glibc is and quite probably has to be written in C (the clue is in the name), and it has to work on a great many platforms and architectures, serving an absolutely huge amount of software. Is there any other software linked with and providing vital infrastructure to tens of thousands of other programs? (Maybe Windows MSVCRT.)

I've written tons and tons of software for Linux, and I believe only once have I seen a bug in glibc[1].

[1] And it was both a horrible bug, and also an extremely weird corner case: https://bugzilla.redhat.com/show_bug.cgi?id=563103#c8