clock clicks ?-option?
clock clicks returns a high-resolution time value as a system-dependent integer value. See the clock reference page for more detail.
If -milliseconds is provided as an option, then the command is synonomous with clock milliseconds. This usage is OBSOLETE.
If -microseconds is provided as an option, then the command is synonomous with clock microseconds. This usage is OBSOLETE.
In Classic MacOS, the clicks were typically as long as 1/60 second, but in recent MacOS X, they are instead so fast that
expr {[clock clicks] == [clock clicks]}never seems to return true (more than µs resolution). Other platforms still have had clicks = milliseconds, and this assumption is sometimes made in Tcl code.
In other words, the uniqueness of this value is often high but notoriously unreliable, especially for cross-platform comparisons. An alternative source when generating unique IDs (cf. GUID and UUID) is info cmdcount.
Scott Beasley: I feel that info cmdcount is totally useless for Unique key generation. It stays very small in size for too long and changes at a MUCH lower rate then does clock clicks in a modern tcl (8.4-8.5) system. Not saying it couldn't be used as an "enhancement" number though in the generation process.
Lars H: Why is it a problem to be small? The need (as I understand it) is for uniqueness of values, not for having them look random.
Scott Beasley: A uuid is 128 bits. You need enough in terms of size and variation to generate one. I do see the value of info cmdcount in combination with clock clicks on older systems. I may look into that for my needs as well and publish my results.
Lars H: OK, so essentially a randomness argument — I'm generating these things, others are too, and we want to be reasonably sure there won't be a collision. Not all unique-IDs are supposed to be "universally" unique however, many just need to be locally unique.