As random as I wanna be: Why cmd.exe's %RANDOM% isn't so random
Somebody on my team reported that
a particular script in our project’s build process
would fail with bizarre output maybe
once in fifty tries.
This script was run from a
and the result was a failed build.
Rerunning make fixed the problem,
but that’s not much consolation when the build lab encounters it
approximately every other day.
The strange thing about the bizarre output was that it appeared
to contain a mix of two different runs.
How could the output of two runs be mixed into one output file?
The script was a batch file, and it generated its output in
a few different steps,
storing the intermediate output in randomly-named temporary files,
taking advantage of the
pseudovariable to generate the name of those temporary files.
%TEMP%\%RANDOM%.tmp2, you get the idea.)
Cutting to the chase: The reason for the mixed output was that
%RANDOM% pseudo-variable wasn’t random enough.
If two copies of the script are running at the same time,
they will get the same “random” number and end up
mixing their output together.
(And running multiple builds at the same time is something the
people in the build lab are wont to do.)
It turns out that the Windows command processor uses the standard naïve algorithm for seeding the random number generator:
time has a resolution of one second,
two command prompts launched in rapid succession have a good
chance of seeding the random number generator with the same
timestamp, which means that they will have the same random number
C> copy con notsorandom.cmd @pause @echo %RANDOM% ^Z 1 file(s) copied. C> for /l %i in (1,1,3000) do @cmd /c notsorandom.cmd // hold down the space bar Press any key to continue . . . 14153 Press any key to continue . . . 14153 Press any key to continue . . . 14153 Press any key to continue . . . 14153 Press any key to continue . . . 14156 Press any key to continue . . . 14156 Press any key to continue . . . 14156 Press any key to continue . . . 14156 Press any key to continue . . . 14156 Press any key to continue . . . 14160 Press any key to continue . . . 14160 Press any key to continue . . . 14160
Notice that the
%RANDOM% pseudovariable generates
the same “random” number until the clock ticks over another second.
(Notice also that the “random” numbers don’t look all that random.)
We fixed the script so it generated its temporary file in the
project’s output directory rather than in the (shared)
That way, even if two copies of the project are building at the same
time, they will generate their temporary files in different directories
and not step on each other.
Exercise: There is much subtlety in that
Describe alternative formulations of the
both those that work and those that don’t.
To get you started: Explain the output of this variation:
for /l %i in (1,1,300) do @(pause&echo %RANDOM%)
Obligatory batch file bashing: Every time I write an entry about batch files, you can count on people complaining about how insane the batch programming language is. The batch language wasn’t designed; it evolved. (And according to commenter Daev, it followed a form of parallel evolution from what most people are familiar with.) I doubt anybody actually enjoy writing batch files. At best you tolerate it. I’m just trying to make it slightly more tolerable. I bet these are the same people who complain to their tax preparer about the complexity of tax law.