compiler/clib/lseek.c

   1 /*
   2     Copyright © 1995-2012, 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 "__fdesc.h"
  14
  15 /*****************************************************************************
  16
  17     NAME */
  18 #include <unistd.h>
  19
  20         off_t lseek (
  21
  22 /*  SYNOPSIS */
  23         int    filedes,
  24         off_t  offset,
  25         int    whence)
  26
  27 /*  FUNCTION
  28         Reposition read/write file offset
  29
  30     INPUTS
  31         filedef - the filedescriptor being modified
  32         offset, whence -
  33                   How to modify the current position. whence
  34                   can be SEEK_SET, then offset is the absolute position
  35                   in the file (0 is the first byte), SEEK_CUR then the
  36                   position will change by offset (ie. -5 means to move
  37                   5 bytes to the beginning of the file) or SEEK_END.
  38                   SEEK_END means that the offset is relative to the
  39                   end of the file (-1 is the last byte and 0 is
  40                   the EOF).
  41
  42     RESULT
  43         The new position on success and -1 on error. If an error occurred, the global
  44         variable errno is set.
  45
  46     NOTES
  47
  48     EXAMPLE
  49
  50     BUGS
  51         File is extended with zeros if desired position is beyond the end of
  52         file.
  53
  54         Since it's not possible to use Seek() for directories, this
  55         implementation fails with EISDIR for directory file descriptors.
  56
  57     SEE ALSO
  58         fopen(), fwrite()
  59
  60     INTERNALS
  61
  62 ******************************************************************************/
  63 {
  64     int  cnt;
  65     fdesc *fdesc = __getfdesc(filedes);
  66
  67     if (!fdesc)
  68     {
  69         errno = EBADF;
  70         return -1;
  71     }
  72
  73     if(fdesc->fcb->isdir)
  74     {
  75         errno = EISDIR;
  76         return -1;
  77     }
  78
  79     switch (whence)
  80     {
  81         case SEEK_SET: whence = OFFSET_BEGINNING; break;
  82         case SEEK_CUR: whence = OFFSET_CURRENT; break;
  83         case SEEK_END: whence = OFFSET_END; break;
  84
  85         default:
  86             errno = EINVAL;
  87             return -1;
  88     }
  89
  90     cnt = Seek ((BPTR)fdesc->fcb->fh, offset, whence);
  91
  92     if (cnt == -1)
  93     {
  94         if(IoErr() == ERROR_SEEK_ERROR)
  95         {
  96             LONG saved_error = IoErr();
  97             /* Most likely we tried to seek behind EOF. POSIX lseek allows
  98                that, and if anything is written at the end on the gap,
  99                reads from the gap should return 0 unless some real data
 100                is written there. Since implementing it would be rather
 101                difficult, we simply extend the file by writing zeros
 102                and hope for the best. */
 103             LONG abs_cur_pos = Seek(fdesc->fcb->fh, 0, OFFSET_CURRENT);
 104             if(abs_cur_pos == -1)
 105                 goto error;
 106             LONG file_size = Seek(fdesc->fcb->fh, 0, OFFSET_END);
 107             if(file_size == -1)
 108                 goto error;
 109             /* Now compute how much we have to extend the file */
 110             LONG abs_new_pos = 0;
 111             switch(whence)
 112             {
 113                 case OFFSET_BEGINNING: abs_new_pos = offset; break;
 114                 case OFFSET_CURRENT: abs_new_pos = abs_cur_pos + offset; break;
 115                 case OFFSET_END: abs_new_pos = file_size + offset; break;
 116             }
 117             if(abs_new_pos > abs_cur_pos)
 118             {
 119                 ULONG bufsize = 4096;
 120                 APTR zeros = AllocMem(bufsize, MEMF_ANY | MEMF_CLEAR);
 121                 if(!zeros)
 122                 {
 123                     /* Restore previous position */
 124                     Seek(fdesc->fcb->fh, abs_cur_pos, OFFSET_BEGINNING);
 125                     errno = ENOMEM;
 126                     return -1;
 127                 }
 128
 129                 LONG towrite = abs_new_pos - abs_cur_pos;
 130                 do
 131                 {
 132                     Write(fdesc->fcb->fh, zeros, MIN(towrite, bufsize));
 133                     towrite -= bufsize;
 134                 }
 135                 while(towrite > 0);
 136
 137                 FreeMem(zeros, bufsize);
 138             }
 139             else
 140             {
 141                 /* Hmm, that's strange. Looks like ERROR_SEEK_ERROR has
 142                    been caused by something else */
 143                 SetIoErr(saved_error);
 144                 goto error;
 145             }
 146         }
 147         else
 148             goto error;
 149     }
 150
 151     return Seek((BPTR)fdesc->fcb->fh, 0, OFFSET_CURRENT);
 152 error:
 153     errno = __arosc_ioerr2errno (IoErr ());
 154     return (off_t) -1;
 155 } /* lseek */