asm generic termbits.h macro "B0" collides with other macros

[matthewn4444]

Description

Compiling with libraries such as (newest) FFMPEG with arm64-v8a collides with several functions of the macro B0 (such as hevc_mvs.c). You can see how it may cause issues in this person's blog: http://alientechlab.com/how-to-build-ffmpeg-for-android/

Basically what happens is sysroot/usr/include/asm-generic/termbits.h defines a macro definition of

#define B0 0000000000

and FFMPEG uses variables such as

 is_available_B0 = AVAILABLE(cand_up_right, B0) &&
            xb0 < s->ps.sps->width &&
            PRED_BLOCK_AVAILABLE(B0) &&
            !is_diff_mer(s, xB0, yB0, x0, y0);

And then replaces them with 0000000000 as

 is_available_0000000000  = AVAILABLE(cand_up_right, 0000000000 ) &&
            x0000000000  < s->ps.sps->width &&
            PRED_BLOCK_AVAILABLE(0000000000 ) &&
            !is_diff_mer(s, x0000000000 , y0000000000 , x0, y0);

Which then leads to compile issues like

libavcodec/hevc_mvs.c: In function 'derive_spatial_merge_candidates':
libavcodec/hevc_mvs.c:368:23: error: 'y0000000' undeclared (first use in this function)
libavcodec/hevc_mvs.c:368:23: note: each undeclared identifier is reported only once for each function it appears in
libavcodec/hevc_mvs.c:368:23: error: 'x0000000' undeclared (first use in this function)
libavcodec/hevc_mvs.c: In function 'ff_hevc_luma_mv_mvp_mode':
libavcodec/hevc_mvs.c:683:24: error: 'y0000000' undeclared (first use in this function)
libavcodec/hevc_mvs.c:683:24: error: 'x0000000' undeclared (first use in this function)
ffbuild/common.mak:60: recipe for target 'libavcodec/hevc_mvs.o' failed

Sorry for my ignorance but does B0 have some special reason for being 00000000? Could this be changed in the future since this could potentially have collisions with other libraries (for only being 2 letters). (I am not sure if FFMPEG wants to rewrite their variables).

For now, I am using a script to comment that line out (from the termbits.h).

To reproduce you can follow the instructions in that blog (clone FFMPEG, configure, make and make install)

Environment Details

• NDK Version: r16b
• Build sytem: ndk-build, standalone tool chain
• Host OS: Linux (Windows 10 Ubuntu shell, I think it is 16.04)
• Compiler: Clang
• ABI: arm64-v8a
• STL: libc++
• NDK API level: 27
• Device API level: N/A, not compiling for device

[rprichard]

From the blog post, the issue appears to be:

• FFMPEG has a B0 local variable.
• It includes sys/ioctl.h, which includes asm/termbits.h.
• asm/termbits.h defines a B0 macro.

This code doesn't compile with Bionic, but it does with glibc, musl, and FreeBSD:

#include <sys/ioctl.h>
int main() {
  int B0 = 0;
}

Maybe sys/ioctl.h shouldn't include asm/termbits.h.

[enh]

Maybe sys/ioctl.h shouldn't include asm/termbits.h.

sounds plausible to me, based on what that #include gets us. there was no motivation in the git history either.

the only issue will be compatibility with ourselves. testing that for the platform now: https://android-review.googlesource.com/#/c/platform/bionic/+/600968/

[enh]

Sorry for my ignorance but does B0 have some special reason for being 00000000?

(it's one of a number of baud rate constants for serial lines. there's B0, B300, B2400, et cetera. it's a legit POSIX constant, but POSIX says it's in <termios.h>.)

[matthewn4444]

Cool, would the change make it in the next version of the NDK?

Edit: I applied that change to the standalone toolchain similar to the change above and I still get the error. Based on the review change above, also removing #include <linux/termios.h> from sys/ioctl.h allowed it to build.

[enh]

that's more of a problem because <sys/ioctl.h> should expose struct winsize and the TIOCM_ constants, and they come from #include <linux/termios.h>. i'm not sure there's anything we can do about that, other than suggest that ffmpeg shouldn't try to reuse names reserved by POSIX. (<sys/ioctl.h> isn't a POSIX header, so there are no guarantees about which other headers it causes to be included.) closing this bug as "infeasible" unless someone has a clever idea...

btw, that page you linked to isn't the right way to build configure-based projects with the NDK (you can tell because it installs a different compiler). see https://developer.android.com/ndk/guides/standalone_toolchain.html#building_open_source_projects_using_standalone_toolchains for how it should actually work.

[matthewn4444]

For anyone reading this in the future, to prevent this without modifying the ndk or ffmpeg code, use

--disable-linux-perf

It will make /ffmpeg/ffbuild/config.mak comment out the line CONFIG_LINUX_PERF. This prevents the code in /libavutil/timer.h to include the header <sys/ioctl.h> and it will build (which imports <linux/termios.h> and has the B0 constant).

[jiangjie8]

@matthewn4444 Very good solution