function nullptr in shared library data section

[bnoordhuis]

export const init_array section(".init_array") = [1]extern fn() void { init };
extern fn init() void {}
pub fn main() void {}

$ ./build/zig --verbose-link build-lib --output lib.so lib.zig
lld -m elf_x86_64 -shared -o lib.so -dynamic-linker /lib64/ld-linux-x86-64.so.2 -soname liblib.so.0 ./zig-cache/lib.o ./zig-cache/builtin.o ./zig-cache/compiler_rt.o

$ objdump -s lib.so | grep -A1 'section .init_array'
Contents of section .init_array:
 3b7a0 00000000 00000000                    ........

Trying to load lib.so crashes with a nullptr dereference in glibc's dynamic linker. Changing the type to [1]usize and using @ptrToInt(init) isn't accepted ("unable to evaluate constant expression".)

The right address is filled in (and called) with zig --library c build-exe lib.zig.

(Aside: something like __attribute__((constructor)) would be nice.)

[bnoordhuis]

It magically started working after I did a rebuild against a newer llvm version so this might be a llvm bug. The .init_array field is still 0 according to objdump but the constructor is called.

[andrewrk]

What's the use case for this?

I would generally recommend against trying to have a shared library constructor. It's quite intentional that we don't have static constructors.

But if it solves a problem, let's talk about that.

[bnoordhuis]

The use case is auto-registration of Node.js add-ons: https://github.com/nodejs/node/blob/v9.5.0/src/node.h#L485-L515

[andrewrk]

It looks like the goal is to call node_module_register. Would it work to have some kind of initialization function in your module that calls this function? like this

[kmeaw]

I am also trying to export .init_array. I need a shared library which modifies the code of the application it is being loaded into, so I am not able to change the application's code - it is forced to load the library by the user passing a value into LD_PRELOAD.

pub fn init() callconv(.C) void {
    std.debug.print("{s}", .{"init()"});
}

export const init_array linksection(".init_array") = [1]fn() callconv(.C) void { init };

This code works as expected.