libf2c/libI77/backspace.c

   1 #include <sys/types.h>
   2 #include "f2c.h"
   3 #include "fio.h"
   4 #ifdef KR_headers
   5 integer f_back(a) alist *a;
   6 #else
   7 integer f_back(alist *a)
   8 #endif
   9 {       unit *b;
  10         long v, w, x, y, z;
  11         uiolen n;
  12         FILE *f;
  13
  14         f__curunit = b = &f__units[a->aunit];   /* curunit for error messages */
  15         if (f__init & 2)
  16                 f__fatal (131, "I/O recursion");
  17         if(a->aunit >= MXUNIT || a->aunit < 0)
  18                 err(a->aerr,101,"backspace");
  19         if(b->useek==0) err(a->aerr,106,"backspace");
  20         if(b->ufd == NULL) {
  21                 fk_open(1, 1, a->aunit);
  22                 return(0);
  23                 }
  24         if(b->uend==1)
  25         {       b->uend=0;
  26                 return(0);
  27         }
  28         if(b->uwrt) {
  29                 t_runc(a);
  30                 if (f__nowreading(b))
  31                         err(a->aerr,errno,"backspace");
  32                 }
  33         f = b->ufd;     /* may have changed in t_runc() */
  34         if(b->url>0)
  35         {
  36                 x=ftell(f);
  37                 y = x % b->url;
  38                 if(y == 0) x--;
  39                 x /= b->url;
  40                 x *= b->url;
  41                 (void) fseek(f,x,SEEK_SET);
  42                 return(0);
  43         }
  44
  45         if(b->ufmt==0)
  46         {       fseek(f,-(long)sizeof(uiolen),SEEK_CUR);
  47                 fread((char *)&n,sizeof(uiolen),1,f);
  48                 fseek(f,-(long)n-2*sizeof(uiolen),SEEK_CUR);
  49                 return(0);
  50         }
  51         w = x = ftell(f);
  52         z = 0;
  53  loop:
  54         while(x) {
  55                 x -= x < 64 ? x : 64;
  56                 fseek(f,x,SEEK_SET);
  57                 for(y = x; y < w; y++) {
  58                         if (getc(f) != '\n')
  59                                 continue;
  60                         v = ftell(f);
  61                         if (v == w) {
  62                                 if (z)
  63                                         goto break2;
  64                                 goto loop;
  65                                 }
  66                         z = v;
  67                         }
  68                 err(a->aerr,(EOF),"backspace");
  69                 }
  70  break2:
  71         fseek(f, z, SEEK_SET);
  72         return 0;
  73 }