STRUCT IB_USER_MAD_HDR(9)

✨ struct ib_user_mad_hdr(9) ✨

okay bestie, welcome to the Kernel API Manual entry for struct ib_user_mad_hdr — she's giving MAD packet header energy and honestly? iconic. 🎀

This layout is the whole reason you can specify and receive the P_Key index. But listen — to actually use that capability, your app must call the IB_USER_MAD_ENABLE_PKEY ioctl on the user MAD file handle before it does literally anything else with that file handle. no cap. first move or nothing. 💅

✨ SYNOPSIS ✨

struct ib_user_mad_hdr {
    __u32 id;
    __u32 status;
    __u32 timeout_ms;
    __u32 retries;
    __u32 length;
    __be32 qpn;
    __be32 qkey;
    __be16 lid;
    __u8 sl;
    __u8 path_bits;
    __u8 grh_present;
    __u8 gid_index;
    __u8 hop_limit;
    __u8 traffic_class;
    __u8 gid[16];
    __be32 flow_label;
    __u16 pkey_index;
    __u8 reserved[6];
 };

🦋 Members 🦋

time for the member-by-member vibe check, and every single one of them ate:

• id — the ID of the agent MAD, whether it's coming in or going out.
• status — 0 on a successful receive, ETIMEDOUT if no response ever showed up (in that case the transaction ID in data[] gets set to the TID of the original request). On send? this one's ignored, she's not relevant.
• timeout_ms — how many milliseconds to wait for a response. Unset on receive.
• retries — the number of automatic retries to attempt. go get 'em.
• qpn — the remote QP number, received from or to be sent to.
• qkey — the remote Q_Key to be sent with. Unset on receive.
• lid — the remote lid, received from or to be sent to.
• sl — the service level, received with or to be sent with.
• path_bits — the local path bits, received with or to be sent with.
• grh_present — if this is set, a GRH was received / should be sent. 🔥
• gid_index — the local GID index to send with. Unset on receive.
• hop_limit — the hop limit in the GRH.
• traffic_class — the traffic class in the GRH.
• gid — the remote GID in the GRH.
• flow_label — the flow label in the GRH.
• pkey_index — the P_Key index. (the moment we've all been waiting for 💖)
• reserved[6] — reserved. she's just sitting there, minding her business, do not touch.

👑 SEE ALSO 👑

go read these next, i'm obsessed:

• the kernel file ./include/uapi/rdma/ib_user_mad.h
• struct ib_user_mad_hdr_old(9)
• struct ib_user_mad(9)
• struct ib_user_mad_reg_req(9)
• struct ib_user_mad_reg_req2(9)

and that's the whole page, bestie 🫶 huge shoutout to ./include/uapi/rdma for housing this masterpiece, dated October 2026 — literally stamped and everything. periodt. 💜🌸