ftruncate is a command in TclX:
From the man pages:
Truncate 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.
MHo 10-Nov-2008: what about a file truncate <filename> subcommand in the core?
DKF: Also see chan truncate.