PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult 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
       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, ftruncate() shall mark for update the
       last data modification and last file status change timestamps of the file and  the  S_ISUID  and  S_ISGID
       bits  of  the  file  mode  may  be  cleared.  If  the  ftruncate()  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 memory mapped file or a shared memory object
       and whole pages beyond the new end were previously mapped, then the whole pages beyond the new end  shall
       be discarded.

       References to discarded pages shall result in the generation of a SIGBUS signal.

       If  the  effect  of ftruncate() is to increase the size of a 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.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       open(), truncate()

       The Base Definitions volume of POSIX.1‐2017, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                     FTRUNCATE(3POSIX)