Android編譯環境(2) – 手工編譯C模組

 

http://www.top-e.org/jiaoshi/html/?161.html

 

上一篇中（即《Android編譯環境(1) - 編譯Native C的helloworld模組》），我們試用了通過標準的Android.mk檔案來編譯C模組，下面我們來試試如何直接運用gcc命令列來編譯，從而瞭解Android編譯環境的細節。

Android編譯環境提供了”showcommands”選項來顯示編譯命令列，我們可以通過開啟這個選項來查看一些編譯時間的細節。當然，在這之前要把上一篇中的helloworld模組clean:

# make clean-helloworld

上面的“make clean-$(LOCAL_MODULE)”是Android編譯環境提供的make clean的方式。

 

接下來使用showcommands選項重新編譯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

 

從上面的命令列可以看到，Android編譯環境所用的交叉編譯工具鏈是prebuilt/linux-x86/toolchain/arm-eabi-4.2.1/bin/arm-eabi-gcc，-I和-L參數指定了所用的C庫標頭檔和動態庫檔案路徑分別是bionic/libc/include 和out/target/product/generic/obj/lib，其他還包括很多編譯選項以及-D所定義的先行編譯宏。

 

我們可以利用上面的編譯命令，稍加簡化來手工編譯helloworld程式。先手工刪除上次編譯得到的helloworld程式：

# rm out/target/product/generic/obj/EXECUTABLES/helloworld_intermediates/hello.o # rm out/target/product/generic/system/bin/helloworld

再用gcc編譯，產生目標檔案：

# 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

與Android.mk編譯參數比較，上面主要減少了不必要的-I參數。

接下來產生可執行檔：

# 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

這裡值得留意的是參數“-Wl,-dynamic-linker,/system/bin/linker”，它指定了Android專用的動態連結器/system/bin/linker，而不是通常所用的ld.so。

 

產生的可執行程式可用file和readelf命令來查看一下：

# 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]

這是ARM格式的動態連結可執行檔，運行時需要libc.so和libm.so。“not stripped”表示它還沒被STRIP。嵌入式系統中為節省空間的通常將編譯完成的可執行檔或動態庫進行STRIP，即去掉其中多餘的符號表資訊。在前面“make helloworld showcommands”命令的最後我們也可以看到，Android編譯環境中使用了out/host/linux-x86/bin/soslim工具進行STRIP。

有關Android Toolchain的其他一些內容可參考：Android Toolchain與Bionic Libc