c - Why do some compilers use "a.out" as the default name for executables?

Question

Most UNIX C compilers link executables by default to a file called "a.out". Why? Is this a written standard or just de-facto standard behavior? What would break if these compilers would just produce an error message (or use a different default name) instead of producing "a.out"?

Answer 1

a.out stands for assembler output.
I think that justifies why most compilers have this name as default. More info here.