Use case: Ability to create global constructor/destructor functions about zig

[dweiller]

Disclaimer: I have no idea what happens if you're not making an ELF file.

It is possible to register code to run before/after main (including if std.process.exit() is called, but not if a panic occurs, or it exits due to an unhandled signal) when linking libc by placing an array of function pointers into the .init_array and .fini_array sections. I'm not sure if this is exactly equivalent to the constructor/destructor but it may work for you.

Here's a minimal example:

pub fn main() !void {
    try std.io.getStdOut().writeAll("this is main\n");
    std.process.exit(2);
}

fn constructor1() callconv(.C) void {
    std.io.getStdOut().writeAll("this is constructor1\n") catch @panic("constructor1 failed");
}

fn constructor2() callconv(.C) void {
    std.io.getStdOut().writeAll("this is constructor2\n") catch @panic("constructor2 failed");
}

fn destructor() callconv(.C) void {
    std.io.getStdOut().writeAll("this is destructor\n") catch @panic("destructor failed");
}

export const init_array: [2]*const fn () callconv(.C) void linksection(".init_array") = .{&constructor1, &constructor2};
export const fini_array: [1]*const fn () callconv(.C) void linksection(".fini_array") = .{&destructor};

const std = @import("std");

Which produces the following:

> zig run init_fini.zig -lc
this is constructor1
this is constructor2
this is main
this is destructor

I think this is basically how the constructor/destructor attributes work though someone could correct me.

[alexrp]

I think this is basically how the constructor/destructor attributes work though someone could correct me.

I believe you're right.

The thing is, I compile this library for Windows, macOS, and Linux, so doing this for each OS would get a bit hairy. I think it's also tricky in the more general case because the linker is supposed to have "append semantics" (if memory serves) for these arrays. It's not clear how that interacts with Zig's language semantics.