FTRUNCATE(P) POSIX Programmer's Manual FTRUNCATE(P)
ftruncate - 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 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 modi-
fied 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 ftruncate() function is unsuccessful, the file
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.
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 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.
The ftruncate() function is part of IEEE Std 1003.1-2001 as it was deemed to be more useful than truncate(). The truncate() function is
provided as an XSI extension.
open() , truncate() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (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.opengroup.org/unix/online.html .
/The Open Group 2003 FTRUNCATE(P)