Clang编译FFmpeg常见问题

最近闲来无事，就开始研究下和ffmpeg相关都知识，看了网上关于ndk编译ffmpeg的教程，基本上都是使用gcc来编译，而ndk从r18b开始就正式移除gcc来，因此很有必要研究下clang编译ffmpeg，在此过程中遇到了不少奇怪的问题。

系统：macos
ffmpeg版本：4.1.3
ndk版本：r19c
编译器：clang

1、命令找不到

错误信息：

./build_android.sh: line 18: --enable-shared: command not found
./build_android.sh: line 20: --disable-static: command not found
./build_android.sh: line 22: --disable-doc: command not found
./build_android.sh: line 24: --disable-ffmpeg: command not found
./build_android.sh: line 26: --disable-ffplay: command not found
./build_android.sh: line 28: --disable-ffprobe: command not found
./build_android.sh: line 30: --disable-ffserver: command not found
./build_android.sh: line 32: --disable-avdevice: command not found

解决：
如果是直接copy网上的shell脚本，可能会是dos格式，请使用dos2unix build_android.sh 转换一下，删掉多余空格(这一点非常重要dos2unix 是一个工具，如果没有安装的话请先安装一下：brew install dos2unix ，很快就完事。

2、xmakefile 文件没有生成

错误信息：

./android_config.sh: line 36: --enable-shared: command not found
Makefile:2: ffbuild/config.mak: No such file or directory
Makefile:40: /tools/Makefile: No such file or directory
Makefile:41: /ffbuild/common.mak: No such file or directory
Makefile:91: /libavutil/Makefile: No such file or directory
Makefile:91: /ffbuild/library.mak: No such file or directory
Makefile:93: /fftools/Makefile: No such file or directory
Makefile:94: /doc/Makefile: No such file or directory
Makefile:95: /doc/examples/Makefile: No such file or directory
Makefile:160: /tests/Makefile: No such file or directory
make: *** No rule to make target `/tests/Makefile'. Stop.
Makefile:2: ffbuild/config.mak: No such file or directory
Makefile:40: /tools/Makefile: No such file or directory
Makefile:41: /ffbuild/common.mak: No such file or directory
Makefile:91: /libavutil/Makefile: No such file or directory
Makefile:91: /ffbuild/library.mak: No such file or directory
Makefile:93: /fftools/Makefile: No such file or directory
Makefile:94: /doc/Makefile: No such file or directory
Makefile:95: /doc/examples/Makefile: No such file or directory
Makefile:160: /tests/Makefile: No such file or directory

解决：
执行./configure --disable-x86asm 生成config.mak文件

3、arm-linxu-androideabi-gcc is unable to create an executable file

错误信息：

/Users/aria/dev/android/sdk/ndk-bundle/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-gcc is unable to create an executable file.

原因：
检查ndk版本，android官方从r18b开始，已经移除了gcc这个编译工具详情见ndk r18b修订内容
解决：
使用clang进行编译，详情见：https://medium.com/@ilja.kosynkin/building-ffmpeg-4-0-for-android-with-clang-642e4911c31e

4、/android_config.sh: line 32: xxxxx No such file or directory

原因：
.configure后面的命令不能有注释

解决：
删除注释的哪一行代码

5、static declaration of 'xxx' follows non-static declaration

解决：
config.h 搜索 lrint、lrintf、round、roundf 等对于的字符，将0修改为1

#define HAVE_LLRINT 1
#define HAVE_LLRINTF 1
#define HAVE_LRINT 1
#define HAVE_LRINTF 1
#define HAVE_ROUND 1
#define HAVE_ROUNDF 1
#define HAVE_CBRT 1
#define HAVE_CBRTF 1
#define HAVE_COPYSIGN 1
#define HAVE_TRUNC 1
#define HAVE_TRUNCF 1
#define HAVE_RINT 1
#define HAVE_HYPOT 1
#define HAVE_ERF 1

或直接使用sed来修改config.h文件

sed -i -e 's/#define HAVE_LLRINT 0/#define HAVE_LLRINT 1/g' config.h
sed -i -e 's/#define HAVE_LLRINTF 0/#define HAVE_LLRINTF 1/g' config.h
sed -i -e 's/#define HAVE_LRINT 0/#define HAVE_LRINT 1/g' config.h
sed -i -e 's/#define HAVE_LRINTF 0/#define HAVE_LRINTF 1/g' config.h
sed -i -e 's/#define HAVE_ROUND 0/#define HAVE_ROUND 1/g' config.h
sed -i -e 's/#define HAVE_ROUNDF 0/#define HAVE_ROUNDF 1/g' config.h
sed -i -e 's/#define HAVE_CBRT 0/#define HAVE_CBRT 1/g' config.h
sed -i -e 's/#define HAVE_CBRTF 0/#define HAVE_CBRTF 1/g' config.h
sed -i -e 's/#define HAVE_COPYSIGN 0/#define HAVE_COPYSIGN 1/g' config.h
sed -i -e 's/#define HAVE_TRUNC 0/#define HAVE_TRUNC 1/g' config.h
sed -i -e 's/#define HAVE_TRUNCF 0/#define HAVE_TRUNCF 1/g' config.h
sed -i -e 's/#define HAVE_RINT 0/#define HAVE_RINT 1/g' config.h
sed -i -e 's/#define HAVE_HYPOT 0/#define HAVE_HYPOT 1/g' config.h
sed -i -e 's/#define HAVE_ERF 0/#define HAVE_ERF 1/g' config.h
sed -i -e 's/#define HAVE_GMTIME_R 0/#define HAVE_GMTIME_R 1/g' config.h
sed -i -e 's/#define HAVE_LOCALTIME_R 0/#define HAVE_LOCALTIME_R 1/g' config.h
sed -i -e 's/#define HAVE_INET_ATON 0/#define HAVE_INET_ATON 1/g' config.h

6、 xxxxxxxxxx error: expected ')'

错误信息：

#define getenv(x) NULL
^
/home/cd008/diska/android-ndk-r9/platforms/android-18/arch-arm/usr/include/stdlib.h:54:14: note: in expansion of macro 'getenv'
extern char *getenv(const char *);
^
./config.h:17:19: error: expected ')' before numeric constant
#define getenv(x) NULL
^
/home/cd008/diska/android-ndk-r9/platforms/android-18/arch-arm/usr/include/stdlib.h:54:14: note: in expansion of macro 'getenv'
extern char *getenv(const char *);

解决：
在config.h中注释掉#define getenv(x) NULL
/#define getenv(x) NULL/

sed -i -e 's/#define getenv(x) NULL/\/\*#define getenv(x) NULL\*\//g' config.h

7、arm-linux-androideabi-ld -Wl,-soname,libavutil.so unknown option

错误信息：

Users/aria/dev/android/sdk/ndk-bundle/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-ld -Wl,-soname

原因：
gcc 构建 .so 的命令是 -shared -wl,soname,xxxx.so 而 clang 的是 -shared -soname xxx.so

解决：
修改ffbuild/config.mak文件，将SHFLAGS=-shared -Wl,-soname,$(SLIBNAME)修改为SHFLAGS=-shared -soname $(SLIBNAME)

8、arm-linux-androideabi-ld: fatal error: -shared and -pie are incompatible

解决：
修改ffbuild/config.mak文件，将LDEXEFLAGS= -fPIE -pie 去掉-pie

9、member at 2760 is not an ELF object

原因1：没有使用ndk带的ar来进行打包
解决：检查下ar命令的路径是否正确
原因2: macos ranlib 和ndk的ranlib不兼容
解决：见3.8

10、the table of contents is empty (no object file members in the library define global symbols)

原因：
macos的ranlib和ndk的ranlib不兼容
解决：
使用ndk自带的ranlib

ranlib=xxxx/arm-linux-androideabi-runlib

11、undefined reference to '__aeabi_memcpy'

原因：
ld合并多个.a静态库参数错误
解决：
将-static改为-shared

12、undefined reference xxxxx

原因：
ffmpeg内联方式导致的问题
解决：
暂时没有有效的解决方案

总结

编译过程中出现最多的是clang、ld、ar 等工具的配置，当出现这些错误的时候，应该使用各个工具的--help来检查配置是否正确，特别是从gcc过度到clang，gcc和clang的很多配置都不同，而网上大多数教程都是使用gcc来编译ffmpeg。

clang --help
xxxxxx-ld --help
xxxxxx-ar --help