I agree with your broad point that you can write a usable C library with a very minimal amount of assembly, but I am going to quibble on some of the details.
> You can write pretty much the entire standard library in C, ... varargs, setjmp/longjmp and sbrk are 100% library.
If you're saying "These four functions can be written in C" then you are mistaken. If you are saying "These four functions can be written in assembly" then you are not saying anything interesting about C, because it is true of any language that it is possible to write its library in assembly if it's possible to write it at all.
In more detail.
Whether you can write va_arg in C depends on the platform's calling convention. On most platforms you can (because the default calling convention has to support varargs, and the easiest way to do that is to push the arguments on the stack) but on x86-64 you can't.
setjmp and longjmp cannot be written in C on any platform I'm familiar with, unless you're writing them on top of something like setcontext, which has a slight superset of their functionality. They are, however, very simple to write in assembly.
You can write sbrk in one line of C, and the glibc implementation is only ten: http://koala.cs.pub.ro/lxr/glibc/misc/sbrk.c#L29
But sbrk is simply a convenience wrapper around brk, which is necessarily a system call. And C doesn't have a "system call" statement.
(Actually, you can write brk in C too if you just want to allocate out of some fixed blob of address space, which is a reasonable choice on, say, a microcontroller.)
> You can write pretty much the entire standard library in C, ... varargs, setjmp/longjmp and sbrk are 100% library.
If you're saying "These four functions can be written in C" then you are mistaken. If you are saying "These four functions can be written in assembly" then you are not saying anything interesting about C, because it is true of any language that it is possible to write its library in assembly if it's possible to write it at all.
In more detail.
Whether you can write va_arg in C depends on the platform's calling convention. On most platforms you can (because the default calling convention has to support varargs, and the easiest way to do that is to push the arguments on the stack) but on x86-64 you can't.
setjmp and longjmp cannot be written in C on any platform I'm familiar with, unless you're writing them on top of something like setcontext, which has a slight superset of their functionality. They are, however, very simple to write in assembly.
You can write sbrk in one line of C, and the glibc implementation is only ten: http://koala.cs.pub.ro/lxr/glibc/misc/sbrk.c#L29 But sbrk is simply a convenience wrapper around brk, which is necessarily a system call. And C doesn't have a "system call" statement.
(Actually, you can write brk in C too if you just want to allocate out of some fixed blob of address space, which is a reasonable choice on, say, a microcontroller.)