What can create a lexical error in C?

Question

Besides not closing a comment /*..., what constitutes a lexical error in C?

Answer 1

Aren't [@$`] and other symbols like that (maybe from unicode) lexical errors in C if put anywhere outside of string or comment? They are not constituting any valid lexical sequence of that language. They cannot pass the lexer because the lexer cannot recognize them as any kind of valid token. Usually lexers are FSMs or regex based so these symbols are just unrecognized input.

For example in the following code there are several lexical errors:

int main(void){
` int a = 3;
@ —
return 0;
}

We can support it by feeding this to gcc, which gives

../a.c: In function ‘main’:
../a.c:2: error: stray ‘`’ in program
../a.c:3: error: stray ‘@’ in program
../a.c:3: error: stray ‘\342’ in program  
../a.c:3: error: stray ‘\200’ in program
../a.c:3: error: stray ‘\224’ in program

GCC is smart and does error-recovery so it parsed a function definition (it knows we are in 'main') but these errors definitely look like lexical errors, they are not syntax errors and rightly so. GCC's lexer doesn't have any types of tokens that can be built from these symbols. Note that it even treats a three-byte UTF-8 symbol as three unrecognized symbols.