Android NDK在v5版本后不再提供全部的POSIX线程库的API(比如pthread_cancel和pthread_setcancelstate)。原因之一是线程被标记结束后不一定会把自己拥有的资源释放掉，甚至不一定会结束，因此很可能造成内存泄露或死锁等问题，而这些问题在移动设备上更加突出[1]。
比较安全的方法是使用更安全pthread_kill函数代替，有关pthread_kill的功能讲解和替换方式可以参见这篇博客。
此处仅记录ffmpeg编译android版本时对应的问题及解决。使用ndk-r14b编译arm64时报错：
libavformat/udp.c: In function 'circular_buffer_task_rx':
libavformat/udp.c:499:5: error: implicit declaration of function 'pthread_setcancelstate' [-Werror=implicit-function-declaration]
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &old_cancelstate);
^
libavformat/udp.c:499:28: error: 'PTHREAD_CANCEL_DISABLE' undeclared (first use in this function)
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &old_cancelstate);
^
libavformat/udp.c:499:28: note: each undeclared identifier is reported only once for each function it appears in
libavformat/udp.c:513:32: error: 'PTHREAD_CANCEL_ENABLE' undeclared (first use in this function)
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, &old_cancelstate);
^
libavformat/udp.c: In function 'circular_buffer_task_tx':
libavformat/udp.c:561:28: error: 'PTHREAD_CANCEL_DISABLE' undeclared (first use in this function)
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &old_cancelstate);
^
libavformat/udp.c:596:32: error: 'PTHREAD_CANCEL_ENABLE' undeclared (first use in this function)
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, &old_cancelstate);
^
libavformat/udp.c: In function 'udp_close':
libavformat/udp.c:1142:13: error: implicit declaration of function 'pthread_cancel' [-Werror=implicit-function-declaration]
pthread_cancel(s->circular_buffer_thread);
解决方案：修改ffmpeg/libavformat/udp.c文件函数开始部分，添加：
#define SIG_CANCEL_SIGNAL SIGUSR1
#define PTHREAD_CANCEL_ENABLE 1
#define PTHREAD_CANCEL_DISABLE 0
typedef long pthread_t;
static int pthread_setcancelstate(int state, int *oldstate) {
sigset_t new, old;
int ret;
sigemptyset (&new);
sigaddset (&new, SIG_CANCEL_SIGNAL);
ret = pthread_sigmask(state == PTHREAD_CANCEL_ENABLE ? SIG_BLOCK : SIG_UNBLOCK, &new , &old);
if(oldstate != NULL)
{
*oldstate =sigismember(&old,SIG_CANCEL_SIGNAL) == 0 ? PTHREAD_CANCEL_DISABLE : PTHREAD_CANCEL_ENABLE;
}
return ret;
}
static inline int pthread_cancel(pthread_t thread) {
return pthread_kill(thread, SIG_CANCEL_SIGNAL);
}
另：在Android NDK中是没有libpthread.a的，有关的功能实现已经被集成到了libc.a中，所以使用时不需要使用-lpthread，而应该使用-lc链接参数。
附：ffmpeg添加libdavs2模块并合并成一个.so文件输出的arm64版编译脚本：
#!/bin/sh
build_dir=$(pwd)
AVS2SRC=$build_dir/cavs2dec_lib # path to libdavs2 install dictionary
ARCH=arm64
SDK_VERSION=21
NDK="/Android/android-ndk-r14b"
export PKG_CONFIG_PATH=$AVS2SRC/lib/pkgconfig
if [ "$ARCH" = "arm64" ]
then
PLATFORM_PREFIX="aarch64-linux-android-"
HOST="aarch64"
PLATFORM_VERSION=4.9
EXTRA_CFLAGS="-march=armv8-a -O3 -D__ARM_ARCH_7__ -D__ARM_ARCH_7A__ -DPIC \
-flax-vector-conversions -Wunused-variable -fPIE -pie -pthread"
else
PLATFORM_PREFIX="arm-linux-androideabi-"
HOST="arm"
PLATFORM_VERSION=4.9
EXTRA_CFLAGS="-march=armv7-a -mfloat-abi=softfp -mfpu=neon -marm -mtune=cortex-a8 -D__ARM_ARCH_7__ -D__ARM_ARCH_7A__ \
-flax-vector-conversions -Wunused-variable -fPIE -pie"
fi
NDKROOT=$NDK/platforms/android-${SDK_VERSION}/arch-${ARCH}
TOOLCHAIN=$NDK/toolchains/${PLATFORM_PREFIX}${PLATFORM_VERSION}/prebuilt/linux-x86_64
CROSS_PREFIX=$TOOLCHAIN/bin/${PLATFORM_PREFIX}
EXTRA_LDFLAGS="-fPIE -pie -llog -landroid -shared"
# cp $NDKROOT/usr/include/linux/sysctl.h $NDKROOT/usr/include/sys/sysctl.h
build_one(){
./configure \
--target-os=linux \
--prefix=$build_dir/avs2_lib \
--enable-cross-compile \
--extra-libs="-lgcc -lstdc++ -lc" \
--arch=$ARCH \
--cross-prefix=$CROSS_PREFIX \
--cc=${CROSS_PREFIX}gcc \
--nm=${CROSS_PREFIX}nm \
--sysroot=$NDKROOT \
--extra-cflags="-I$NDKROOT/usr/include -I$AVS2SRC/include -O3 -fpic \
-DANDROID -DHAVE_SYS_UIO_H=1 -Dipv6mr_interface=ipv6mr_ifindex -fasm \
-Wno-psabi -fno-short-enums -fno-strict-aliasing -finline-limit=300 \
-DHAVE_CLOSESOCKET=0 -fvisibility=hidden -fdata-sections -ffunction-sections $EXTRA_CFLAGS" \
--extra-ldflags="-Wl,-rpath-link=$NDKROOT/usr/lib -L$NDKROOT/usr/lib -lc \
-lm -ldl -llog -lstdc++ $AVS2SRC/lib/libdavs2.a -L$AVS2SRC/lib -ldavs2 \
$EXTRA_LDFLAGS" \
--enable-gpl \
--enable-libdavs2 \
--disable-shared \
--enable-static \
--disable-linux-perf \
--enable-pic \
--disable-armv5te \
--disable-armv6 \
--disable-armv6t2 \
--pkg-config=pkg-config
}
cd FFmpegAVS2
make distclean
build_one
make -j16 && make install
# use to package all part into single .so lib
${CROSS_PREFIX}ld -z muldefs -rpath-link=$NDKROOT/usr/lib \
-L$NDKROOT/usr/lib -L$PREFIX/lib -soname libffmpeg.so -shared -nostdlib -Bsymbolic \
--whole-archive --no-undefined -o $AVS2SRC/libffmpeg.so $AVS2SRC/lib/libdavs2.a \
libavcodec/libavcodec.a libavfilter/libavfilter.a libswresample/libswresample.a \
libavformat/libavformat.a libavutil/libavutil.a libswscale/libswscale.a \
libpostproc/libpostproc.a libavdevice/libavdevice.a -fPIC -lc -lm -lz -ldl -llog \
$NDK/sources/cxx-stl/gnu-libstdc++/4.9/libs/arm64-v8a/libgnustl_shared.so -lstdc++ \
--dynamic-linker=/system/bin/linker $TOOLCHAIN/lib/gcc/aarch64-linux-android/4.9.x/libgcc.a
