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#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);
}
}
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