4.1cBSD/usr/src/usr.lib/libI77/backspace.c

/*
char id_backspace[] = "@(#)backspace.c	1.2";
 *
 * Backspace records
 */

#include "fio.h"

char *bksp = "backspace";
char last_char();

f_back(a) alist *a;
{	unit *b;
	int n,i;
	long x,y;
	lfname = NULL;
	elist = NO;
	external = YES;
	errflag = a->aerr;
	lunit = a->aunit;
	if (not_legal(lunit)) err(errflag,F_ERUNIT,bksp)
	b= &units[lunit];
	if(!b->ufd && (n=fk_open(READ,SEQ,FMT,(ftnint)lunit)) )
		err(errflag,n,bksp)
	lfname = b->ufnm;
	if(b->uend)
	{	b->uend = NO;
		return(OK);
	}
	if((x=ftell(b->ufd))==0) return(OK);
	if(!b->useek) err(errflag,F_ERNOBKSP,bksp)
	if(b->uwrt) t_runc(b,errflag);
	if(b->url)		/* direct access, purely academic */
	{	y = x%(long)b->url;
		x -= y?y:b->url;
		fseek(b->ufd,x,0);
		return(OK);
	}
	if(!b->ufmt)		/* unformatted sequential */
	{	fseek(b->ufd,-(long)sizeof(int),1);
		fread((char *)&n,sizeof(int),1,b->ufd);
		fseek(b->ufd,-(long)n-2*sizeof(int),1);
		return(OK);
	}
	if(x==1)			/* formatted sequential */
	{	rewind(b->ufd);
		return(OK);
	}
	while(last_char(b->ufd)!='\n');	/* slow but simple */
	return(OK);
}