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