TRUNCATE(2) Linux Programmer's Manual TRUNCATE(2)
truncate, ftruncate - truncate a file to a specified length
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
The truncate and ftruncate functions cause the regular file named by path or referenced by
fd to be truncated to a size of precisely length bytes.
If the file previously was larger than this size, the extra data is lost. If the file
previously was shorter, it is extended, and the extended part reads as zero bytes.
The file pointer is not changed.
If the size changed, then the ctime and mtime fields for the file are updated, and suid
and sgid mode bits may be cleared.
With ftruncate, the file must be open for writing; with truncate, the file must be
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EACCES Search permission is denied for a component of the path prefix, or the named file
is not writable by the user.
EFAULT Path points outside the process's allocated address space.
EFBIG The argument length is larger than the maximum file size. (XSI)
EINTR A signal was caught during execution.
EINVAL The argument length is negative or larger than the maximum file size.
EIO An I/O error occurred updating the inode.
EISDIR The named file is a directory.
ELOOP Too many symbolic links were encountered in translating the pathname.
A component of a pathname exceeded 255 characters, or an entire path name exceeded
ENOENT The named file does not exist.
A component of the path prefix is not a directory.
EROFS The named file resides on a read-only file system.
The file is a pure procedure (shared text) file that is being executed.
For ftruncate the same errors apply, but instead of things that can be wrong with path, we
now have things that can be wrong with fd:
EBADF The fd is not a valid descriptor.
EBADF or EINVAL
The fd is not open for writing.
EINVAL The fd does not reference a regular file.
4.4BSD, SVr4 (these function calls first appeared in BSD 4.2). POSIX 1003.1-1996 has
ftruncate. POSIX 1003.1-2001 also has truncate, as an XSI extension.
SVr4 documents additional truncate error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.
SVr4 documents for ftruncate an additional EAGAIN error condition.
The above description is for XSI-compliant systems. For non-XSI-compliant systems, the
POSIX standard allows two behaviours for ftruncate when length exceeds the file length
(note that truncate is not specified at all in such an environment): either returning an
error, or extending the file. (Most Unices follow the XSI requirement.)