select readhandles [writehandles] [excepthandles] [timeout] This command allows an Extended Tcl program to wait on zero or more files being ready for for reading, writing, have an exceptional condition pending, or for a timeout period to expire. readhandles, writehandles, excepthandles are each lists of file handles, as returned from open, to query. An empty list ({}) may be specified if a category is not used. The files specified by the readhandles list are checked to see if data is available for reading. The writehandles are checked if the specified files are clear for writing. The excepthandles are checked to see if an exceptional condition has occured (typically, an error). The write and exception checking is most useful on devices, however, the read checking is very useful when communicating with multiple processes through pipes. Select considers data pending in the stdio input buffer for read files as being ready for reading, the files do. not have to be unbuffered. Timeout is a floating point timeout value, in seconds. If an empty list is supplied (or the parameter is omitted), then no timeout is set. If the value is zero, then the select command functions as a poll of the files, returning immediately even if none are ready. If the timeout period expires with none of the files becomming ready, then the command returns an empty list. Otherwise the command returns a list of three elements, each of those elements is a list of the file handles that are ready in the read, write and exception classes. If none are ready in a class, then that element will be the null list. For example: select {file3 file4 file5} {file6 file7} {} 10.5 could return {file3 file4} {file6} {} or perhaps file3 {} {}