support dynamic linking musl libc

[necessarily-equal]

downstream bug report https://gitlab.alpinelinux.org/alpine/aports/-/issues/11399.

Programs built by zig can't initialize video output (audio is fine, though): SDL_Init(SDL_INIT_VIDEO) fails.

I wrote a minimal C program to test this further, but I originally noticed the issue when compiling zig code.

to test this, I used the following:

• sdl.c
• zig-logo.bmp (I'm not using attachments because github refuses bmp images)
• Install zig, sdl2+headers, sdl_image+headers, gcc and clang. (apk add -t .zig zig sdl2 sdl2-dev sdl2_image-dev build-base clang on Alpine)

(using the SDL_image pkg-config to ensure correctness)

> zig cc sdl.c $(pkg-config SDL2_image --cflags --libs) && ./a.out
Could not initialize SDL. SDL_Error: No available video device

Yet the C code is functional:

> gcc sdl.c $(pkg-config SDL2_image --cflags --libs) && ./a.out
# works

and clang (LLVM) also works:

> clang sdl.c $(pkg-config SDL2_image --cflags --libs) && ./a.out
# works

The cause seems to reside at link time:

> zig cc sdl.c $(pkg-config SDL2_image --cflags)
> gcc sdl.o $(pkg-config SDL2_image --libs)
> ./a.out
# works

The issue was tested with Zig 0.6, and was already present with Zig 0.5. The same code works with Zig on Debian.

I don't know how to debug this further, but feel free to ask any question.

[necessarily-equal]

Using the static version of the SDL also doesn't work, unfortunately

EDIT: note that the executable wasn't fully static though

[andrewrk]

I think it is fair to also call this a bug because it makes zig unusable on musl-based linux distros for applications that require dynamic linking.

[alexrp]

@ifreund from what I can gather, #7406 only allows dynamically linking musl when doing native builds, right? Is there a way to extend this support for cross scenarios? (I'm not overly familiar with how musl distros work - I just happened to notice that cross-compiling an x86_64-linux-musl executable always results in musl being statically linked.)

[andrewrk]

Static is the default; you need to add the -dynamic flag.

[alexrp]

Ah, the issue was that I tried -lc and -dynamic separately, not together. Combining them did the trick, thanks!

[alexrp]

Is there a way to achieve the same with zig cc?

$ zig cc main.c -target x86_64-linux-musl -fPIE -lc
$ wsl musl-ldd a.out
        musl-ldd (0x7f5ffe36e000)

[motiejus]

Full repro:

main.c

#include <stdio.h>
#include <features.h>

int main() {
    #ifdef __GLIBC__
    printf("glibc_%d.%d\n", __GLIBC__, __GLIBC_MINOR__);
    #else
    printf("non-glibc\n");
    #endif
    return 0;
}

Compiling:

$ zig cc -v -target x86_64-linux-musl main.c -fPIE -lc -dynamic -o main
<...>
LLD Link... ld.lld -error-limit=0 -O0 -z stack-size=16777216 --gc-sections --eh-frame-hdr -znow -m elf_x86_64 -static -pie -o /home/motiejus/.cache/zig/o/503c6bf2ee43b2d33014a8d71f6bc1d2/main /home/motiejus/.cache/zig/o/60b545a849e93516862f973801e6307b/rcrt1.o /home/motiejus/.cache/zig/o/1f9266dba05df20689bccc6b184442ec/crti.o /home/motiejus/.cache/zig/o/6471a2fd822202eae06273e85f08f5c6/main.o /home/motiejus/.cache/zig/o/baf786ce8c76ce866d19a60d75637d88/libcompiler_rt.a --as-needed /home/motiejus/.cache/zig/o/a111f160cb9815a5d7c8128bcf0c7af8/libc.a /home/motiejus/.cache/zig/o/c8866bc9544d3faae13fa04a4728ac91/crtn.o --allow-shlib-undefined

Inspect the resulting main. It is static:

$ objdump --dynamic-syms main
main:     file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
no symbols


$ file main
main: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), static-pie linked, with debug_info, not stripped

Looking at the linker line above, you can see that -static is passed to the linker, and at least --dynamic-linker option is missing. So I believe this should still be open, at least for binaries produced with zig cc.

[ifreund]

@motiejus Would you mind opening a new issue for that with that example? It seems like a bug in the zig cc frontend not in the underlying feature.