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struct ibv_device** ibv_get_device_list(int *num_devices);


ibv_fork_init() should be called before calling any other function in libibverbs.


ibv_get_device_list() returns a NULL-terminated array of RDMA devices currently available. The array should be released with ibv_free_device_list().

The array entries shouldn't be accessed directly. Instead, they should be used with the following service verbs: ibv_get_device_name(), ibv_get_device_guid() and ibv_open_device().


Name Direction Description
num_devices out (optional) If not NULL, it is set to the number of devices returned in the array

Return Values

ibv_get_device_list() returns the array of available RDMA devices on success, returns NULL and sets errno if the request fails. If no devices are found, then num_devices is set to 0, and non-NULL is returned.

Possible errno values are:

  • EPERM - Permission denied.
  • ENOMEM - Insufficient memory to complete the operation.
  • ENOSYS - No kernel support for RDMA.


Get device list without a parameter:

struct ibv_device **dev_list;
dev_list = ibv_get_device_list(NULL);
if (!dev_list)

Get device list with a parameter:

struct ibv_device **dev_list;
int num_devices;
dev_list = ibv_get_device_list(&num_devices);
if (!dev_list)



I called ibv_get_device_list() and it returned NULL, what does it mean?

This is a basic verb that shouldn't fail, check if the module ib_uverbs is loaded.

I called ibv_get_device_list() and it didn't find any RDMA device at all (empty list), what does it mean?

The driver couldn't find any RDMA device.

- Check with lspci, if you have any RDMA device in your machine

- Check if the low-level driver for your RDMA device is loaded, using lsmod

- Check dmesg/var/log/messages for errors

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Tell us what do you think.

  1. Sagar Jha says: January 20, 2016

    Hi Dotan,
    There are two RDMA devices on my machines, but ibv_get_device_list return only one of them (which is not active). They used to work before, but this was observed only today.
    The NIC on the devices is :
    Mellanox Technologies MT27500 Family [ConnectX-3]

    ibstat shows both the devices, but ibv_devinfo shows only one.
    Here are the output of the relevant linux commands :
    $ ibstat
    CA 'mlx4_0'
    CA type: MT26428
    Number of ports: 1
    Firmware version: 2.7.0
    Hardware version: a0
    Node GUID: 0x0002c903000f8ab0
    System image GUID: 0x0002c903000f8ab3
    Port 1:
    State: Down
    Physical state: Polling
    Rate: 10
    Base lid: 1
    LMC: 0
    SM lid: 1
    Capability mask: 0x0251086a
    Port GUID: 0x0002c903000f8ab1
    CA 'mlx4_1'
    CA type: MT4099
    Number of ports: 2
    Firmware version: 2.32.5100
    Hardware version: 1
    Node GUID: 0xe41d2d03000b1d50
    System image GUID: 0xe41d2d03000b1d50
    Port 1:
    State: Active
    Physical state: LinkUp
    Rate: 10
    Base lid: 0
    LMC: 0
    SM lid: 0
    Capability mask: 0x0c010000
    Port GUID: 0xe61d2dfffe0b1d50
    Port 2:
    State: Down
    Physical state: Disabled
    Rate: 10
    Base lid: 0
    LMC: 0
    SM lid: 0
    Capability mask: 0x0c010000
    Port GUID: 0xe41d2d00010b1d51

    $ ibv_devinfo
    libibverbs: Warning: no userspace device-specific driver found for /sys/class/infiniband_verbs/uverbs1
    hca_id: mlx4_0
    transport: InfiniBand (0)
    fw_ver: 2.7.000
    node_guid: 0002:c903:000f:8ab0
    sys_image_guid: 0002:c903:000f:8ab3
    vendor_id: 0x02c9
    vendor_part_id: 26428
    hw_ver: 0xA0
    board_id: MT_0C40110009
    phys_port_cnt: 1
    port: 1
    state: PORT_DOWN (1)
    max_mtu: 4096 (5)
    active_mtu: 4096 (5)
    sm_lid: 1
    port_lid: 1
    port_lmc: 0x00

    $ ibv_devices
    libibverbs: Warning: no userspace device-specific driver found for /sys/class/infiniband_verbs/uverbs1
    device node GUID
    ------ ----------------
    mlx4_0 0002c903000f8ab0

    As you can see, port1 of mlx4_1 is active, but ibv_devices detects only mlx4_0 which is not active. So, the RDMA operations (read and write) do not work with no completion found in the completion queue.

    All the modules seem to be loaded as well (As I said, everything worked before).

    Any help will be appreciated. Thanks.


    • Dotan Barak says: January 29, 2016

      Hi Sagar.

      I would suggest to check /var/log/messages for any error message related to the mlx4 driver.
      Can you update the driver/FW of the adapters?


      • Sagar Jha says: February 1, 2016

        Thanks Dotan for the reply.
        The error actually was that the port for mlx4_0 was down. After reloading the drivers, RDMA started working. I still don't know though, the reason for different outputs by ibv_devinfo and ibstat.

      • Dotan Barak says: February 5, 2016


        Restarting the driver can do miracles

        Anyway, ibstat and ibv_devinfo get their information using different methods;
        usually the result of them is equal, unless there is a problem ...

        I would suggest to search for error messages in /var/log/messages.


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