> Modularizing

Can't be done. The libc is legacy, it can't be changed without breaking everything. It's also mandatory on every operating system other than Linux.

A change in paradigm is necessary. Freestanding C, not hosted C. This completely gets rid of the libc and is a surprisingly clean language. Linux only, because it's the only kernel with a stable binary interface. Every other OS forces a C runtime.

I once worked on a liblinux project that embodied this... Stopped because Linux itself has a nolibc thing in the kernel tree and I didn't want to compete with it. Now I'm working on the Rust version.

> what is necessary for users of the C language to actually do stuff

Surprisingly little. I wrote an entire lisp interpreter in freestanding C with Linux system calls. It managed to survive for a rather long time without any memory allocation at all.

The system layer is refreshingly tiny. It consists of a memory allocator and extremely basic functions like memmove and strlen. I successfully got rid of total nonsense like thread local errno, locales, implicit buffering, cached global state, possibly more. All that stuff is gone! Exactly one global survived: the stack canary generated by GCC and clang. Every other symbol in the ELF is controlled by me.

Wasn't able to get rid of the NUL terminator. Linux itself needs it. To get rid of that little billion dollar mistake requires an entirely new kernel with zero UNIX/POSIX influence. I had to make my peace with that one. All my buffers maintain an extra NUL byte at the end.