int ftruncate(int fildes, off_t length);
IEEE Std 1003.1-2017
The purpose is to truncate a file to a specified 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 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
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_ISGID bits of the file mode may be cleared.
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
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 object.
Upon successful completion,
ftruncate() shall return
-1 shall be returned and
errno set to indicate the error.
ftruncate() function shall fail if:
EINTR- A signal was caught during execution.
EINVAL- The length argument was less than
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.
The fildes argument is not a file descriptor open for writing.