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C/C++ Source or Header  |  1994-03-20  |  1.7 KB  |  97 lines
  1. #include "f2c.h"
  2. #include "fio.h"
  3. #ifdef KR_headers
  4. integer f_back(a) alist *a;
  5. #else
  6. integer f_back(alist *a)
  7. #endif
  8. {    unit *b;
  9.     int i, n, ndec;
  10. #ifdef MSDOS
  11.     int j, k;
  12.     long w, z;
  13. #endif
  14.     long x, y;
  15.     char buf[32];
  16.     if(a->aunit >= MXUNIT || a->aunit < 0)
  17.         err(a->aerr,101,"backspace")
  18.     b= &f__units[a->aunit];
  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.         (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(b->ufd);
  36.         y = x % b->url;
  37.         if(y == 0) x--;
  38.         x /= b->url;
  39.         x *= b->url;
  40.         (void) fseek(b->ufd,x,SEEK_SET);
  41.         return(0);
  42.     }
  43.  
  44.     if(b->ufmt==0)
  45.     {    (void) fseek(b->ufd,-(long)sizeof(int),SEEK_CUR);
  46.         (void) fread((char *)&n,sizeof(int),1,b->ufd);
  47.         (void) fseek(b->ufd,-(long)n-2*sizeof(int),SEEK_CUR);
  48.         return(0);
  49.     }
  50. #ifdef MSDOS
  51.     w = -1;
  52. #endif
  53.     for(ndec = 2;; ndec = 1)
  54.     {
  55.         y = x=ftell(b->ufd);
  56.         if(x<sizeof(buf)) x=0;
  57.         else x -= sizeof(buf);
  58.         (void) fseek(b->ufd,x,SEEK_SET);
  59.         n=fread(buf,1,(int)(y-x), b->ufd);
  60.         for(i=n-ndec;i>=0;i--)
  61.         {
  62.             if(buf[i]!='\n') continue;
  63. #ifdef MSDOS
  64.             for(j = k = 0; j <= i; j++)
  65.                 if (buf[j] == '\n')
  66.                     k++;
  67.             fseek(b->ufd,x,SEEK_SET);
  68.             do {
  69.                 if (getc(b->ufd) == '\n') {
  70.                     --k;
  71.                     if ((z = ftell(b->ufd)) >= y) {
  72.                         if (w == -1)
  73.                             goto break2;
  74.                         break;
  75.                         }
  76.                     w = z;
  77.                     }
  78.                 } while(k > 0);
  79.             fseek(b->ufd, w, SEEK_SET);
  80. #else
  81.             fseek(b->ufd,(long)(i+1-n),SEEK_CUR);
  82. #endif
  83.             return(0);
  84.         }
  85. #ifdef MSDOS
  86.  break2:
  87. #endif
  88.         if(x==0)
  89.             {
  90.             (void) fseek(b->ufd, 0L, SEEK_SET);
  91.             return(0);
  92.             }
  93.         else if(n<=0) err(a->aerr,(EOF),"backspace")
  94.         (void) fseek(b->ufd, x, SEEK_SET);
  95.     }
  96. }
  97.