-Wdeprecated-non-prototype warning in musl's dlstart.c with zig cc main.c -target aarch64-linux-musl

[alexrp]

Zig Version

0.10.0

Steps to Reproduce and Observed Behavior

$ cat main.c
#include <stdio.h>

int main()
{
    printf("Hello World\n");
}
$ zig cc main.c -target aarch64-linux-musl
In file included from /home/alexrp/sysroot/lib/zig/lib/libc/musl/crt/rcrt1.c:3:
/home/alexrp/sysroot/lib/zig/lib/libc/musl/crt/../ldso/dlstart.c:146:20: warning: a function declaration without a prototype is deprecated in all versions of C and is treated as a zero-parameter prototype in C2x, conflicting with a subsequent definition [-Wdeprecated-non-prototype]
        GETFUNCSYM(&dls2, __dls2, base+dyn[DT_PLTGOT]);
                          ^
/home/alexrp/sysroot/lib/zig/lib/libc/musl/crt/rcrt1.c:11:13: note: conflicting prototype is here
hidden void __dls2(unsigned char *base, size_t *sp)
            ^
1 warning generated.

Also happens for arm-linux-musleabihf.

This did not occur in 0.9.1.

Expected Behavior

No warning.

[ehaas]

This is due to the switch to clang 15 for Zig 0.10.0, since support for old-style (K&R) function declarations is being removed: https://discourse.llvm.org/t/rfc-enabling-wstrict-prototypes-by-default-in-c/60521

You can manually disable it with -Wno-deprecated-non-prototype, otherwise it will have to be fixed upstream in musl.

[sylr]

Got the same thing using zig as CC for a golang crossbuild:

CGO_ENABLED=1 GOOS=linux GOARCH=arm64 CC="zig cc -target aarch64-linux" CXX="zig c++ -target aarch64-linux" /opt/homebrew/bin/go build -tags ,crossbuild -trimpath -ldflags '-s -w -X sylr.dev/fix/cmd.Version=c3ca56c-dirty -extldflags ""' -o dist/fix-c3ca56c-dirty-linux-arm64
# sylr.dev/fix
In file included from /opt/homebrew/Cellar/zig/0.10.0/lib/zig/libc/musl/crt/rcrt1.c:3:
/opt/homebrew/Cellar/zig/0.10.0/lib/zig/libc/musl/crt/../ldso/dlstart.c:146:20: warning: a function declaration without a prototype is deprecated in all versions of C and is treated as a zero-parameter prototype in C2x, conflicting with a subsequent definition [-Wdeprecated-non-prototype]
        GETFUNCSYM(&dls2, __dls2, base+dyn[DT_PLTGOT]);
                          ^
/opt/homebrew/Cellar/zig/0.10.0/lib/zig/libc/musl/crt/rcrt1.c:11:13: note: conflicting prototype is here
hidden void __dls2(unsigned char *base, size_t *sp)
            ^
1 warning generated.

Reproducing steps:

mkdir -p ~/git && (cd ~/git && git clone https://github.com/sylr/fix)
cd git/fix
make crossbuild

[markus-oberhumer]

Would this issue get fixed by upgrading to musl 1.2.5 ?

[alexrp]

Would this issue get fixed by upgrading to musl 1.2.5 ?

Doesn't seem to be the case. This will need an upstream patch.

[alexrp]

That said, it's not actually obvious to me what the patch should be.

https://git.musl-libc.org/cgit/musl/tree/ldso/dlstart.c?id=6f3ead0ae16deb9f0004b275e29a276c9712ee3c#n14

This GETFUNCSYM definition is a fallback for when an architecture doesn't provide a definition with inline assembly. Short of passing the whole function signature into the macro, I'm not sure how else it could be written to avoid that warning. Suppressing the warning is probably not desirable (there seems to be no precedent for this in musl).

[alexrp]

For whatever reason, I'm just completely unable to reproduce this outside the context of Zig's own musl build logic:

$ zig cc --version
clang version 18.1.6
$ ./configure --target=aarch64-linux-gnu CC="zig cc" CFLAGS="-target aarch64-linux -Wall -Wextra -Werror"
$ make -j
$ make clean
$ clang --version
Ubuntu clang version 16.0.6
$ ./configure --target=aarch64-linux-gnu CC=clang CFLAGS="--target=aarch64-linux-gnu -Wall -Wextra -Werror"
$ make -j
$ make clean
$ /opt/llvm/bin/clang --version
clang version 18.1.8
$ ./configure --target=aarch64-linux-gnu CC=/opt/llvm/bin/clang CFLAGS="--target=aarch64-linux-gnu -Wall -Wextra -Werror"
$ make -j

They all build successfully. At this point, I strongly suspect the bug is actually on the Zig side somehow.

[markus-oberhumer]

@alexrp

This is might be because musl configure disables warnings with -w when building with clang.

Try the diff below and run something like CC="clang" CFLAGS="-std=c17 -Werror=deprecated-non-prototype" ../../musl-1.2.5/configure. This should now fail.

$ git diff
diff --git a/configure b/configure
index bc9fbe48..c0fdeffe 100755
--- a/configure
+++ b/configure
@@ -534,7 +534,7 @@ fi
 # to start from a clean slate. So use -w if building with clang. Also
 # turn off a common on-by-default cast warning regardless of compiler.
 #
-test "$cc_family" = clang && tryflag CFLAGS_AUTO -w
+#### test "$cc_family" = clang && tryflag CFLAGS_AUTO -w

 tryflag CFLAGS_AUTO -Wno-pointer-to-int-cast

[alexrp]

@markus-oberhumer thank you, that's super helpful! I hadn't spotted that. This means we just need to adjust our CFLAGS when building musl to use the same warning settings as upstream. So it was indeed on the Zig side. 🙂