struct ibv_ah *ibv_create_ah(struct ibv_pd *pd, struct ibv_ah_attr *attr);

Description

ibv_create_ah() creates an Address Handle (AH) associated with a Protection Domain.

This AH will later be used when a Send Request (SR) will be posted to a Unreliable Datagram QP.

The struct ibv_ah_attr describes the requested attributes of the newly created AH.

struct ibv_ah_attr {
	struct ibv_global_route	grh;
	uint16_t		dlid;
	uint8_t			sl;
	uint8_t			src_path_bits;
	uint8_t			static_rate;
	uint8_t			is_global;
	uint8_t			port_num;
};

Here is the full description of struct ibv_ah_attr:

struct ibv_global_route describes the values to be used in the GRH of the packets that will be sent when using this AH.

struct ibv_global_route {
	union ibv_gid		dgid;
	uint32_t		flow_label;
	uint8_t			sgid_index;
	uint8_t			hop_limit;
	uint8_t			traffic_class;
};

Here is the full description of struct ibv_global_route:

Parameters

Return Values

Examples

Create an Address Handle and destroy it (this example assumed that the variables dlid, sl and port are declared and initialized with valid values):

struct ibv_pd *pd;
struct ibv_ah *ah;
struct ibv_ah_attr ah_attr;
 
memset(&ah_attr, 0, sizeof(ah_attr));
 
ah_attr.is_global     = 0;
ah_attr.dlid          = dlid;
ah_attr.sl            = sl;
ah_attr.src_path_bits = 0;
ah_attr.port_num      = port;
 
ah = ibv_create_ah(pd, &ah_attr);
if (!ah) {
	fprintf(stderr, "Error, ibv_create_ah() failed\n");
	return -1;
}
 
if (ibv_destroy_ah(ah)) {
	fprintf(stderr, "Error, ibv_destroy_ah() failed\n");
	return -1;
}

FAQs

Why is an AH good for anyway?

AH is used when calling ibv_post_send() to an UD QP. This object describes the path from the requester side to the responder side.

Can I use one AH with different QPs?

Yes. This can be done as long as all of those QPs and the AH are associated with the same PD.

How can I get the needed information for the AH when calling ibv_create_ah()?

There are several ways to obtain this information:

• Perform path query to the Subnet Administrator (SA)
• Out of band connection to the remote node, for example: using socket
• Using well-known values, for example: this can be done in a static subnet (which all of the addresses are predefined) or using multicast groups

Written by: Dotan Barak on September 22, 2012.on February 17, 2015.