compiler/clib/lseek.c

   1 /*
   2     Copyright © 1995-2003, The AROS Development Team. All rights reserved.
   3     $Id$
   4
   5     Reposition read/write file offset.
   6 */
   7 #include <errno.h>
   8 #include <dos/dos.h>
   9 #include <proto/dos.h>
  10 #include <exec/exec.h>
  11 #include <proto/exec.h>
  12 #include <clib/macros.h>
  13 #include "__errno.h"
  14 #include "__open.h"
  15
  16 /*****************************************************************************
  17
  18     NAME */
  19 #include <unistd.h>
  20
  21         off_t lseek (
  22
  23 /*  SYNOPSIS */
  24         int    filedes,
  25         off_t  offset,
  26         int    whence)
  27
  28 /*  FUNCTION
  29         Reposition read/write file offset
  30
  31     INPUTS
  32         filedef - the filedescriptor being modified
  33         offset, whence -
  34                   How to modify the current position. whence
  35                   can be SEEK_SET, then offset is the absolute position
  36                   in the file (0 is the first byte), SEEK_CUR then the
  37                   position will change by offset (ie. -5 means to move
  38                   5 bytes to the beginning of the file) or SEEK_END.
  39                   SEEK_END means that the offset is relative to the
  40                   end of the file (-1 is the last byte and 0 is
  41                   the EOF).
  42
  43     RESULT
  44         The new position on success and -1 on error. If an error occurred, the global
  45         variable errno is set.
  46
  47     NOTES
  48
  49     EXAMPLE
  50
  51     BUGS
  52         File is extended with zeros if desired position is beyond the end of
  53         file.
  54
  55         Since it's not possible to use Seek() for directories, this
  56         implementation fails with EISDIR for directory file descriptors.
  57
  58     SEE ALSO
  59         fopen(), fwrite()
  60
  61     INTERNALS
  62
  63 ******************************************************************************/
  64 {
  65     int  cnt;
  66     fdesc *fdesc = __getfdesc(filedes);
  67
  68     if (!fdesc)
  69     {
  70         errno = EBADF;
  71         return -1;
  72     }
  73
  74     if(fdesc->fcb->isdir)
  75     {
  76         errno = EISDIR;
  77         return -1;
  78     }
  79
  80     switch (whence)
  81     {
  82         case SEEK_SET: whence = OFFSET_BEGINNING; break;
  83         case SEEK_CUR: whence = OFFSET_CURRENT; break;
  84         case SEEK_END: whence = OFFSET_END; break;
  85
  86         default:
  87             errno = EINVAL;
  88             return -1;
  89     }
  90
  91     cnt = Seek ((BPTR)fdesc->fcb->fh, offset, whence);
  92
  93     if (cnt == -1)
  94     {
  95         if(IoErr() == ERROR_SEEK_ERROR)
  96         {
  97             LONG saved_error = IoErr();
  98             /* Most likely we tried to seek behind EOF. POSIX lseek allows
  99                that, and if anything is written at the end on the gap,
 100                reads from the gap should return 0 unless some real data
 101                is written there. Since implementing it would be rather
 102                difficult, we simply extend the file by writing zeros
 103                and hope for the best. */
 104             LONG abs_cur_pos = Seek(fdesc->fcb->fh, 0, OFFSET_CURRENT);
 105             if(abs_cur_pos == -1)
 106                 goto error;
 107             LONG file_size = Seek(fdesc->fcb->fh, 0, OFFSET_END);
 108             if(file_size == -1)
 109                 goto error;
 110             /* Now compute how much we have to extend the file */
 111             LONG abs_new_pos = 0;
 112             switch(whence)
 113             {
 114                 case OFFSET_BEGINNING: abs_new_pos = offset; break;
 115                 case OFFSET_CURRENT: abs_new_pos = abs_cur_pos + offset; break;
 116                 case OFFSET_END: abs_new_pos = file_size + offset; break;
 117             }
 118             if(abs_new_pos > abs_cur_pos)
 119             {
 120                 ULONG bufsize = 4096;
 121                 APTR zeros = AllocMem(bufsize, MEMF_ANY | MEMF_CLEAR);
 122                 if(!zeros)
 123                 {
 124                     /* Restore previous position */
 125                     Seek(fdesc->fcb->fh, abs_cur_pos, OFFSET_BEGINNING);
 126                     errno = ENOMEM;
 127                     return -1;
 128                 }
 129
 130                 LONG towrite = abs_new_pos - abs_cur_pos;
 131                 do
 132                 {
 133                     Write(fdesc->fcb->fh, zeros, MIN(towrite, bufsize));
 134                     towrite -= bufsize;
 135                 }
 136                 while(towrite > 0);
 137
 138                 FreeMem(zeros, bufsize);
 139             }
 140             else
 141             {
 142                 /* Hmm, that's strange. Looks like ERROR_SEEK_ERROR has
 143                    been caused by something else */
 144                 SetIoErr(saved_error);
 145                 goto error;
 146             }
 147         }
 148         else
 149             goto error;
 150     }
 151
 152     return Seek((BPTR)fdesc->fcb->fh, 0, OFFSET_CURRENT);
 153 error:
 154     errno = IoErr2errno (IoErr ());
 155     return (off_t) -1;
 156 } /* lseek */