Why # is required before #include<stdio.h>?

Question

What is the function of #?  

Answer 1

It denotes a preprocessor directive:

One important thing you need to remember is that the C preprocessor is not part of the C compiler.

The C preprocessor uses a different syntax. All directives in the C preprocessor begin with a pound sign (#). In other words, the pound sign denotes the beginning of a preprocessor directive, and it must be the first nonspace character on the line.

# was probably chosen arbitrarily as an otherwise unused character in C syntax. @ would have worked just as well, I presume.

If there wasn't a character denoting it, then there would probably be trouble differentiating between code intended for the preprocessor -- how would you tell whether if (FOO) was meant to be preprocessed or not?

Answer 2

Because # is the standard prefix for introducing preprocessor statements.

In early C compilers, the pre-processor was a separate program which would handle all the preprocessor statements (similar to the way early C++ "compilers" such as cfront generated C code) and generate C code for the compiler (it may still be a separate program but it may also be just a phase of the compiler nowadays).

The # symbol is just a useful character that can be recognised by the preprocessor and acted upon, such as:

#include <stdio.h>
#if 0
#endif
#pragma treat_warnings_as_errors
#define USE_BUGGY_CODE

and so on.