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((f = 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                 (void) t_runc(a);
  30                 if (f__nowreading(b))
  31                         err(a->aerr,errno,"backspace");
  32                 }
  33         if(b->url>0)
  34         {
  35                 x=ftell(f);
  36                 y = x % b->url;
  37                 if(y == 0) x--;
  38                 x /= b->url;
  39                 x *= b->url;
  40                 (void) fseek(f,x,SEEK_SET);
  41                 return(0);
  42         }
  43
  44         if(b->ufmt==0)
  45         {       fseek(f,-(long)sizeof(uiolen),SEEK_CUR);
  46                 fread((char *)&n,sizeof(uiolen),1,f);
  47                 fseek(f,-(long)n-2*sizeof(uiolen),SEEK_CUR);
  48                 return(0);
  49         }
  50         w = x = ftell(f);
  51         z = 0;
  52  loop:
  53         while(x) {
  54                 x -= x < 64 ? x : 64;
  55                 fseek(f,x,SEEK_SET);
  56                 for(y = x; y < w; y++) {
  57                         if (getc(f) != '\n')
  58                                 continue;
  59                         v = ftell(f);
  60                         if (v == w) {
  61                                 if (z)
  62                                         goto break2;
  63                                 goto loop;
  64                                 }
  65                         z = v;
  66                         }
  67                 err(a->aerr,(EOF),"backspace");
  68                 }
  69  break2:
  70         fseek(f, z, SEEK_SET);
  71         return 0;
  72 }