{"id":13673,"date":"2010-06-17T07:00:00","date_gmt":"2010-06-17T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2010\/06\/17\/as-random-as-i-wanna-be-why-cmd-exes-random-isnt-so-random\/"},"modified":"2010-06-17T07:00:00","modified_gmt":"2010-06-17T07:00:00","slug":"as-random-as-i-wanna-be-why-cmd-exes-random-isnt-so-random","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20100617-00\/?p=13673","title":{"rendered":"As random as I wanna be: Why cmd.exe's %RANDOM% isn't so random"},"content":{"rendered":"

\nSomebody on my team reported that\na particular script in our project’s build process\nwould fail with bizarre output maybe\nonce in fifty tries.\nThis script was run from a makefile<\/code>,\nand the result was a failed build.\nRerunning make<\/i> fixed the problem,\nbut that’s not much consolation when the build lab encounters it\napproximately every other day.\nThe strange thing about the bizarre output was that it appeared\nto contain a mix of two different runs.\nHow could the output of two runs be mixed into one output file?\n<\/p>\n

\nThe script was a batch file, and it generated its output in\na few different steps,\nstoring the intermediate output in randomly-named temporary files,\ntaking advantage of the\n\n%RANDOM%<\/code><\/a>\npseudovariable to generate the name of those temporary files.\n(They were %TEMP%\\%RANDOM%.tmp1<\/code>,\n%TEMP%\\%RANDOM%.tmp2<\/code>, you get the idea.)\n<\/p>\n

\nCutting to the chase: The reason for the mixed output was that\nthe %RANDOM%<\/code> pseudo-variable wasn’t random enough.\nIf two copies of the script are running at the same time,\nthey will get the same<\/i> “random” number and end up\nmixing their output together.\n(And running multiple builds at the same time is something the\npeople in the build lab are wont to do.)\n<\/p>\n

\nIt turns out that the Windows command processor uses\n\nthe standard naïve algorithm for seeding the random number generator<\/a>:\n<\/p>\n

\n   srand((unsigned)time(NULL));\n<\/pre>\n
\nSince time<\/code> has a resolution of one second,\ntwo command prompts launched in rapid succession have a good\nchance of seeding the random number generator with the same\ntimestamp, which means that they will have the same random number\nstream.\n<\/p>\n
\nC> copy con notsorandom.cmd\n@pause\n@echo %RANDOM%\n^Z\n        1 file(s) copied.\nC> for \/l %i in (1,1,3000) do @cmd \/c notsorandom.cmd\n\/\/ hold down the space bar\nPress any key to continue . . .\n14153\nPress any key to continue . . .\n14153\nPress any key to continue . . .\n14153\nPress any key to continue . . .\n14153\nPress any key to continue . . .\n14156\nPress any key to continue . . .\n14156\nPress any key to continue . . .\n14156\nPress any key to continue . . .\n14156\nPress any key to continue . . .\n14156\nPress any key to continue . . .\n14160\nPress any key to continue . . .\n14160\nPress any key to continue . . .\n14160\n<\/pre>\n
\nNotice that the %RANDOM%<\/code> pseudovariable generates\nthe same “random” number until the clock ticks over another second.\n(Notice also that the “random” numbers don’t look all that random.)\n<\/p>\n
\nWe fixed the script so it generated its temporary file in the\nproject’s output directory rather than in the (shared) %TEMP%<\/code>\ndirectory.\nThat way, even if two copies of the project are building at the same\ntime, they will generate their temporary files in different directories\nand not step on each other.\n<\/p>\n
\nExercise<\/b>: There is much subtlety in that for<\/code>\ncommand.\nDescribe alternative formulations of the for<\/code> command,\nboth those that work and those that don’t.\nTo get you started: Explain the output of this variation:\n<\/p>\n
\nfor \/l %i in (1,1,300) do @(pause&echo %RANDOM%)<\/code>\n<\/pre>\n
\nObligatory batch file bashing<\/b>:\nEvery time I write an entry about batch files,\nyou can count on people complaining about how insane\nthe batch programming language is.\nThe batch language wasn’t designed; it evolved.\n(And according to commenter Daev,\n\nit followed a form of parallel evolution<\/a> from what most people\nare familiar with.)\nI doubt anybody actually enjoy writing batch files.\nAt best you tolerate it.\nI’m just trying to make it slightly more tolerable.\nI bet these are the same people who complain to their tax preparer\nabout the complexity of tax law.<\/p>\n","protected":false},"excerpt":{"rendered":"
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 makefile, 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 […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[104],"class_list":["post-13673","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-tipssupport"],"acf":[],"blog_post_summary":"
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 makefile, 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 […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/13673","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=13673"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/13673\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=13673"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=13673"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=13673"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}