Why is the use of rand() considered bad?

问题:

I heard some guys telling that the use of rand() is bad EVEN AFTER USING srand() to get a seed. Why is that so? I want to know how the stuff happens... And sorry for another question.. but what is an alternative to this then?

回答1:

There are two parts to this story.

First, rand is a pseudorandom number generator. This means it depends on a seed. For a given seed it will always give the same sequence (assuming the same implementation). This makes it not suitable for certain applications where security is of a great concern. But this is not specific to rand. It's a problem of any pseudo-random generator. And there are most certainly a lot of classes of problems where a pseudo-random generator is acceptable. A true random generator has its own problems (efficiency, implementation, entropy) so for problems that are not security related most often a pseudo-random generator is used.

So you analyzed your problem and you conclude a pseudo-random generator is the solution. And here we arrive to the real problems of the C random library (which includes rand and srand) who are specific to it and make it obsolete (a.k.a.: the reasons you should never use rand and the C random library).

• One problem is that it has a global state (set by srand). This makes it impossible to use multiple random engines at the same time. It also greatly complicates multithreaded tasks.

• The most visible problem of it is that it lacks a distribution engine: rand gives you a number in interval [0 RAND_MAX]. It is uniform in this interval, which means that each number in this interval has the same probability to appear. But most often you need a random number in a specific interval. Let's say [0, 1017]. A commonly (and naive) used formula is rand() % 1018. But the problem with this is that unless RAND_MAX is an exact multiple of 1018 you won't get an uniform distribution.

• Another problem is the Quality of Implementation of rand. There are other answers here detailing this better than I could, so please read them.

In modern C++ you should definitely use the C++ library from <random> which comes with multiple random well-defined engines and various distributions for integer and floating point types.

回答2:

None of the answers here explains the real reason of being rand() bad.

rand() is a pseudo-random number generator (PRNG), but this doesn't mean it must be bad. Actually, there are very good PRNGs, which are statistically hard or impossible to distinguish from true random numbers.

rand() is completely implementation defined, but historically it is implemented as a Linear Congruential Generator (LCG), which is usually a fast, but notoriously bad class of PRNGs. The lower bits of these generators have much lower statistical randomness than the higher bits and the generated numbers can produce visible lattice and/or planar structures (the best example of that is the famous RANDU PRNG). Some implementations try to reduce the lower bits problem by shifting the bits right by a pre-defined amount, however this kind of solution also reduces the range of the output.

Still, there are notable examples of excellent LCGs, like L'Ecuyer's 64 and 128 bits multiplicative linear congruential generators presented in Tables of Linear Congruential Generators of Different Sizes and Good Lattice Structure, Pierre L'Ecuyer, 1999.

The general rule of thumb is that don't trust rand(), use your own pseudo-random number generator which fits your needs and usage requirements.

回答3:

What is bad about rand/srand is that rand—

• uses an unspecified RNG algorithm, yet
• allows that RNG to be initialized with srand for repeatable "randomness".

These two points, taken together, hamper the ability of implementations to improve on the RNG's implementation (e.g., to use a cryptographic or otherwise "better" RNG). For example, JavaScript's Math.random and FreeBSD's arc4random don't have this problem, since they don't allow applications to seed them for repeatable "randomness" — it's for exactly this reason that the V8 JavaScript engine was able to change its Math.random implementation to a variant of xorshift128+ while preserving backward compatibility. (On the other hand, letting applications supply additional data to supplement randomness, as in BCryptGenRandom, is less problematic; even so, however, this is generally seen only in cryptographic RNGs.)

Also:

• The fact that the RNG algorithm and the seeding procedure are unspecified means that even reproducible "randomness" is not guaranteed between rand/srand implementations, between versions of the same standard library, between operating systems, etc.
• If srand is not called before rand is, rand behaves similarly as though srand(1) were first called. In practice, this means that rand can only be implemented as a PRNG rather than as a nondeterministic RNG, and that rand's PRNG algorithm can't differ in a given implementation whether the application calls srand or not.

回答4:

Firstly, srand() doesn't get a seed, it sets a seed. Seeding is part of the use of any pseudo random number generator (PRNG). When seeded the sequence of numbers that the PRNG produces from that seed is strictly deterministic because (most?) computers have no means to generate true random numbers. Changing your PRNG won't stop the sequence from being repeatable from the seed and, indeed, this is a good thing because the ability to produce the same sequence of pseudo-random numbers is often useful.

So if all PRNGs share this feature with rand() why is rand() considered bad? Well, it comes down to the "psuedo" part of pseudo-random. We know that a PRNG can't be truly random but we want it to behave as close to a true random number generator as possible, and there are various tests that can be applied to check how similar a PRNG sequence is to a true random sequence. Although its implementation is unspecified by the standard, rand() in every commonly used compiler uses a very old method of generation suited for very weak hardware, and the results it produces fair poorly on these tests. Since this time many better random number generators have been created and it is best to choose one suited to your needs rather than relying on the poor quality one likely to provided by rand().

Which is suitable for your purposes depends on what you are doing, for example you may need cryptographic quality, or multi-dimensional generation, but for many uses where you simply want things to be fairly uniformly random, fast generation, and money is not on the line based on the quality of the results you likely want the xoroshiro128+ generator. Alternatively you could use one of the methods in C++'s <random> header but the generators offered are not state of the art, and much better is now available, however, they're still good enough for most purposes and quite convenient.

If money is on the line (e.g. for card shuffling in an online casino, etc.), or you need cryptogaphic quality, you need to carefully investigation appropriate generators and ensure they exactly much your specific needs.

回答5:

If you use rand(), you will basically have the same result after generating your random number. So even after using srand(), it will be easy to predict the number generated if someone can guess the seed you use. This is because the function rand() uses a specific algorithm to produce such numbers

With some time to waste, you can figure out how to predict numbers generated by the function, given the seed. All you need now is to guess the seed. Some people refer to the seed as the current time. So if can guess the time at which you run the application, I ll be able to predict the number

IT IS BAD TO USE RAND()!!!!

回答6:

rand is usually -but not always-, for historical reasons, a very bad pseudo-random number generator (PRNG). How bad is it is implementation specific.

C++11 has nice, much better, PRNGs. Use its <random> standard header. See notably std::uniform_int_distribution here which has a nice example above std::mersenne_twister_engine.

PRNGs are a very tricky subject. I know nothing about them, but I trust the experts.