Linux 与 Windows 中的系统库

Question

My background is in Windows and I'm a Linux noob. Still trying to wrap my head around some basic concepts, and specifically the system libraries:

1. Windows has ntdll.dll which wraps system calls, and a CRT dll which interface between the C syntax to the ntdll OS-exposed services.
   (for simplification I ignore the intermediate layer of user32, kernel32, kernalbase etc. I also realize the CRT is several dlls, this is not the point).
2. It seems Unix/Linux has pretty much just libc, which wraps system calls and called directly from your application code.

Is this the right analogy? (ntdll + CRT) <===> libc?

I realize that C & Unix evolved together, but am still surprised. Can it be that the C interface is hard-wired into the OS for Unix/Linux? In Windows non-C programs link against the underlying OS provided dlls. Is it possible that in Linux there is no OS/C-runtime border?

Answer 1

In general, most programs link against libc, even if they are written in another language. It provides the C standard library interface (like MSVCRT), POSIX features (the equivalent of certain parts of the Win32 subsystem), and wrappers around system calls. For example, Rust uses libc because it provides a portable environment to link against.

However, on Linux, you don't have to link against libc. Go chooses to make system calls directly, which means it can ship static binaries that have no runtime dependencies. This is possible because Linux guarantees a stable kernel ABI, but not all operating systems do this (eg, macOS). So unless you have significant resources (like an entire programming language team), this isn't generally a smart move unless you're only working with a handful of syscalls.

I should point out that even Windows is intrinsically wired into the C language: it uses C strings (granted, usually wide C strings) for its system calls, and much of the kernel is written in C. Even if you were starting a kernel from scratch, you'd still need a general C interface, because virtually every programming language has a way to interact with C.

Answer 2

The Linux system calls are documented in syscalls(2) and are the foundation of user-land programs. The calling conventions are documented in the ABI specifications. The ELF executable format is documented, eg in elf(5) .

Read also Advanced Linux Programming and about the Unix philosophy .

You can make system calls directly in assembler. The Linux Assembly HowTo explains that. You'll prefer to use the C interface, and for that reason the libc is preferable. In practice, the libc.so is the cornerstone of most Linux systems.

Play with ldd(1) , pmap(1) , strace(1) , BusyBox

The GCC compiler enables useful language extensions , and mixing C and assembler code.

Some programming languages implementations are barely using C and could call system calls directly (look into SBCL or Go ...)

Both the Linux kernel and usual GNU libc (or musl-libc ), and also the GCC compiler and the binutils are free software or open source , and you can study their source code .

Things become trickier with systemd and vdso(7) .

See also http://linuxfromscratch.org/

Graphical applications are using some display server , often Xorg orWayland . Read about X11 . You would want to use GUI toolkits like GTK or Qt to code them.