C's rand() (rather, your libcs rand()) is perfectly fine for what it's supposed to be: fast random data, with no particular guarantees to cryptographic security, for things like randomized algorithms or the 99% of cases where you're picking what powerup to spawn instead of generating super important key material.
To make it into a CSPRNG would be a detriment. The only thing I would change is to have a minimum cycle length.
No, C's rand is unsuited for most randomized algorithms. MCMC algorithms won't work with rand unless you're lucky, it's even unsuited for choosing pivots for quicksort. The only application where it might be ok is games.
> The only application where it might be ok is games.
Not even that. Different types of games require different types of RNGs (e.g. in an RPG or strategy game you probably want a stable seeded RNG to preclude RNG save-scumming), the C standard requires almost no guarantee of rand().
> The only application where it might be ok is games.
This is silly, there are plenty of ways randomness is used to communicate randomness to the user (think games, song shuffling, visualizations, art, etc. Algorithms using randomness ≠ randomized algorithms.
To make it into a CSPRNG would be a detriment. The only thing I would change is to have a minimum cycle length.