What is the reason function names are prefixed with an underscore by the compiler?

Question

When I see the assembly code of a C app, like this:

emacs hello.c
clang -S -O hello.c -o hello.s
cat hello.s

Function names are prefixed wi

Answer 1

From Linkers and Loaders:

At the time that UNIX was rewritten in C in about 1974, its authors already had extensive assember language libraries, and it was easier to mangle the names of new C and C-compatible code than to go back and fix all the existing code. Now, 20 years later, the assembler code has all been rewritten five times, and UNIX C compilers, particularly ones that create COFF and ELF object files, no longer prepend the underscore.

Prepending an underscore in the assembly results of C compilation is just a name-mangling convention that arose as a workaround. It stuck around for (as far as I know) no particular reason, and has now made its way into Clang.

Outside of assembly, the C standard library often has implementation-defined functions prefixed with an underscore to convey notions of magicalness and don't touch this to the ordinary programmers that stumble across them.