#include "f2c.h"
#include "fio.h"
#ifdef KR_headers
integer f_back(a) alist *a;
#else
integer f_back(alist *a)
#endif
{	unit *b;
	int i, n, ndec;
#ifdef MSDOS
	int j, k;
	long w, z;
#endif
	long x, y;
	char buf[32];
	if(a->aunit >= MXUNIT || a->aunit < 0)
		err(a->aerr,101,"backspace")
	b= &f__units[a->aunit];
	if(b->useek==0) err(a->aerr,106,"backspace")
	if(b->ufd==NULL) {
		fk_open(1, 1, a->aunit);
		return(0);
		}
	if(b->uend==1)
	{	b->uend=0;
		return(0);
	}
	if(b->uwrt) {
		(void) t_runc(a);
		if (f__nowreading(b))
			err(a->aerr,errno,"backspace")
		}
	if(b->url>0)
	{
		x=ftell(b->ufd);
		y = x % b->url;
		if(y == 0) x--;
		x /= b->url;
		x *= b->url;
		(void) fseek(b->ufd,x,SEEK_SET);
		return(0);
	}

	if(b->ufmt==0)
	{	(void) fseek(b->ufd,-(long)sizeof(int),SEEK_CUR);
		(void) fread((char *)&n,sizeof(int),1,b->ufd);
		(void) fseek(b->ufd,-(long)n-2*sizeof(int),SEEK_CUR);
		return(0);
	}
#ifdef MSDOS
	w = -1;
#endif
	for(ndec = 1;; ndec = 0)
	{
		y = x = ftell(b->ufd);
		if(x < sizeof(buf))
			x = 0;
		else
			x -= sizeof(buf);
		(void) fseek(b->ufd,x,SEEK_SET);
		n=fread(buf,1,(int)(y-x), b->ufd);
		for(i = n - ndec; --i >= 0; )
		{
			if(buf[i]!='\n') continue;
#ifdef MSDOS
			for(j = k = 0; j <= i; j++)
				if (buf[j] == '\n')
					k++;
			fseek(b->ufd,x,SEEK_SET);
			for(;;)
				if (getc(b->ufd) == '\n') {
					if ((z = ftell(b->ufd)) >= y && ndec) {
						if (w == -1)
							goto break2;
						break;
						}
					if (--k <= 0)
						return 0;
					w = z;
					}
			fseek(b->ufd, w, SEEK_SET);
#else
			fseek(b->ufd,(long)(i+1-n),SEEK_CUR);
#endif
			return(0);
		}
#ifdef MSDOS
 break2:
#endif
		if(x==0)
			{
			(void) fseek(b->ufd, 0L, SEEK_SET);
			return(0);
			}
		else if(n<=0) err(a->aerr,(EOF),"backspace")
		(void) fseek(b->ufd, x, SEEK_SET);
	}
}