ftruncate is a command in TclX:
From the man pages: ---
ftruncate \[-fileid\] file newsize Truncate a file to have a length of at most newsize bytes. If the option -fileid is specified, file is an open file identifier, otherwise it is a file path. This command is not available or not fully functional if the underlying operating system support is not available. The command infox have_truncate will indicate if this command may truncate by file path. The command infox have_ftruncate will indicate if this command may truncate by file id. The -fileid option is not available on Windows 95/NT.