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 (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEftruncate — truncate a file to a specified length
int ftruncate(int fildes, off_t length);
If fildes is not a valid file descriptor open for writing, the ftrun‐
cate() 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
Upon successful completion, if fildes refers to a regular file, ftrun‐
cate() 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 unsuc‐
cessful, 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
References to discarded pages shall result in the generation of a SIG‐
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
Upon successful completion, ftruncate() shall return 0; otherwise, −1
shall be returned and errno set to indicate the error.
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
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.
SEE ALSOopen(), truncate()
The Base Definitions volume of POSIX.1‐2008, <unistd.h>
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 Electri‐
cal 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 .
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IEEE/The Open Group 2013 FTRUNCATE(3P)