Conditional build tags

[astral grotto]

In Golang, there is a build tag in the form of:

// +build tag_name

Is there something similar in zig?

https://www.digitalocean.com/community/tutorials/customizing-go-binaries-with-build-tags

[kindred blaze]

not really similar

but you can achieve the same effect via different mechanisms

in zig, all code is lazily analysed

so you can have a file, imported and everything

but so long as you dont reference the code, it's never analysed, and therefore never compiled

this means you can do things like

fn foo() void {
    if (comptime_bool_constant) {
        fooImpl1();
    } else {
        fooImpl2();
    }
}

if comptime_bool_constant is true, fooImpl1() will be analysed and compiled, and fooImpl2() won't

and vice versa if it's false

[astral grotto]

Thanks for the exposition, this makes me understand now why I didn't get any error about having the function main() defined in an imported file!

[kindred blaze]

well, that's actually moreso to do with the fact that main isn't specially recognised by the language or anything

there's a file called start.zig, where all of the platform-specific code for setting up the entry point is

that imports @import("root"), which will be the file you passed to zig build-exe, or in build.zig the file you passed as the root_source_file

it checks if you have a public main function via @hasDecl(root, "main")

if you do, then it runs your main function from the platform-specific entry point, which is just an @exported function

if you don't, it then checks if you're defining a platform-specific entry point yourself

if you're not doing that either, it'll issue a compile error about missing a main entry point

[astral grotto]

mm, I thought I needed to define the file directly as zig run main.zig and in this one I have to use the main function, any reference how I can define platform-dpecific entry points.

[kindred blaze]

generally you just define it as an export

e.g. on linux, pub export fn _start() noreturn {...}

[kindred blaze]

to be clear it's just the code that sets up stuff for being able to just have pub fn main() T {}, instead of doing it all yourself

the same type of code would exist for Go or C

it's just it's all visible in the zig stdlib

[astral grotto]

so do you mean pub export fn _start() is equivalent to Go 'func main()`, sorry I got confused, is it the file name to be start.zig and inside it an exported function regardless of its name, or it is the external _start function regardless if the file name, or both, in Go the filename is not a big deal.

[kindred blaze]

no, I'm just referencing an implementation detail

you can continue to use pub fn main as normal

start.zig is a file in the stdlib

[wheat jackal]

As mentioned, main is just something the language provides so you don't have to think about platform-specific details…
The reason you can have more than one main() function in Zig but not in Go is because in Zig each file has its own namespace, whereas in Go the namespace extends across the entire directory (tho not sub directories?)