#include <sys/types.h>
#include "f2c.h"
#include "fio.h"
#ifdef KR_headers
integer f_back(a) alist *a;
#else
integer f_back(alist *a)
#endif
{	unit *b;
	long v, w, x, y, z;
	uiolen n;
	FILE *f;

	f__curunit = b = &f__units[a->aunit];	/* curunit for error messages */
	if (f__init & 2)
		f__fatal (131, "I/O recursion");
	if(a->aunit >= MXUNIT || a->aunit < 0)
		err(a->aerr,101,"backspace");
	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) {
		t_runc(a);
		if (f__nowreading(b))
			err(a->aerr,errno,"backspace");
		}
	f = b->ufd;	/* may have changed in t_runc() */
	if(b->url>0)
	{
		x=ftell(f);
		y = x % b->url;
		if(y == 0) x--;
		x /= b->url;
		x *= b->url;
		(void) fseek(f,x,SEEK_SET);
		return(0);
	}

	if(b->ufmt==0)
	{	fseek(f,-(long)sizeof(uiolen),SEEK_CUR);
		fread((char *)&n,sizeof(uiolen),1,f);
		fseek(f,-(long)n-2*sizeof(uiolen),SEEK_CUR);
		return(0);
	}
	w = x = ftell(f);
	z = 0;
 loop:
	while(x) {
		x -= x < 64 ? x : 64;
		fseek(f,x,SEEK_SET);
		for(y = x; y < w; y++) {
			if (getc(f) != '\n')
				continue;
			v = ftell(f);
			if (v == w) {
				if (z)
					goto break2;
				goto loop;
				}
			z = v;
			}
		err(a->aerr,(EOF),"backspace");
		}
 break2:
	fseek(f, z, SEEK_SET);
	return 0;
}
