c++ dlopen、dlsym

// 加载动态库
Status LoadLibrary(const char* library_filename, void** handle) {
  *handle = dlopen(library_filename, RTLD_NOW | RTLD_LOCAL);
  if (!*handle) {
    return errors::NotFound(dlerror());
  }
  return Status::OK();
}
// 加载symbol
Status GetSymbolFromLibrary(void* handle, const char* symbol_name,
                            void** symbol) {
  *symbol = dlsym(handle, symbol_name);
  if (!*symbol) {
    return errors::NotFound(dlerror());
  }
  return Status::OK();
}

加载库

#include "third_party/hadoop/hdfs.h"
Status BindFunc(void* handle, const char* name,
                std::function<R(Args...)>* func) {
  void* symbol_ptr = nullptr;
  TF_RETURN_IF_ERROR(
      Env::Default()->GetSymbolFromLibrary(handle, name, &symbol_ptr));
  *func = reinterpret_cast<R (*)(Args...)>(symbol_ptr);
  return Status::OK();
}

class LibHDFS {
 public:
  static LibHDFS* Load() {
    static LibHDFS* lib = []() -> LibHDFS* {
      LibHDFS* lib = new LibHDFS;
      lib->LoadAndBind();
      return lib;
    }();

    return lib;
  }

  // The status, if any, from failure to load.
  Status status() { return status_; }

  std::function<hdfsFS(hdfsBuilder*)> hdfsBuilderConnect;
  std::function<hdfsBuilder*()> hdfsNewBuilder;
  std::function<void(hdfsBuilder*, const char*)> hdfsBuilderSetNameNode;
  std::function<int(const char*, char**)> hdfsConfGetStr;
  std::function<int(hdfsFS, hdfsFile)> hdfsCloseFile;
  std::function<tSize(hdfsFS, hdfsFile, tOffset, void*, tSize)> hdfsPread;
  std::function<tSize(hdfsFS, hdfsFile, const void*, tSize)> hdfsWrite;
  std::function<int(hdfsFS, hdfsFile)> hdfsHFlush;
  std::function<int(hdfsFS, hdfsFile)> hdfsHSync;
  std::function<tOffset(hdfsFS, hdfsFile)> hdfsTell;
  std::function<hdfsFile(hdfsFS, const char*, int, int, short, tSize)>
      hdfsOpenFile;
  std::function<int(hdfsFS, const char*)> hdfsExists;
  std::function<hdfsFileInfo*(hdfsFS, const char*, int*)> hdfsListDirectory;
  std::function<void(hdfsFileInfo*, int)> hdfsFreeFileInfo;
  std::function<int(hdfsFS, const char*, int recursive)> hdfsDelete;
  std::function<int(hdfsFS, const char*)> hdfsCreateDirectory;
  std::function<hdfsFileInfo*(hdfsFS, const char*)> hdfsGetPathInfo;
  std::function<int(hdfsFS, const char*, const char*)> hdfsRename;

 private:
  void LoadAndBind() {
    auto TryLoadAndBind = [this](const char* name, void** handle) -> Status {
      TF_RETURN_IF_ERROR(Env::Default()->LoadLibrary(name, handle));
#define BIND_HDFS_FUNC(function) \
  TF_RETURN_IF_ERROR(BindFunc(*handle, #function, &function));

      BIND_HDFS_FUNC(hdfsBuilderConnect);
      BIND_HDFS_FUNC(hdfsNewBuilder);
      BIND_HDFS_FUNC(hdfsBuilderSetNameNode);
      BIND_HDFS_FUNC(hdfsConfGetStr);
      BIND_HDFS_FUNC(hdfsCloseFile);
      BIND_HDFS_FUNC(hdfsPread);
      BIND_HDFS_FUNC(hdfsWrite);
      BIND_HDFS_FUNC(hdfsHFlush);
      BIND_HDFS_FUNC(hdfsTell);
      BIND_HDFS_FUNC(hdfsHSync);
      BIND_HDFS_FUNC(hdfsOpenFile);
      BIND_HDFS_FUNC(hdfsExists);
      BIND_HDFS_FUNC(hdfsListDirectory);
      BIND_HDFS_FUNC(hdfsFreeFileInfo);
      BIND_HDFS_FUNC(hdfsDelete);
      BIND_HDFS_FUNC(hdfsCreateDirectory);
      BIND_HDFS_FUNC(hdfsGetPathInfo);
      BIND_HDFS_FUNC(hdfsRename);
#undef BIND_HDFS_FUNC
      return Status::OK();
    };

// libhdfs.so won't be in the standard locations. Use the path as specified
// in the libhdfs documentation.
#if defined(PLATFORM_WINDOWS)
    const char* kLibHdfsDso = "hdfs.dll";
#elif defined(MACOS) || defined(TARGET_OS_MAC)
    const char* kLibHdfsDso = "libhdfs.dylib";
#else
    const char* kLibHdfsDso = "libhdfs.so";
#endif
    char* hdfs_home = getenv("HADOOP_HDFS_HOME");
    if (hdfs_home != nullptr) {
      string path = io::JoinPath(hdfs_home, "lib", "native", kLibHdfsDso);
      status_ = TryLoadAndBind(path.c_str(), &handle_);
      if (status_.ok()) {
        return;
      }
    }

    // Try to load the library dynamically in case it has been installed
    // to a in non-standard location.
    status_ = TryLoadAndBind(kLibHdfsDso, &handle_);
  }

  Status status_;
  void* handle_ = nullptr;
};

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