The standard library provides a level of abstraction to system calls
and functionality that all operating systems should provide (according to
the ANSI standard). For portability and consistency, an operating system
should provide access to functions in the C
standard library, at least at some level. This is usually done
through either static or shared libraries, which can be
created with the ar command:
ar vq libc.a strlen.o strcpy.o
for example.
Then, when linking applications for use by your kernel, specify that it should statically link (for now, until your operating system can handle shared libraries) the application with your standard library:
ld -o app.elf app.o -lc