---
name: "rdma_join_multicast"
section: "3"
platform: "linux"
package: "rdma-core"
version: "65.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/rdma_join_multicast"
---

# rdma\_join\_multicast(3)

Joins a multicast group.

## NAME

rdma\_join\_multicast - Joins a multicast group.

## SYNOPSIS

**#include \<rdma/rdma\_cma.h\>**

**int rdma\_join\_multicast** **(struct rdma\_cm\_id \****id***,** **struct sockaddr \****addr***,** **void \****context***);**

## ARGUMENTS

- id

  Communication identifier associated with the request.

- addr

  Multicast address identifying the group to join.

- context

  User-defined context associated with the join request.

## DESCRIPTION

Joins a multicast group and attaches an associated QP to the group.

## RETURN VALUE

Returns 0 on success, or -1 on error.  If an error occurs, errno will be set to indicate the failure reason.

## NOTES

Before joining a multicast group, the rdma\_cm\_id must be bound to an RDMA device by calling rdma\_bind\_addr or rdma\_resolve\_addr.  Use of rdma\_resolve\_addr requires the local routing tables to resolve the multicast address to an RDMA device, unless a specific source address is provided.  The user must call rdma\_leave\_multicast to leave the multicast group and release any multicast resources.  After the join operation completes, if a QP is associated with the rdma\_cm\_id, it is automatically attached to the multicast group when the multicast event is retrieved by the user.  Otherwise, the user is responsible for calling ibv\_attach\_mcast to bind the QP to the multicast group. The join context is returned to the user through the private\_data field in the rdma\_cm\_event.

## SEE ALSO

[rdma\_leave\_multicast(3)](/rdma_leave_multicast/3.md), [rdma\_bind\_addr(3)](/rdma_bind_addr/3.md), [rdma\_resolve\_addr(3)](/rdma_resolve_addr/3.md), [rdma\_create\_qp(3)](/rdma_create_qp/3.md), [rdma\_get\_cm\_event(3)](/rdma_get_cm_event/3.md)