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.