Re: [Bug 1895471] [NEW] compilation error with clang in util/async.c

[Stefan Hajnoczi]

On Sun, Sep 13, 2020 at 06:56:12PM -0000, Amey Narkhede wrote:
> configured with ` CC=clang CXX=clang++ ../configure --target-
> list=x86_64-softmmu --enable-kvm --enable-curl --enable-debug --enable-
> jemalloc --enable-fuzzing --enable-sdl` and after make I get the
> following error related to c11 atomics. I'm using clang because I'm
> experimenting with fuzzer
> 
> [glitz@archlinux /code/qemu/build]$ ninja -j5
> [479/2290] Compiling C object libqemuutil.a.p/util_async.c.o
> FAILED: libqemuutil.a.p/util_async.c.o
> clang -Ilibqemuutil.a.p -I. -I.. -Iqapi -Itrace -Iui -Iui/shader 
> -I/usr/include/p11-kit-1 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include 
> -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/gio-unix-2.0 
> -Ilinux-headers -Xclang -fcolor-diagnostics -pipe -Wall -Winvalid-pch -Werror 
> -std=gnu99 -g -m64 -mcx16 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 
> -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wundef 
> -Wwrite-strings -Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv 
> -Wold-style-definition -Wtype-limits -Wformat-security -Wformat-y2k 
> -Winit-self -Wignored-qualifiers -Wempty-body -Wnested-externs -Wendif-labels 
> -Wexpansion-to-defined -Wno-initializer-overrides -Wno-missing-include-dirs 
> -Wno-shift-negative-value -Wno-string-plus-int -Wno-typedef-redefinition 
> -Wno-tautological-type-limit-compare -fstack-protector-strong 
> -fsanitize=fuzzer-no-link -iquote /code/qemu/tcg/i386 -isystem 
> /code/qemu/linux-headers -iquote . -iquote /code/qemu -iquote 
> /code/qemu/accel/tcg -iquote /code/qemu/include -iquote 
> /code/qemu/disas/libvixl -pthread -fPIC -MD -MQ 
> libqemuutil.a.p/util_async.c.o -MF libqemuutil.a.p/util_async.c.o.d -o 
> libqemuutil.a.p/util_async.c.o -c ../util/async.c
> ../util/async.c:79:17: error: address argument to atomic operation must be a 
> pointer to _Atomic type ('unsigned int *' invalid)
>     old_flags = atomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
>                 ^               ~~~~~~~~~~
> /usr/lib/clang/10.0.1/include/stdatomic.h:138:42: note: expanded from macro 
> 'atomic_fetch_or'
> #define atomic_fetch_or(object, operand) __c11_atomic_fetch_or(object, 
> operand, __ATOMIC_SEQ_CST)
>                                          ^                     ~~~~~~
> ../util/async.c:105:14: error: address argument to atomic operation must be a 
> pointer to _Atomic type ('unsigned int *' invalid)
>     *flags = atomic_fetch_and(&bh->flags,
>              ^                ~~~~~~~~~~
> /usr/lib/clang/10.0.1/include/stdatomic.h:144:43: note: expanded from macro 
> 'atomic_fetch_and'
> #define atomic_fetch_and(object, operand) __c11_atomic_fetch_and(object, 
> operand, __ATOMIC_SEQ_CST)
>                                           ^                      ~~~~~~
> 2 errors generated.
> [483/2290] Compiling C object libqemuutil.a.p/util_qemu-error.c.o
> ninja: build stopped: subcommand failed.

This happens when a system header file includes <stdatomic.h>. QEMU's
"atomic.h" conflicts with <stdatomic.h> in that QEMU atomic variables do
not need to be declared _Atomic.

Please rerun the full clang command-line above from your meson build
directory with -E instead of -c. Then upload the
libqemuutil.a.p/util_async.c.o so we can see why stdatomic.h was
included.

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