ibv_qp_a...comp_cntr(3)

💅 ibv_qp_attach_comp_cntr — NAME ✨

okay so bestie, welcome to the ibv_qp_attach_comp_cntr page from the Libibverbs Programmer's Manual (yes, that's the manual name, we're fancy here 🎀). The whole vibe of this function? Attach a completion counter to a QP. That's it, that's the tweet. 🦋

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🌸 SYNOPSIS 🌸

#include <infiniband/verbs.h>

int ibv_qp_attach_comp_cntr(struct ibv_qp *qp,
                            struct ibv_comp_cntr *comp_cntr,
                            struct ibv_qp_attach_comp_cntr_attr *attr);

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✨ DESCRIPTION ✨

So ibv_qp_attach_comp_cntr() does exactly what it says on the tin: it attaches the completion counter comp_cntr to the queue pair qp. And the attr argument? That's the little icon that tells it which operation types should update the counter. iykyk 💖

🚨 vibe check on the QP state, bestie: the QP must be in IBV_QPS_RESET or IBV_QPS_INIT state when you attach a completion counter. Try to attach to a QP in literally any other state and it will fail with EINVAL. no cap, it's not gonna work 😭

The completion counter starts collecting values for the specified QP once attached. And here's the cute part — attaching the same completion counter to multiple QPs will accumulate values from all attached QPs into the same counter. it's giving "sharing is caring" 🫶

The op_mask field is the main character here — it controls which operation completions get counted:

• Local operations — IBV_QP_ATTACH_COMP_CNTR_OP_SEND, IBV_QP_ATTACH_COMP_CNTR_OP_RECV, IBV_QP_ATTACH_COMP_CNTR_OP_RDMA_READ, IBV_QP_ATTACH_COMP_CNTR_OP_RDMA_WRITE — count completions initiated by the local QP.
• Remote operations — IBV_QP_ATTACH_COMP_CNTR_OP_REMOTE_RDMA_READ, IBV_QP_ATTACH_COMP_CNTR_OP_REMOTE_RDMA_WRITE — count completions of incoming RDMA operations initiated by the remote side. 🌈

Supported op_mask values may vary by device and can be queried using ibv_query_comp_cntr_caps(3); unsupported values will result in an ENOTSUP error. so like, check before you commit, bestie 🙈

Multiple completion counters can be attached to the same QP — provided their op_mask values do not overlap. Each QP and operation type pair can be associated with at most one completion counter. Attempting to attach a counter with an op_mask that conflicts with an already attached counter will fail. 💀

And the tea on detaching: there is no explicit detach operation. A completion counter is implicitly detached when the QP it is attached to is destroyed. Also — a completion counter cannot be destroyed while it is still attached to any QP. The QP must be destroyed first. periodt. 👑

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🎀 ARGUMENTS 🎀

• qp — The queue pair to attach the completion counter to. your fave QP 💜
• comp_cntr — The completion counter to attach, previously created with ibv_create_comp_cntr(). gotta make it first, babe ✨
• attr — Attach attributes specifying which operation types update the counter.

🧚 ibv_qp_attach_comp_cntr_attr 🧚

enum ibv_qp_attach_comp_cntr_op {
    IBV_QP_ATTACH_COMP_CNTR_OP_SEND                    = 1 << 0,
    IBV_QP_ATTACH_COMP_CNTR_OP_RECV                    = 1 << 1,
    IBV_QP_ATTACH_COMP_CNTR_OP_RDMA_READ               = 1 << 2,
    IBV_QP_ATTACH_COMP_CNTR_OP_REMOTE_RDMA_READ        = 1 << 3,
    IBV_QP_ATTACH_COMP_CNTR_OP_RDMA_WRITE              = 1 << 4,
    IBV_QP_ATTACH_COMP_CNTR_OP_REMOTE_RDMA_WRITE       = 1 << 5,
};

struct ibv_qp_attach_comp_cntr_attr {
    uint32_t comp_mask;
    uint32_t op_mask;
};

• comp_mask — Bitmask specifying what fields in the structure are valid. It's used for extensibility, so it should be set to 0. keep it zero, bestie, it's not that deep 🍓
• op_mask — Bitmask of ibv_qp_attach_comp_cntr_op values specifying which operation types should update the counter. the important one 💅

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⭐ RETURN VALUE ⭐

ibv_qp_attach_comp_cntr() returns 0 on success, or the value of errno on failure (which indicates the failure reason). slay when it's 0 💖

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🔥 ERRORS 🔥

• EINVAL — Invalid argument(s) passed. 💀
• ENOTSUP — Requested operation is not supported on this device. your device said "not today" 😭
• EBUSY — The op_mask overlaps with a completion counter already attached to this QP. someone else got there first 🙈

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👑 SEE ALSO 👑

ibv_create_comp_cntr(3), ibv_query_comp_cntr_caps(3), ibv_create_qp(3)

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🦋 AUTHORS 🦋

Michael Margolin mrgolin@amazon.com

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okay that's the whole page, bestie, you ate that 🫶✨ shoutout to libibverbs, dated 2026-02-09 — and yes, the footer says ibv_qp_a...comp_cntr(3) too, we love a consistent icon 💅🎀