What is so bad about C standard library?

Many HN readers might think this is joke question, but is serious question. "C Standard Library sucks" is almost a gnrd (geeky nerd) meme. But I don't get this joke!

I am student/hobbyist, and I like to study programming languages. C is amazing. Also are some other languages.

Many times, when I study about !C languages, I see them say things about how XYZlang has much better standard lib than C, and they assume everybody knows how bad is C standard lib, but they never provide any specific references. (Please don't reply rtfd, tyvm)

Maybe some HN people can post some comments about or links to some references, comparisons, or articles about what sucks in C standard lib.

Also please if you know about such please say some alternatives to C standard lib, not in other language, but for use in C.

Thank you HN people!