lib/ftruncate.c

   1 /* ftruncate emulations that work on some System V's.
   2    This file is in the public domain.  */
   3
   4 #if HAVE_CONFIG_H
   5 # include <config.h>
   6 #endif
   7
   8 #include <sys/types.h>
   9 #include <fcntl.h>
  10
  11 #ifdef F_CHSIZE
  12
  13 int
  14 ftruncate (int fd, off_t length)
  15 {
  16   return fcntl (fd, F_CHSIZE, length);
  17 }
  18
  19 #else /* not F_CHSIZE */
  20 # ifdef F_FREESP
  21
  22 /* By William Kucharski <kucharsk@netcom.com>.  */
  23
  24 #  include <sys/stat.h>
  25 #  include <errno.h>
  26 #  if HAVE_UNISTD_H
  27 #   include <unistd.h>
  28 #  endif
  29
  30 int
  31 ftruncate (int fd, off_t length)
  32 {
  33   struct flock fl;
  34   struct stat filebuf;
  35
  36   if (fstat (fd, &filebuf) < 0)
  37     return -1;
  38
  39   if (filebuf.st_size < length)
  40     {
  41       /* Extend file length. */
  42       if (lseek (fd, (length - 1), SEEK_SET) < 0)
  43         return -1;
  44
  45       /* Write a "0" byte. */
  46       if (write (fd, "", 1) != 1)
  47         return -1;
  48     }
  49   else
  50     {
  51
  52       /* Truncate length. */
  53
  54       fl.l_whence = 0;
  55       fl.l_len = 0;
  56       fl.l_start = length;
  57       fl.l_type = F_WRLCK;      /* write lock on file space */
  58
  59       /* This relies on the *undocumented* F_FREESP argument to fcntl,
  60          which truncates the file so that it ends at the position
  61          indicated by fl.l_start.  Will minor miracles never cease?  */
  62
  63       if (fcntl (fd, F_FREESP, &fl) < 0)
  64         return -1;
  65     }
  66
  67   return 0;
  68 }
  69
  70 # else /* not F_CHSIZE nor F_FREESP */
  71 #  if HAVE_CHSIZE
  72
  73 int
  74 ftruncate (int fd, off_t length)
  75 {
  76   return chsize (fd, length);
  77 }
  78
  79 #  else /* not F_CHSIZE nor F_FREESP nor HAVE_CHSIZE */
  80
  81 #   include <errno.h>
  82 #   ifndef errno
  83 extern int errno;
  84 #   endif
  85
  86 int
  87 ftruncate (int fd, off_t length)
  88 {
  89   errno = EIO;
  90   return -1;
  91 }
  92
  93 #  endif /* not HAVE_CHSIZE */
  94 # endif /* not F_FREESP */
  95 #endif /* not F_CHSIZE */