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Build ijkplayer with Clang

liblaf7/16/22ResearchAndroidClangijkplayerNDKAbout 4 min

How to Build ijkplayer

Before Build

  1. Download NDK: Unsupported Downloads · android/ndk Wiki
  2. Download Android Studio: Android Studio download archives | Android Developers

Configure Environment

export ANDROID_SDK=~/Android/Sdk
export ANDROID_NDK=~/programs/ndk/r14b

Build

If no argument is pass to compile-ffmpeg.sh and compile-ijk.sh, only armv7a target will be build instead of all targets including armv5, armv7a, arm64, x86, x86_64.

./init-android.sh

# build ffmpeg
cd android/contrib
./compile-ffmpeg.sh clean
./compile-ffmpeg.sh # compile armv7a only

# build ijkplayer
cd ..
./compile-ijk.sh clean
./compile-ijk.sh # compile armv7a only

# Android Studio:
# Select android/ijkplayer/ and import

Build ffmpeg with Clang

compile-ffmpeg.sh calls tools/do-compile-ffmpeg.sh to do the real compilation job, thus the main build process is present in tools/do-compile-ffmpeg.sh.

Make NDK Standalone Toolchain

do-compile-ffmpeg.sh calls $ANDROID_NDK/build/tools/make-standalone-toolchain.sh to make toolchain on line 202. make-standalone-toolchain.sh then calls $ANDROID_NDK/build/tools/make-standalone-toolchain.py to install toolchain under android/contrib/build/ffmpeg-armv7a/toolchain/.

Under android/contrib/build/ffmpeg-armv7a/toolchain/bin/, you will find clang, clang38, arm-linux-androideabi-clang. clang and arm-linux-androideabi-clang are identical, both of which are shell script wrapper ( see below ) for clang38, a binary program.

#!/bin/bash
if [ "$1" != "-cc1" ]; then
    `dirname $0`/clang38 -target armv7a-none-linux-androideabi --sysroot `dirname $0`/../sysroot "$@"
else
    # target/triple already spelled out.
    `dirname $0`/clang38 "$@"
fi

clang38 is directly copied from $ANDROID_NDK. In the python script make-standalone-toolchain.py, we can find the following function on line 133:

def get_clang_path_or_die(host_tag):
    """Return the Clang path for our host or die."""
    clang_toolchain_path = os.path.join(
        NDK_DIR, 'toolchains/llvm/prebuilt', host_tag)
    if not os.path.exists(clang_toolchain_path):
        sys.exit('Could not find Clang: {}'.format(clang_toolchain_path))
    return clang_toolchain_path

That is to say, clang38 probably comes from $ANDROID_NDK/toolchains/llvm/prebuilt/linux-x86_64/. And in fact, that’s true! If we compare clang38 and $ANDROID_NDK/toolchains/llvm/prebuilt/clang:

cmp $ANDROID_NDK/toolchains/llvm/prebuilt/linux-x86_64/bin/clang android/contrib/build/ffmpeg-armv7a/toolchain/bin/clang38

The two compilers are identical!

configurate ffmpeg

This step is quite short in do-compile-ffmpeg.sh, staring from line 300:

cd $FF_SOURCE
if [ -f "./config.h" ]; then
    echo 'reuse configure'
else
    which $CC
    ./configure $FF_CFG_FLAGS \
        --extra-cflags="$FF_CFLAGS $FF_EXTRA_CFLAGS" \
        --extra-ldflags="$FF_DEP_LIBS $FF_EXTRA_LDFLAGS"
    make clean
fi

configure is a bash script under android/contrib/ffmpeg-armv7a/. Help messages can be found in function show_help() on line 59. Toolchain options starts from line 331. By default, arm-linux-androideabi-gcc is used as C compiler ( $cc ). To use clang, we need to pass --cc=clang or --cc=arm-linux-androideabi-clang to ./configure. The two clang wrapper are identical.

--- a/android/contrib/tools/do-compile-ffmpeg.sh
+++ b/android/contrib/tools/do-compile-ffmpeg.sh
@@ -214,7 +214,7 @@ echo "[*] check ffmpeg env"
 echo "--------------------"
 export PATH=$FF_TOOLCHAIN_PATH/bin/:$PATH
 #export CC="ccache ${FF_CROSS_PREFIX}-gcc"
-export CC="${FF_CROSS_PREFIX}-gcc"
+export CC="${FF_CROSS_PREFIX}-clang"
 export LD=${FF_CROSS_PREFIX}-ld
 export AR=${FF_CROSS_PREFIX}-ar
 export STRIP=${FF_CROSS_PREFIX}-strip
@@ -303,6 +303,8 @@ if [ -f "./config.h" ]; then
 else
     which $CC
     ./configure $FF_CFG_FLAGS \
+        --cc='clang' \
+        --host-cc='clang38' \
         --extra-cflags="$FF_CFLAGS $FF_EXTRA_CFLAGS" \
         --extra-ldflags="$FF_DEP_LIBS $FF_EXTRA_LDFLAGS"
     make clean

However, you would probably get an error message like this:

GNU assembler not found, install/update gas-preprocessor

Here is the reason. By default, ./configure uses $cc as the assembler ( $as ). However, Clang 3.8 is somewhat incompatible with GNU when it comes to assembly code.

Even if I skip the assembler test ( function check_as() ) in configure, it will fail when compiling.

One possible solution is to add -fno-integrated-as to Clang flags.[1]

+        --cc='clang -fno-integrated-as' \

Or, explicitly tell configure to use gcc as assembler:

+        --as='arm-linux-androideabi-gcc' \
+        --cc='clang' \

It works!

Suppress Warnings

Clang generates lots of warnings. To suppress them, add the following flags to Clang flags.

Suppress “deprecated declarations”

Add -Wno-deprecated-declarations to Clang flags.

Suppress “unknown warning option”

Add -Wno-unknown-warning-option to Clang flags.

Suppress “unused function”

Add -Wno-unused-function to Clang flags. I tried to add the flag directly in do-compile-ffmpeg.sh as following, but didn’t work. Perhaps -Wunused-function is added back somewhere else in configure.

--- a/android/contrib/tools/do-compile-ffmpeg.sh
+++ b/android/contrib/tools/do-compile-ffmpeg.sh
@@ -214,7 +214,7 @@ echo "[*] check ffmpeg env"
 echo "--------------------"
 export PATH=$FF_TOOLCHAIN_PATH/bin/:$PATH
 #export CC="ccache ${FF_CROSS_PREFIX}-gcc"
-export CC="${FF_CROSS_PREFIX}-gcc"
+export CC="${FF_CROSS_PREFIX}-clang"
 export LD=${FF_CROSS_PREFIX}-ld
 export AR=${FF_CROSS_PREFIX}-ar
 export STRIP=${FF_CROSS_PREFIX}-strip
@@ -303,6 +303,8 @@ if [ -f "./config.h" ]; then
 else
     which $CC
     ./configure $FF_CFG_FLAGS \
+        --cc='clang -fno-integrated-as -Wno-deprecated-declarations -Wno-unknown-warning-option -Wno-unused-function' \
+        --host-cc='clang38' \
         --extra-cflags="$FF_CFLAGS $FF_EXTRA_CFLAGS" \
         --extra-ldflags="$FF_DEP_LIBS $FF_EXTRA_LDFLAGS"
     make clean

Upgrade Clang to Higher Version

The version of Clang which NDK r14b uses is 3.8. It’s a little bit old. I failed to compile ffmpeg with NDK r15 or higher. Instead, I find a tricky method to use later version of Clang.

As mentioned in the previous, the Clang used to compile ffmpeg comes from NDK toolchain. If we replace the Clang in NDK, we may be able to use a different version of Clang. NDK r25 comes with Clang 14, so I can:

(base) liblaf@xps:~/programs/ndk/r14b/toolchains/llvm/prebuilt/linux-x86_64/bin$ mv clang clang.old # backup clang
(base) liblaf@xps:~/programs/ndk/r14b/toolchains/llvm/prebuilt/linux-x86_64/bin$ ln --symbolic ~/programs/ndk/r25/toolchains/llvm/prebuilt/linux-x86_64/bin/clang clang # link clang in NDK r14 to clang in NDK r25

Do Not Copy! That won’t work! Link Instead!

Even better, Clang 14 is compatible with GNU assembler, so -fno-integrated-as is unnecessary any more.

Compatibility Test

SDKNDKGradleAndroid Studiocompile ffmpegcompile ijkEmulator
25.0.3r10e2.14.12.3.2✅✅✅
25.0.3r11c2.14.12.3.2✅✅✅
25.0.3r12b2.14.12.3.2✅✅✅
25.0.3r13b2.14.12.3.2✅✅✅
25.0.3r14b2.14.12.3.2⭕✅✅
25.0.3r15c2.14.12.3.2❌
28.0.3r14b2.14.13.6.3✅❌
25.0.3r21e2.14.12.3.3⭕✅poor performance

When building with NDK r14b, you may encounter an error like this:

BFD: libncurses.so.5: cannot open shared object file: No such file or directory

The solution is to install the corresponding dependency:[2]

sudo apt install libncurses5

Discussion on GitHub

Here are some discussions on GitHub. I tried some of their methods but it didn’t work.

  1. 编译出错 · Issue #3378 · bilibili/ijkplayer
  2. You need the NDKr10e or later · Issue #2752 · bilibili/ijkplayer

Reference

  1. c - Unknown directive .altmacro error happens when using android-ndk standalone toolchain - Stack Overflow ↩︎

  2. linux - error while loading shared libraries: libncurses.so.5: - Stack Overflow ↩︎

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