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



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

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(3)
