Why is `rand()` Considered a Substandard Random Number Generator in C?

Why is the use of rand() Considered Bad?

While rand() is a commonly encountered pseudorandom number generator, its usage is generally discouraged despite the option to set a seed with srand(). This preference for alternative generators stems from several reasons related to the limitations and shortcomings of the C random library.

Pseudorandom Number Generation Limitation

Rand is a pseudorandom number generator, meaning its output depends on an initial seed. Consequently, the same sequence of numbers is produced for a given seed, making it unsuitable for security-sensitive applications that require true randomness. This is a characteristic of pseudorandom generators, not unique to rand.

C Random Library Deficiencies

Beyond its inherent limitations, the C random library, including rand and srand, exhibits specific issues that make it obsolete:

• Global State: The random library maintains a global state that prevents simultaneous usage of multiple random engines and complicates multithreaded operations.
• Lack of Distribution Engine: Rand generates numbers within the interval [0, RAND_MAX], but most scenarios require a specific distribution. Attempts to convert to a different distribution often result in non-uniform outcomes.
• Implementation Quality: Rand's implementation has known quality concerns, as detailed in other responses.

Alternatives in C

Modern C provides a robust alternative in the library. It offers:

• Multiple Random Engines: Supports various random number engines, allowing for simultaneous usage.
• Distribution Support: Provides distribution engines that generate numbers within specified intervals and distributions.
• Improved Quality: The library is better implemented, ensuring a higher level of quality.

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