FTRUNCATE(3P)                                       POSIX Programmer's Manual                                      FTRUNCATE(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       ftruncate - truncate a file to a specified length

SYNOPSIS
       #include <unistd.h>

       int ftruncate(int fildes, off_t length);


DESCRIPTION
       If fildes is not a valid file descriptor open for writing, the ftruncate() function shall fail.

       If  fildes refers to a regular file, the ftruncate() function shall cause the size of the file to be truncated to length.
       If the size of the file previously exceeded length, the extra data shall no longer be available to reads on the file.  If
       the  file  previously  was smaller than this size, ftruncate() shall either increase the size of the file or fail.   XSI-
       conformant systems shall increase the size of the file.  If the file size is increased, the extended area shall appear as
       if it were zero-filled. The value of the seek pointer shall not be modified by a call to ftruncate().

       Upon  successful  completion,  if  fildes  refers  to  a regular file, the ftruncate() function shall mark for update the
       st_ctime and st_mtime fields of the file and the S_ISUID and S_ISGID bits of the file mode may be cleared. If the  ftrun-
       cate() function is unsuccessful, the file is unaffected.

       If  the  request would cause the file size to exceed the soft file size limit for the process, the request shall fail and
       the implementation shall generate the SIGXFSZ signal for the thread.

       If fildes refers to a directory, ftruncate() shall fail.

       If fildes refers to any other file type, except a shared memory object, the result is unspecified.

       If fildes refers to a shared memory object, ftruncate() shall set the size of the shared memory object to length.

       If the effect of ftruncate() is to decrease the size of a shared memory object or memory  mapped  file  and  whole  pages
       beyond the new end were previously mapped, then the whole pages beyond the new end shall be discarded.

       If  the  Memory  Protection option is supported, references to discarded pages shall result in the generation of a SIGBUS
       signal; otherwise, the result of such references is undefined.

       If the effect of ftruncate() is to increase the size of a shared memory object, it is unspecified whether the contents of
       any mapped pages between the old end-of-file and the new are flushed to the underlying object.

RETURN VALUE
       Upon  successful  completion,  ftruncate()  shall return 0; otherwise, -1 shall be returned and errno set to indicate the
       error.

ERRORS
       The ftruncate() function shall fail if:

       EINTR  A signal was caught during execution.

       EINVAL The length argument was less than 0.

       EFBIG or EINVAL

              The length argument was greater than the maximum file size.

       EFBIG  The file is a regular file and length is greater than the offset maximum established in the open file  description
              associated with fildes.

       EIO    An I/O error occurred while reading from or writing to a file system.

       EBADF or EINVAL

              The fildes argument is not a file descriptor open for writing.

       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 trun-
       cate() function is provided as an XSI extension.

FUTURE DIRECTIONS
       None.

SEE ALSO
       open(), truncate(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                 FTRUNCATE(3P)