The following code sample is the librados client-side application, which will read the object from the OSD, calculate the MD5 hash of the object on the client, and write it back as an attribute to the object. This is doing the calculation and storage in the same way as the RADOS class, with the only difference being the location of the processing.

Create a new file named rados_md5.cc and place the following into it:

#include <cctype>
#include <rados/librados.hpp>
#include <iostream>
#include <string>
#include <openssl/md5.h>

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;
  }
  for(int i = 0; i < 1000; ++i)
  {
    size_t size;
    int ret = io_ctx.stat(object_name, &size, NULL);
    if (ret < 0)
      return ret;

    librados::bufferlist data;
    ret = io_ctx.read(object_name, data, size, 0);
    if (ret < 0)
      return ret;
    unsigned char md5out[16];
    MD5((unsigned char*)data.c_str(), data.length(), md5out);
    char md5string[33];
    for(int i = 0; i < 16; ++i)
      sprintf(&md5string[i*2], "%02x", (unsigned int)md5out[i]);
    librados::bufferlist attrbl;
    attrbl.append(md5string);
    ret = io_ctx.setxattr(object_name, "MD5", attrbl);
    if (ret < 0)
    {
      exit_func(1);
    }
  }
  exit_func(0);
}

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