rdma_get_devices usage

["Liu, Changcheng" <changcheng.liu@xxxxxxxxx>]

Hi all,
    I’m a software engineer, working on storage system.
    The system use rdma/iWARP for async communication.

    There's one program model is used in the system. It always trigger
    segmental fault because of failing to query device in the child
    process.
    Could anyone tell me what's the right way to use rdma_get_devices?
    How to free/close the devices before forking the child process to
    make that child process could successfully query the devices?
      /*
       * Link: https://github.com/iotcg/rdma_error/blob/master/x722_mcx4.cpp
       * Build command: g++ x722_mcx4.cpp -libverbs -lrdmacm -o x722_mcx4
       *
       * Run:
       *   ./x722_mcx4 mlx5_0
       * Output:
       *    parent start query
       *    query device: mlx5_0
       *    parent end query
       *
       *    child start query
       *    query device: mlx5_0
       *    child end query
       *
       *
       * Run:
       *    ./x722_mcx4 i40iw0
       * Output:
       *    parent start query
       *    query device: i40iw0
       *    parent end query
       *
       *    child start query
       *    query device: i40iw0
       *    libi40iw-i40iw_uquery_device: query device failed and returned status code: 13
       *    child end query
       */
      #include <sys/types.h>
      #include <unistd.h>
      #include <sys/types.h>
      #include <sys/wait.h>

      #include <infiniband/verbs.h>
      #include <rdma/rdma_cma.h>

      #include <string>
      #include <iostream>

      using namespace std;

      int query_i40iw_mlx5_device(ibv_context* *device_context_list, const char * const dev_name)
      {
        int result = 0;
        ibv_device_attr *device_attr(new ibv_device_attr);

        while(*device_context_list) {
          ibv_context* ibv_ctx = *device_context_list;
          const ibv_device* ibv_device = ibv_ctx->device;
          if (0 == strncmp(ibv_device->name, dev_name, strlen("i40iw0")) ||
              0 == strncmp(ibv_device->name, dev_name, strlen("mlx5_0"))) {
            cout << " query device: " << ibv_device->name << endl;
            result = ibv_query_device(ibv_ctx, device_attr);
            break;
          }
          device_context_list++;
        }

        return result;
      }

      int main(int argc, char* *args)
      {
        const char* device_name = args[1];
      parent:
        int device_count = 0;
        ibv_context* *device_context_list(rdma_get_devices(NULL));

        cout << " parent start query" << endl;
        query_i40iw_mlx5_device(device_context_list, device_name);
        cout << " parent end query" << endl << endl;

        pid_t fork_pid = fork();
        if (fork_pid != 0) {
          int status = 0;
          int err = waitpid(fork_pid, &status, 0);
          return err;
        } else {
      child:
          int device_count = 0;
          ibv_context* *device_context_list(rdma_get_devices(NULL));

          cout << " child start query" << endl;
          query_i40iw_mlx5_device(device_context_list, device_name);
          cout << " child end query" << endl << endl;
        }
        return 0;
      }

B.R.
Changcheng