Do c/c preprocessors process all lines that begin with #? Does is errors out when encountering unknown macros or will it just ignore them?

for an example,

#include <stdio.h>

#hello
int main(){
    printf("Hello World!");
    return 0;
}

what happens in this situation?will it produce an error or will it work (ignoring #hello line)?

CodePudding user response：

The language grammar specifies all pre-processor directives that exist in the language. If the program doesn't conform to the grammar, then the program has a syntax error. Hence the program is ill-formed and the language implementation is required to issue a diagnostic message and is free to refuse to proceed.

CodePudding user response：

C

Syntactically, #hello is a "non-directive" preprocessing directive.

C17/C18 section 6.10 paragraph 9 (newly added in C17/C18) says:

The execution of a non-directive preprocessing directive results in undefined behavior.

"Undefined behavior" does not necessarily mean that the compiler will fail to translate the code or issue a diagnostic. It could behave in a documented manner, for example if the directive is part of an extension to the C language.

C

Syntactically, #hello is a "conditionally-supported-directive" preprocessing directive.

C 20 section 15.1 paragraph 2 says:

A conditionally-supported-directive is conditionally-supported with implementation-defined semantics.

"Conditionally-supported" means that an implementation is not required to support it. Implementations need to document all conditionally-supported constructs that they do not support. (In the case of conditionally-supported-directives, I guess that would amount to documenting that none of them are supported, or documenting the semantics of those that are supported.)

CodePudding user response：

An unrecognized preprocessing directive error will rise, and your code won't compile