Uint32_t arc4random_uniform(uint32_t limit) Void arc4random_buf(void *buf, size_t bytes) The arc4random family includes: uint32_t arc4random(void) If your system supports the arc4random family of functions I would recommend using those instead the standard rand function. Java API for describes algorithms for removing bias from random integers, and packing 53 bits into random floats.OpenBSD's arc4random_uniform() explains modulo bias.It mixes bits from the current time, the process ID, and some pointers, if it can't read /dev/urandom.
Perl's Perl_seed() is an example of how to calculate a seed in C for srand().It is faster to allow OpenSSL to generate more random numbers from a seed. If we need many random numbers, it would be too slow to read them all from /dev/urandom, because they must be copied from the kernel. OpenSSL's RAND_bytes() seeds itself, perhaps by reading /dev/urandom in Linux.
Some programmers write code like rand() / (double)RAND_MAX, but rand() might return only 31 bits, or only 15 bits in Windows. If we use more than 53 bits, we get rounding bias. To remove the bias, we can retry rand() while the value is below 2768, because the 30000 values from 2768 to 32767 map uniformly onto the 10000 values from 0 to 9999.įor floats, we want 53 random bits, because a double holds 53 bits of precision (assuming it's an IEEE double). Each number from 0 to 2767 would appear more often than each number from 2768 to 9999. Suppose that we got some random 4 digit integers from rand() % 10000, but rand() can only return 0 to 32767 (as it does in Microsoft Windows). OpenSSL only gives random bytes, so I try to mimic how Java or Ruby would transform them into integers or floats.įor integers, we want to avoid modulo bias. Why so much code? Other languages like Java and Ruby have functions for random integers or floats. The best way to generate random numbers in C is to use a third-party library like OpenSSL. The way that some programs call rand() is awful, and calculating a good seed to pass to srand() is hard. Many implementations of rand() cycle through a short list of numbers, and the low bits have shorter cycles. It's good enough to deal cards for solitaire, but it's awful. Result = (rand() % (hi_num - low_num)) + low_num Low_num = max_num + 1 // include max_num in output Hi_num = max_num + 1 // include max_num in output Int random_number(int min_num, int max_num) Now, when we call rand(), a new random number will be produced every time. We do this by feeding it the value of the current time with the time() function.
Therefore, we have to seed the randomizer with a value that is always changing. If you gave the same seed value, then the same random numbers would be generated every time. Basically, the computer can generate random numbers based on the number that is fed to srand(). First we use the srand() function to seed the randomizer. so we can expect to bail out of this loop pretty quickly.) (Worst case the loop condition should succeed 50% of the time, and ignore results from rand() that fall above that limit. elaborate implementation that combines multiple calls to rand() Supporting larger values for n would requires an even more
* Uses rand(), and so is affected-by/affects the same seed. If you really care about uniformity you can do something like this: /* Returns an integer in the range [0, n). For example: /* random int between 0 and 19 */ It's common practice to use the % operator in conjunction with rand() to get a different range (though bear in mind that this throws off the uniformity somewhat). You can use srand(unsigned int seed) to set a seed. The rand() function in returns a pseudo-random integer between 0 and RAND_MAX.