assembly - What are .S files?

Question

I've seen .S files in various projects such as in the Linux Kernel, FreeBSD kernel, or in some other projects. What do these files do and why can't we have .c files instead ?

Answer 1

.S files are source code files written in assembly. Assembly is an extremely low-level form of programming. The files contain assembly instructions to the processor in sequential order and are typically compiled based on a selected architecture. Examples of such files are often seen in the linux kernel for specific architectures, e.g. x86, sparc, ARM, etc.

For more information about assembly language:

• X86 Assembly/GAS syntax
• x86 Instruction list
• TLDP Linux Assembly Howto
• Example in the Linux kernel: arch/x86/net/bpf_jit.S