C: what is the second # for in #define FOO(s) #s

Question

What does # do in C? Especially in #define like this

#define FOO(s) #s

Answer 1

From MSDN documentation:

The number-sign or "stringizing" operator (#) converts macro parameters to string literals without expanding the parameter definition. It is used only with macros that take arguments. If it precedes a formal parameter in the macro definition, the actual argument passed by the macro invocation is enclosed in quotation marks and treated as a string literal. The string literal then replaces each occurrence of a combination of the stringizing operator and formal parameter within the macro definition.

The page linked above contains some practical examples for comparison.

Answer 2

It makes a string so

#define FOO(s) #s
FOO(hello)

Is the same as

"hello"