lib/ftruncate.c

   1 /* ftruncate emulations that work on some System V's.
   2    This file is in the public domain. */
   3
   4 #include <sys/types.h>
   5 #include <fcntl.h>
   6
   7 #ifdef F_CHSIZE
   8 int
   9 ftruncate (fd, length)
  10      int fd;
  11      off_t length;
  12 {
  13   return fcntl (fd, F_CHSIZE, length);
  14 }
  15 #else
  16 #ifdef F_FREESP
  17 /* The following function was written by
  18    kucharsk@Solbourne.com (William Kucharski) */
  19
  20 #include <sys/stat.h>
  21 #include <errno.h>
  22 #include <unistd.h>
  23
  24 int
  25 ftruncate (fd, length)
  26      int fd;
  27      off_t length;
  28 {
  29   struct flock fl;
  30   struct stat filebuf;
  31
  32   if (fstat (fd, &filebuf) < 0)
  33     return -1;
  34
  35   if (filebuf.st_size < length)
  36     {
  37       /* Extend file length. */
  38       if (lseek (fd, (length - 1), SEEK_SET) < 0)
  39         return -1;
  40
  41       /* Write a "0" byte. */
  42       if (write (fd, "", 1) != 1)
  43         return -1;
  44     }
  45   else
  46     {
  47       /* Truncate length. */
  48       fl.l_whence = 0;
  49       fl.l_len = 0;
  50       fl.l_start = length;
  51       fl.l_type = F_WRLCK;      /* Write lock on file space. */
  52
  53       /* This relies on the UNDOCUMENTED F_FREESP argument to
  54          fcntl, which truncates the file so that it ends at the
  55          position indicated by fl.l_start.
  56          Will minor miracles never cease? */
  57       if (fcntl (fd, F_FREESP, &fl) < 0)
  58         return -1;
  59     }
  60
  61   return 0;
  62 }
  63 #else
  64 int
  65 ftruncate (fd, length)
  66      int fd;
  67      off_t length;
  68 {
  69   return chsize (fd, length);
  70 }
  71 #endif
  72 #endif