert a file descriptor to a stream */{	int	num1, num2;	char	operation[BUFSIZ], message[BUFSIZ], *fgets();	FILE	*fpout, *fpin, *fdopen();	/* setup */	close(todc[0]);			/* won't read from pipe to dc  */	close(fromdc[1]);		/* won't write to pipe from dc */	fpout = fdopen( todc[1],   "w" );	/* convert file desc-  */	fpin  = fdopen( fromdc[0], "r" );	/* riptors to streams  */	if ( fpout == NULL || fpin == NULL )		fatal("Error convering pipes to streams");	/* main loop */	while ( printf("tinybc: "), fgets(message,BUFSIZ,stdin) != NULL ){		/* parse input */		if ( sscanf(message,"%d%[-+*/^]%d",&num1,operation,&num2)!=3){			printf("syntax error\n");			continue;		}		if ( fprintf( fpout , "%d\n%d\n%c\np\n", num1, num2, 				*operation ) == EOF )					fatal("Error writing");		fflush(  fpout );		if ( fgets( message, BUFSIZ, fpin ) == NULL )			break;		printf("%d %c %d = %s", num1, *operation , num2, message);	}	fclose(fpout);		/* close pipe		*/	fclose(fpin);		/* dc will see EOF	*/}fatal( char *mess ){	fprintf(stderr, "Error: %s\n", mess);	exit(1);}