ftruncate(2) -- Linux man page
NAMEtruncate, ftruncate - truncate a file to a specified length
DESCRIPTIONThe 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.
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errno is set appropriately.
- Search permission is denied for a component of the path prefix, or the named file is not writable by the user.
- Path points outside the process's allocated address space.
- The argument length is larger than the maximum file size. (XSI)
- A signal was caught during execution.
- The argument length is negative or larger than the maximum file size.
- An I/O error occurred updating the inode.
- The named file is a directory.
- Too many symbolic links were encountered in translating the pathname.
- A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
- The named file does not exist.
- A component of the path prefix is not a directory.
- 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:
- The fd is not a valid descriptor.
- EBADF or EINVAL
- The fd is not open for writing.
- The fd does not reference a regular file.
CONFORMING TO4.4BSD, 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.
NOTESThe 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.)