select -- wait for files to come available or satisfy conditions


select readfileIds [writefileIds] [exceptfileIds] [timeout]


This command allows an Extended Tcl program to wait for zero or more files being available for reading, writing, have an exceptional condition pending, or for a timeout period to expire. readFileIds, writeFileIds, exceptFileIds are each lists of fileIds, as returned from open, to query. An empty list ({}) may be specified if a category is not used.

The files specified by the readFileIds list are checked to see if data is available for reading. The writeFileIds are checked if the specified files are clear for writing. The exceptFileIds are checked to see if an exceptional condition has occurred (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 becoming 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 fileIds 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 {} {}