text: SYNOPSIS
  #include <unistd.h>
  int ftruncate(int fildes, off_t length);
  DESCRIPTION
  If fildes is not a valid file descriptor open for writing, the ftrun-
  c either increase the size of the file or
  fail. XSI-conformant systems, the
  ftruncate() function shall mark for update the st_ctime and st_mtime
  fields.
  EINVAL The fildes argument references a file that was opened without
  write permission.
  EROFS The named file resides on a read-only file system.
  The following sections are informative.
  EXAMPLES
  None.
  APPLICATION USAGE
  None.
  RATIONALE
  The ftruncate() function is part of IEEE Std 1003.1-2001 as it was
  deemed to be more useful than truncate(). The truncate() function is
  provided as an XSI extension.
  FUTURE DIRECTIONS
  None.
  SEE ALSO
  open() , truncate() , .