Not emitting definition for INT8_MAX (from stdint)

[Octogonapus]

I'm wrapping the AWS CRT packages and I have encountered some weird behavior. This C code:

// stdint.h is included indirectly through a number of headers which are also passed to Clang.jl
#define AWS_EVENT_STREAM_HEADER_NAME_LEN_MAX (INT8_MAX)

causes this wrapper to be generated:

const AWS_EVENT_STREAM_HEADER_NAME_LEN_MAX = INT8_MAX

There's no definition for INT8_MAX in the generated code. It would be great if Clang.jl could emit:

const AWS_EVENT_STREAM_HEADER_NAME_LEN_MAX = 127

or at least:

const INT8_MAX = 127
const AWS_EVENT_STREAM_HEADER_NAME_LEN_MAX = INT8_MAX

Is Clang.jl expected to fail in this case, or did I misconfigure something?

Clang.jl v0.17.6
Julia v1.9.3

[Gnimuc]

Yes. It's technically impossible to do this because macro expanding is handled by the preprocessor, before the AST is created, and we can only extract info from the AST.¹ The identifier INT8_MAX could be anything, it's legal to undef it and define it again. So, unless we implement a full-featured C preprocessor in Julia, it's hard to do it without any mistakes.

However, the best practice is much simpler. We can add const INT8_MAX = typemax(Int8) to the prologue file and do it once for all.

I know the ultimate solution is something like Rust's https://github.com/rust-lang/libc. But in practice, as Julia is niche at the moment, we really don't have that manpower. ;)

Footnotes

1. to elaborate a bit more, the info is lost after the macro is "expanded" by the preprocessor. the compiler can only see 127, not INT8_MAX. The reason why Clang.jl can handle #define AWS_EVENT_STREAM_HEADER_NAME_LEN_MAX (INT8_MAX) is that it uses a heuristic way to parse the C code.) ↩

[Octogonapus]

Fair enough