Android编译环境(2) - 手工编译C模块
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上一篇中（即《Android编译环境(1) - 编译Native C的helloworld模块》），我们试用了通过标准的Android.mk文件来编译C模块，下面我们来试试如何直接运用gcc命令行来编译，从而了解Android编译环境的细节。
Android编译环境提供了”showcommands”选项来显示编译命令行，我们可以通过打开这个选项来查看一些编译时的细节。当然，在这之前要把上一篇中的helloworld模块clean:
| # make clean-helloworld |
上面的“make clean-$(LOCAL_MODULE)”是Android编译环境提供的make clean的方式。
接下来使用showcommands选项重新编译helloworld:
| # make helloworld showcommands build/core/product_config.mk:229: WARNING: adding test OTA key target thumb C: helloworld <= development/hello/hello.c prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-gcc -I system/core/include -I hardware/libhardware/include -I hardware/ril/include -I dalvik/libnativehelper/include -I frameworks/base/include -I external/skia/include -I out/target/product/generic/obj/include -I bionic/libc/arch-arm/include -I bionic/libc/include -I bionic/libstdc++/include -I bionic/libc/kernel/common -I bionic/libc/kernel/arch-arm -I bionic/libm/include -I bionic/libm/include/arch/arm -I bionic/libthread_db/include -I development/hello -I out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates -c -fno-exceptions -Wno-multichar -march=armv5te -mtune=xscale -msoft-float -fpic -mthumb-interwork -ffunction-sections -funwind-tables -fstack-protector -D__ARM_ARCH_5__ -D__ARM_ARCH_5T__ -D__ARM_ARCH_5E__ -D__ARM_ARCH_5TE__ -include system/core/include/arch/linux-arm/AndroidConfig.h -DANDROID -fmessage-length=0 -W -Wall -Wno-unused -DSK_RELEASE -DNDEBUG -O2 -g -Wstrict-aliasing=2 -finline-functions -fno-inline-functions-called-once -fgcse-after-reload -frerun-cse-after-loop -frename-registers -DNDEBUG -UDEBUG -mthumb -Os -fomit-frame-pointer -fno-strict-aliasing -finline-limit=64 -MD -o out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o development/hello/hello.c target Executable: helloworld (out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld) prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-g++ -nostdlib -Bdynamic -Wl,-T,build/core/armelf.x -Wl,-dynamic-linker,/system/bin/linker -Wl,--gc-sections -Wl,-z,nocopyreloc -o out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld -Lout/target/product/generic/obj/lib -Wl,-rpath-link=out/target/product/generic/obj/lib -lc -lstdc++ -lm out/target/product/generic/obj/lib/crtbegin_dynamic.o out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o -Wl,--no-undefined prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/interwork/libgcc.a out/target/product/generic/obj/lib/crtend_android.o target Non-prelinked: helloworld (out/target/product/generic/symbols/system/bin/helloworld) out/host/linux-x86/bin/acp -fpt out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld out/target/product/generic/symbols/system/bin/helloworld target Strip: helloworld (out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/helloworld) out/host/linux-x86/bin/soslim --strip --shady --quiet out/target/product/generic/symbols/system/bin/helloworld --outfile out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/helloworld Install: out/target/product/generic/system/bin/helloworld out/host/linux-x86/bin/acp -fpt out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/helloworld out/target/product/generic/system/bin/helloworld |
从上面的命令行可以看到，Android编译环境所用的交叉编译工具链是prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-gcc，-I和-L参数指定了所用的C库头文件和动态库文件路径分别是bionic/libc/include 和out/target/product/generic/obj/lib，其他还包括很多编译选项以及-D所定义的预编译宏。
我们可以利用上面的编译命令，稍加简化来手工编译helloworld程序。先手工删除上次编译得到的helloworld程序：
| # rm out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o # rm out/target/product/generic/system/bin/helloworld |
再用gcc编译，生成目标文件：
| # prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-gcc -I bionic/libc/arch-arm/include -I bionic/libc/include -I bionic/libc/kernel/common -I bionic/libc/kernel/arch-arm -c -fno-exceptions -Wno-multichar -march=armv5te -mtune=xscale -msoft-float -fpic -mthumb-interwork -ffunction-sections -funwind-tables -fstack-protector -D__ARM_ARCH_5__ -D__ARM_ARCH_5T__ -D__ARM_ARCH_5E__ -D__ARM_ARCH_5TE__ -include system/core/include/arch/linux-arm/AndroidConfig.h -DANDROID -fmessage-length=0 -W -Wall -Wno-unused -DSK_RELEASE -DNDEBUG -O2 -g -Wstrict-aliasing=2 -finline-functions -fno-inline-functions-called-once -fgcse-after-reload -frerun-cse-after-loop -frename-registers -DNDEBUG -UDEBUG -mthumb -Os -fomit-frame-pointer -fno-strict-aliasing -finline-limit=64 -MD -o out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o development/hello/hello.c |
与Android.mk编译参数比较，上面主要减少了不必要的-I参数。
接下来生成可执行文件：
| # prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-gcc -nostdlib -Bdynamic -Wl,-T,build/core/armelf.x -Wl,-dynamic-linker,/system/bin/linker -Wl,--gc-sections -Wl,-z,nocopyreloc -o out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld -Lout/target/product/generic/obj/lib -Wl,-rpath-link=out/target/product/generic/obj/lib -lc -lm out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o out/target/product/generic/obj/lib/crtbegin_dynamic.o -Wl,--no-undefined ./prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/interwork/libgcc.a out/target/product/generic/obj/lib/crtend_android.o |
这里值得留意的是参数“-Wl,-dynamic-linker,/system/bin/linker”，它指定了Android专用的动态链接器/system/bin/linker，而不是通常所用的ld.so。
生成的可执行程序可用file和readelf命令来查看一下：
| # file out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), not stripped # readelf -d out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/LINKED/helloworld |grep NEEDED 0x00000001 (NEEDED) Shared library: [libc.so] 0x00000001 (NEEDED) Shared library: [libm.so] |
这是ARM格式的动态链接可执行文件，运行时需要libc.so和libm.so。“not stripped”表示它还没被STRIP。嵌入式系统中为节省空间通常将编译完成的可执行文件或动态库进行STRIP，即去掉其中多余的符号表信息。在前面“make helloworld showcommands”命令的最后我们也可以看到，Android编译环境中使用了out/host/linux-x86/bin/soslim工具进行STRIP。
有关Android Toolchain的其他一些内容可参考：Android Toolchain与Bionic Libc