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.
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
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
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
EBADF or EINVAL
The fildes argument is not a file descriptor open for writing.
The following sections are informative.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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
IEEE/The Open Group 2013 FTRUNCATE(3P)