Build android executable so lib with ndk

references：
实现可执行的so动态链接库

Sample

$ tree hello-shared/
hello-shared/
├── CMakeLists.txt
├── build.sh
├── hello.c
├── main.c

• CMakeLists.txt

  cmake_minimum_required(VERSION 3.5)
  project (hello_shared)
  
  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC")
  
  # set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-e,lib_entry")
  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-e__lib_main")
  
  add_library(hello SHARED hello.c)
  add_executable(hello_shared main.c)
  target_link_libraries(hello_shared dl)

• build.sh

  #!/bin/bash
  rm -rf build-android
  mkdir build-android
  cd build-android
  cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
  -DANDROID_ABI="arm64-v8a" \
  -DANDROID_NDK=$ANDROID_NDK \
  -DANDROID_PLATFORM=android-21 ..
  make

• hello.c

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <fcntl.h>
  #include <string.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  
  // Must define the interpretor to be the dynamic linker
  #ifdef __ANDROID__
      #ifdef __LP64__
          const char service_interp[] __attribute__((section(".interp"))) = "/system/bin/linker64";
      #else
          const char service_interp[] __attribute__((section(".interp"))) = "/system/bin/linker";
      #endif
  #endif
  
  struct CmdLine{
      int     argc;
      char ** argv;
  };
   
  void ReadCmdLine(struct CmdLine * pLine){
      pLine->argc = 0;
      pLine->argv = NULL;
   
      int32_t iFileId = open("/proc/self/cmdline", O_RDONLY, 0644);
      if (iFileId < 0){
          return ;
      }
   
      char szBuff[8192];
      int iFileSize = read(iFileId, szBuff, sizeof(szBuff));
      int iBegin    = 0;
      int iTemp     = 0;
      close(iFileId);
   
      for (iTemp = 0; iTemp < iFileSize; iTemp++){
          if (szBuff[iTemp] == '\0'){
              pLine->argc++;
          }
      }
   
      pLine->argv = (char **)malloc(pLine->argc * sizeof(char *));
      for (iTemp = 0, pLine->argc = 0; iTemp < iFileSize; iTemp++){
          if (szBuff[iTemp] == '\0'){
              char * p = (char *)malloc(iTemp - iBegin + 1);
              memcpy(p, szBuff + iBegin, iTemp - iBegin);
              p[iTemp - iBegin] = '\0';
              pLine->argv[pLine->argc++] = p;
              iBegin = iTemp + 1;
          }
      }
  }
  
  void __lib_main(void) {
      printf("Entry point of libhello.so \n");
   
      struct CmdLine cmdLine;
      ReadCmdLine(&cmdLine);
      printf("argc:%d\n", cmdLine.argc);
   
      int iTemp = 0;
      for ( ; iTemp < cmdLine.argc; iTemp++)
      {
          printf("arg:%s\n", cmdLine.argv[iTemp]);
      }
      _exit(0);
  }
  
  void lib_entry() {
      printf("Entry point of lib_entry\n");
      exit(0);
  }
  
  void hello() {
     printf("Entry point of hello\n");
  }

• main.c

  #include <dlfcn.h>
  #include <stdio.h>
  
  int main() {
      void *handle = dlopen("./libhello.so", RTLD_LAZY);
      if (!handle) {
          fprintf(stderr, "Error: %s\n", dlerror());
          return 1;
      }
  
      void (*hello)() = dlsym(handle, "hello");
      if (!hello) {
          fprintf(stderr, "Error: %s\n", dlerror());
          return 1;
      }
  
      hello();
  
      dlclose(handle);
  
      return 0;
  }

Build

hello-shared$ ./build.sh 
-- Check for working C compiler: /home/huangqw/ndk/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin/clang
-- Check for working C compiler: /home/huangqw/ndk/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin/clang -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /home/huangqw/ndk/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++
-- Check for working CXX compiler: /home/huangqw/ndk/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/huangqw/github/hello-shared/build-android
Scanning dependencies of target hello
[ 25%] Building C object CMakeFiles/hello.dir/hello.c.o
[ 50%] Linking C shared library libhello.so
[ 50%] Built target hello
Scanning dependencies of target hello_shared
[ 75%] Building C object CMakeFiles/hello_shared.dir/main.c.o
[100%] Linking C executable hello_shared
[100%] Built target hello_shared

Run in android

$ adb push build-android/libhello.so build-android/hello_shared /data/local/tmp/
$ adb shell
# cd /data/local/tmp/
# ./hello_shared
Entry point of hello

// run so lib
# ./libhello.so 111 222 333 444 555 666 777
Entry point of libhello.so
argc:8
arg:./libhello.so
arg:111
arg:222
arg:333
arg:444
arg:555
arg:666
arg:777