Finally, the last code sample is the librados application, which instructs OSD to calculate the MD5 hash locally without transferring any data to or from the client. You will note that the code given later has no librados read or write statements and relies purely on the exec function to trigger the MD5 hash creation.

Create a new file named rados_class_md5.cc and place the following into it:

#include <cctype>
#include <rados/librados.hpp>
#include <iostream>
#include <string>

void exit_func(int ret);

librados::Rados rados;

int main(int argc, const char **argv)
{
  int ret = 0;

  // Define variables
  const char *pool_name = "rbd";
  std::string object_name("LowerObject");
  librados::IoCtx io_ctx;
  // Create the Rados object and initialize it
  {
    ret = rados.init("admin"); // Use the default client.admin keyring
    if (ret < 0) {
      std::cerr << "Failed to initialize rados! error " << ret <<
      std::endl;
      ret = EXIT_FAILURE;
    }
  }

  // Read the ceph config file in its default location
  ret = rados.conf_read_file("/etc/ceph/ceph.conf");
  if (ret < 0) {
    std::cerr << "Failed to parse config file "
              << "! Error" << ret << std::endl;
    ret = EXIT_FAILURE;
  }

  // Connect to the Ceph cluster
  ret = rados.connect();
  if (ret < 0) {
    std::cerr << "Failed to connect to cluster! Error " << ret <<
    std::endl;
    ret = EXIT_FAILURE;
  } else {
    std::cout << "Connected to the Ceph cluster" << std::endl;
  }

  // Create connection to the Rados pool
  ret = rados.ioctx_create(pool_name, io_ctx);
  if (ret < 0) {
    std::cerr << "Failed to connect to pool! Error: " << ret <<
    std::endl;
    ret = EXIT_FAILURE;
  } else {
    std::cout << "Connected to pool: " << pool_name <<
    std::endl;
  }
  librados::bufferlist in, out;
  io_ctx.exec(object_name, "md5", "calc_md5", in, out);
  exit_func(0);

}
void exit_func(int ret)
{
  // Clean up and exit
  rados.shutdown();
  exit(ret);
}

We can now compile both applications:

If the applications compile successfully, there will be no output.