tools/include/nolibc aims to «provide minimal C-library emulation for small, low-level workloads». crt.h, arch-x86_64.h, arch.h, Makefile. arch.h includes the arch-*.h file corresponding to the target architecture. std.h contains standard definitions and types for NOLIBC, that is, it declares a few common macros and types that usually are found in stdlib.h, stdint.h, ctype.h, unistd.h (sys/types.h?) and a few other common locations (think dev_t, time_t, pid_t, off_t etc.)