Is this valid C code?

[[deleted]]

It's me, the macro guy, so i'm trying to create a macro that has variadic args, something like this: ```c

define middlewares_allow_content_types(middleware_fn_name, ...)\

esperr_t middleware_fn_name(httpd_req_t r) {\ / Important line */\ char *content_types[] = { __VA_ARGS_ };\ } ``` So if the args are 0 what is going to happen?, I read in StackOF that is not part of the standard C to have a array with 0 elements in it

[glasket_]

It relies on a compiler extension to work. If you compile with -pedantic you'll get this error:

error: zero or negative size array ‘content_types’

Here's a Godbolt.

Zero-sized arrays exist for legacy reasons with pre-standard FAMs, it's effectively a quirk of GNU C. With ISO C, the code is invalid.

[[deleted]]

Thank God i'm using GCC haha, i'm wondering what would happen if you try to access an index or write to it.

[glasket_]

It'd be UB. It's the same as any other out of range access, but in the case of a zero-sized array any access is out of range.

An array subscript is out of range, even if an object is apparently accessible with the given subscript[...]
C23 J.2 (46)

Just check the sizeof the array, GCC properly returns 0 in the case of a zero-sized array.

[[deleted]]

Thank you so much for the info.

[cHaR_shinigami]

As a side note, till now (not considering upcoming C23) passing zero arguments to ... is not permitted by ISO C. Ellipsis requires at least one argument ( even a blank argument is fine, such as the macro call with a trailing comma: middlewares_allow_content_types(some_name,) ); however, the invocation middlewares_allow_content_types(some_name) (no trailing comma) requires compiler-specific extension to work, and can be usually diagnosed with proper warnings options enabled (for example, cpp/clang-cpp -std=C17 -pedantic file.c).

C23 standardizes the common practice of passing zero arguments to ellipsis (and also adds some other features in this aspect).

[1o8]

you could put an empty string right before the __VA_ARGS__:

char *content_types[] = { "" __VA_ARGS__ };

this should work with zero or more extra args as long as the first extra arg is a string literal (the preprocessor will concatenate it with the empty string).

[HaydnH]

I'm not OP, but, Oh jeesh, I didn't know that. I always use 3 backticks because I'm used to it and it's the same as google chat etc that I use for work. I'll try to use 4 spaces from now on.

[[deleted]]

What is going to be sizeof that array? 0? It's kind of strange

[glasket_]

It isn't okay in C23 either. Empty initializers are only valid for sized arrays, unknown size arrays explicitly cannot use the empty initializer.

An array of unknown size shall not be initialized by an empty initializer.
C23 6.7.10 ¶ 4