{"id":7448,"date":"2024-09-14T03:00:00","date_gmt":"2024-09-14T07:00:00","guid":{"rendered":"https:\/\/www.both.org\/?p=7448"},"modified":"2024-09-05T15:28:07","modified_gmt":"2024-09-05T19:28:07","slug":"random-numbers-from-the-linux-kernel","status":"publish","type":"post","link":"https:\/\/www.both.org\/?p=7448","title":{"rendered":"Random numbers from the Linux kernel"},"content":{"rendered":"
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\r\n \r\n <\/i>\r\n <\/a>\r\n <\/span>\r\n<\/div><\/div>\n

I recently shared a Guess the number<\/a> game to demonstrate a few \u201cfirst concepts\u201d in C programming. Whenever I learn a new programming language, I like to start with this simple number-guessing game because it exercises the basics of programming, including how to store values in variables<\/em> and how to compare values<\/em>.<\/p>\n\n\n\n

That program used the rand<\/code> function to generate pseudo-random<\/em> values between zero and some large maximum value. Pseudo-random numbers are okay if you aren\u2019t working on anything very sensitive\u2014and writing a number-guessing game is pretty straightforward. But if you need to have more randomness<\/em><\/a> to the random numbers you generate, you should instead use the getrandom<\/code> system call in the Linux kernel.<\/p>\n\n\n\n

Random bits<\/h2>\n\n\n\n

A system call<\/em> acts like a function when you use it in a program, but behind the scenes it interacts directly with the Linux kernel. The getrandom<\/code> system call prompts the kernel to generate a series of random bits<\/em> (ones and zeroes) to \u201cfill\u201d a variable.<\/p>\n\n\n\n

Use the getrandom<\/code> system call like this:<\/p>\n\n\n\n

ssize_t getrandom(void buf, size_t buflen, unsigned int flags);<\/code><\/pre>\n\n\n\n
The buf<\/code> buffer should be the address to a variable, so the system call can fill this variable directly. The buflen<\/code> value tells getrandom<\/code> how many bytes to fill.<\/p>\n\n\n\n
Note that the flags<\/code> value indicates whether the random number should be blocking<\/em> (GRND_RANDOM<\/code>) or nonblocking<\/em> (GRND_NONBLOCK<\/code>). This is important because if the random number source on the system hasn\u2019t been initialized yet, or if the source doesn\u2019t contain enough entropy<\/em>, then using GRND_RANDOM<\/code> with the getrandom<\/code> call will wait for the source to fill up with enough random data before the system call will return its own random bits.<\/p>\n\n\n\n
On a desktop system where the user frequently uses the mouse, clicks buttons in apps, and types on the keyboard, the system entropy should be high enough to instantly generate a few bytes of random data. But if you\u2019re not sure, you can use GRND_NONBLOCK<\/code> to prevent blocking, and the system will generate other random bits on its own.<\/p>\n\n\n\n
You can see this in action by writing a very short sample program:<\/p>\n\n\n\n
#include <stdio.h>\n#include <sys\/random.h>\n\nint main()\n{\n  unsigned int num;\n\n  getrandom(&num, sizeof(unsigned int), GRND_NONBLOCK);\n  printf(\"%u\\n\", num);\n\n  return 0;\n}<\/code><\/pre>\n\n\n\n
Every time you run the program, the program will use the getrandom<\/code> system call to generate random bits in the num<\/code> variable, which it then prints. Note that getrandom<\/code> creates random bits; if you store the result in a signed<\/em> variable such as a regular int<\/code>, the result could be positive or negative. In this sample, I\u2019ve stored the result in an unsigned<\/em> variable, so the result can only be zero or some other positive number.<\/p>\n\n\n\n
Updating the number-guessing program<\/h2>\n\n\n\n
With the getrandom<\/code> system call, we can update the Guess the number<\/a> program to generate a truly random secret number. If you recall the previous number-guessing program, it used srand<\/code> to seed<\/em> the pseudo-random number generator, and rand<\/code> to generate new pseudo-random numbers. We can replace this by using randval<\/code> to generate random data in a variable, then using the modulo<\/em> (%<\/code>) operator to \u201cfold\u201d the random number into the range 0 to 99, then add 1 to make the secret value between 1 and 100:<\/p>\n\n\n\n
    unsigned int randval, secret;\n\n    getrandom(&randval, sizeof(unsigned int), GRND_NONBLOCK);\n    secret = (randval % 100) + 1;<\/code><\/pre>\n\n\n\n
Every time you run the program, the Linux kernel will generate random data in the randval<\/code> variable, so the value in secret<\/code> will always be a truly random value between 1 and 100.<\/p>\n\n\n\n
The rest of the program is the same. Here\u2019s the completed program:<\/p>\n\n\n\n
#include <stdio.h>\n#include <sys\/random.h>\n\nint main()\n{\n    unsigned int randval, secret, guess;\n\n    getrandom(&randval, sizeof(unsigned int), GRND_NONBLOCK);\n    secret = (randval % 100) + 1;\n\n    puts(\"Guess a random number from 1 to 100\");\n\n    do {\n        puts(\"Your guess:\");\n        scanf(\"%d\", &guess);\n\n        if (guess < secret) {\n            puts(\"Too low\");\n        }\n        else if (guess > secret) {\n            puts(\"Too high\");\n        }\n    } while (guess != secret);\n\n    puts(\"That's right!\");\n    return 0;\n}<\/code><\/pre>\n\n\n\n
Guess the number<\/h2>\n\n\n\n
If we save this updated \u201cGuess the number\u201d program as guess2.c<\/code> we can compile it like this:<\/p>\n\n\n\n
$ gcc -o guess2 guess2.c<\/code><\/pre>\n\n\n\n
And now we can play the number-guessing game as before. The program will use if<\/code> and else if<\/code> to indicate if the guess is too low or too high, and exit the program with a \u201cThat\u2019s right\u201d message when the guess equals the secret number:<\/p>\n\n\n\n
$ .\/guess2\nGuess a random number from 1 to 100\nYour guess:\n50\nToo high\nYour guess:\n25\nToo low\nYour guess:\n35\nToo high\nYour guess:\n30\nToo high\nYour guess:\n28\nToo low\nYour guess:\n29\nThat's right!<\/code><\/pre>\n\n\n\n
Using srand<\/code> and rand<\/code> is okay if you don\u2019t mind pseudo-random numbers. But if you need to use random values that are actually random<\/em>, you should use the getrandom<\/code> system call instead. Read the manual page (in section 2) to learn more about the getrandom<\/code> system call:<\/p>\n\n\n\n
$ man 2 getrandom<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
Let the kernel generate random numbers for you using ‘getrandom.’<\/p>\n","protected":false},"author":33,"featured_media":3306,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","footnotes":""},"categories":[69,5,150],"tags":[147,91,152],"class_list":["post-7448","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-fun","category-linux","category-programming","tag-fun","tag-linux","tag-programming"],"modified_by":"Jim Hall","_links":{"self":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/7448","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/users\/33"}],"replies":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7448"}],"version-history":[{"count":2,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/7448\/revisions"}],"predecessor-version":[{"id":7450,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/7448\/revisions\/7450"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/media\/3306"}],"wp:attachment":[{"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7448"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7448"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7448"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}