fi_cq(3) Completion queue operations


#include <rdma/fi_domain.h>
int fi_cq_open(struct fid_domain *domain, struct fi_cq_attr *attr,
    struct fid_cq **cq, void *context);
int fi_close(struct fid *cq);
int fi_control(struct fid *cq, int command, void *arg);
ssize_t fi_cq_read(struct fid_cq *cq, void *buf, size_t count);
ssize_t fi_cq_readfrom(struct fid_cq *cq, void *buf, size_t count,
    fi_addr_t *src_addr);
ssize_t fi_cq_readerr(struct fid_cq *cq, struct fi_cq_err_entry *buf,
    uint64_t flags);
ssize_t fi_cq_sread(struct fid_cq *cq, void *buf, size_t count,
    const void *cond, int timeout);
ssize_t fi_cq_sreadfrom(struct fid_cq *cq, void *buf, size_t count,
    fi_addr_t *src_addr, const void *cond, int timeout);
int fi_cq_signal(struct fid_cq *cq);
const char * fi_cq_strerror(struct fid_cq *cq, int prov_errno,
      const void *err_data, char *buf, size_t len);


domain : Open resource domain

cq : Completion queue

attr : Completion queue attributes

context : User specified context associated with the completion queue.

buf : For read calls, the data buffer to write completions into. For write calls, a completion to insert into the completion queue. For fi_cq_strerror, an optional buffer that receives printable error information.

count : Number of CQ entries.

len : Length of data buffer

src_addr : Source address of a completed receive operation

flags : Additional flags to apply to the operation

command : Command of control operation to perform on CQ.

arg : Optional control argument

cond : Condition that must be met before a completion is generated

timeout : Time in milliseconds to wait. A negative value indicates infinite timeout.

prov_errno : Provider specific error value

err_data : Provider specific error data related to a completion


Completion queues are used to report events associated with data transfers. They are associated with message sends and receives, RMA, atomic, tagged messages, and triggered events. Reported events are usually associated with a fabric endpoint, but may also refer to memory regions used as the target of an RMA or atomic operation.


fi_cq_open allocates a new completion queue. Unlike event queues, completion queues are associated with a resource domain and may be offloaded entirely in provider hardware.

The properties and behavior of a completion queue are defined by struct fi_cq_attr.

struct fi_cq_attr {
    size_t               size;      /* # entries for CQ */
    uint64_t             flags;     /* operation flags */
    enum fi_cq_format    format;    /* completion format */
    enum fi_wait_obj     wait_obj;  /* requested wait object */
    int                  signaling_vector; /* interrupt affinity */
    enum fi_cq_wait_cond wait_cond; /* wait condition format */
    struct fid_wait     *wait_set;  /* optional wait set */

size : Specifies the minimum size of a completion queue. A value of 0 indicates that the provider may choose a default value.

flags : Flags that control the configuration of the CQ. This field must be 0.

format : Completion queues allow the application to select the amount of detail that it must store and report. The format attribute allows the application to select one of several completion formats, indicating the structure of the data that the completion queue should return when read. Supported formats and the structures that correspond to each are listed below. The meaning of the CQ entry fields are defined in the Completion Fields section.

FI_CQ_FORMAT_UNSPEC : If an unspecified format is requested, then the CQ will use a provider selected default format.
FI_CQ_FORMAT_CONTEXT : Provides only user specified context that was associated with the completion.
struct fi_cq_entry {
    void     *op_context; /* operation context */
FI_CQ_FORMAT_MSG : Provides minimal data for processing completions, with expanded support for reporting information about received messages.
struct fi_cq_msg_entry {
    void     *op_context; /* operation context */
    uint64_t flags;       /* completion flags */
    size_t   len;         /* size of received data */
FI_CQ_FORMAT_DATA : Provides data associated with a completion. Includes support for received message length, remote EQ data, and multi-receive buffers.
struct fi_cq_data_entry {
    void     *op_context; /* operation context */
    uint64_t flags;       /* completion flags */
    size_t   len;         /* size of received data */
    void     *buf;        /* receive data buffer */
    uint64_t data;        /* completion data */
FI_CQ_FORMAT_TAGGED : Expands completion data to include support for the tagged message interfaces.
struct fi_cq_tagged_entry {
    void     *op_context; /* operation context */
    uint64_t flags;       /* completion flags */
    size_t   len;         /* size of received data */
    void     *buf;        /* receive data buffer */
    uint64_t data;        /* completion data */
    uint64_t tag;         /* received tag */

wait_obj : CQ's may be associated with a specific wait object. Wait objects allow applications to block until the wait object is signaled, indicating that a completion is available to be read. Users may use fi_control to retrieve the underlying wait object associated with an CQ, in order to use it in other system calls. The following values may be used to specify the type of wait object associated with an CQ: FI_WAIT_NONE, FI_WAIT_UNSPEC, FI_WAIT_SET, FI_WAIT_FD, and FI_WAIT_MUTEX_COND.

  • FI_WAIT_NONE : Used to indicate that the user will not block (wait) for completions on the CQ. When FI_WAIT_NONE is specified, the application may not call fi_cq_sread or fi_cq_sreadfrom.
  • FI_WAIT_UNSPEC : Specifies that the user will only wait on the CQ using fabric interface calls, such as fi_cq_sread or fi_cq_sreadfrom. In this case, the underlying provider may select the most appropriate or highest performing wait object available, including custom wait mechanisms. Applications that select FI_WAIT_UNSPEC are not guaranteed to retrieve the underlying wait object.
  • FI_WAIT_SET : Indicates that the completion queue should use a wait set object to wait for completions. If specified, the wait_set field must reference an existing wait set object.
  • FI_WAIT_FD : Indicates that the CQ should use a file descriptor as its wait mechanism. A file descriptor wait object must be usable in select, poll, and epoll routines. However, a provider may signal an FD wait object by marking it as readable, writable, or with an error.
  • FI_WAIT_MUTEX_COND : Specifies that the CQ should use a pthread mutex and cond variable as a wait object.

signaling_vector : Indicates which processor core interrupts associated with the EQ should target.

wait_cond : By default, when a completion is inserted into an CQ that supports blocking reads (fi_cq_sread/fi_cq_sreadfrom), the corresponding wait object is signaled. Users may specify a condition that must first be met before the wait is satisfied. This field indicates how the provider should interpret the cond field, which describes the condition needed to signal the wait object.

A wait condition should be treated as an optimization. Providers are not required to meet the requirements of the condition before signaling the wait object. Applications should not rely on the condition necessarily being true when a blocking read call returns.

If wait_cond is set to FI_CQ_COND_NONE, then no additional conditions are applied to the signaling of the CQ wait object, and the insertion of any new entry will trigger the wait condition. If wait_cond is set to FI_CQ_COND_THRESHOLD, then the cond field is interpreted as a size_t threshold value. The threshold indicates the number of entries that are to be queued before at the CQ before the wait is satisfied.

This field is ignored if wait_obj is set to FI_WAIT_NONE.

wait_set : If wait_obj is FI_WAIT_SET, this field references a wait object to which the completion queue should attach. When an event is inserted into the completion queue, the corresponding wait set will be signaled if all necessary conditions are met. The use of a wait_set enables an optimized method of waiting for events across multiple event and completion queues. This field is ignored if wait_obj is not FI_WAIT_SET.


The fi_close call releases all resources associated with a completion queue. Any completions which remain on the CQ when it is closed are lost.

When closing the CQ, there must be no opened endpoints, transmit contexts, or receive contexts associated with the CQ. If resources are still associated with the CQ when attempting to close, the call will return -FI_EBUSY.


The fi_control call is used to access provider or implementation specific details of the completion queue. Access to the CQ should be serialized across all calls when fi_control is invoked, as it may redirect the implementation of CQ operations. The following control commands are usable with an CQ.

FI_GETWAIT (void **) : This command allows the user to retrieve the low-level wait object associated with the CQ. The format of the wait-object is specified during CQ creation, through the CQ attributes. The fi_control arg parameter should be an address where a pointer to the returned wait object will be written. See fi_eq.3 for addition details using fi_control with FI_GETWAIT.

fi_cq_read / fi_cq_readfrom

The fi_cq_read and fi_cq_readfrom operations perform a non-blocking read of completion data from the CQ. The format of the completion event is determined using the fi_cq_format option that was specified when the CQ was opened. Multiple completions may be retrieved from a CQ in a single call. The maximum number of entries to return is limited to the specified count parameter, with the number of entries successfully read from the CQ returned by the call.

The fi_cq_readfrom call allows the CQ to return source address information to the user for any received data. Source address data is only available for those endpoints configured with FI_SOURCE capability. If fi_cq_readfrom is called on an endpoint for which source addressing data is not available, the source address will be set to FI_ADDR_NOTAVAIL. The number of input src_addr entries must the the same as the count parameter.

CQs are optimized to report operations which have completed successfully. Operations which fail are reported 'out of band'. Such operations are retrieved using the fi_cq_readerr function. When an operation that completes with an unexpected error is inserted into an CQ, it is placed into a temporary error queue. Attempting to read from an CQ while an item is in the error queue results in an FI_EAVAIL failure. Applications may use this return code to determine when to call fi_cq_readerr.

fi_cq_sread / fi_cq_sreadfrom

The fi_cq_sread and fi_cq_sreadfrom calls are the blocking equivalent operations to fi_cq_read and fi_cq_readfrom. Their behavior is similar to the non-blocking calls, with the exception that the calls will not return until either a completion has been read from the CQ or an error or timeout occurs.

It is invalid for applications to call these functions if the CQ has been configured with a wait object of FI_WAIT_NONE or FI_WAIT_SET.


The read error function, fi_cq_readerr, retrieves information regarding any asynchronous operation which has completed with an unexpected error. fi_cq_readerr is a non-blocking call, returning immediately whether an error completion was found or not.

Error information is reported to the user through struct fi_cq_err_entry. The format of this structure is defined below.

struct fi_cq_err_entry {
    void     *op_context; /* operation context */
    uint64_t flags;       /* completion flags */
    size_t   len;         /* size of received data */
    void     *buf;        /* receive data buffer */
    uint64_t data;        /* completion data */
    uint64_t tag;         /* message tag */
    size_t   olen;        /* overflow length */
    int      err;         /* positive error code */
    int      prov_errno;  /* provider error code */
    void    *err_data;    /*  error data */

The general reason for the error is provided through the err field. Provider specific error information may also be available through the prov_errno and err_data fields. The err_data field, if set, will reference an internal buffer owned by the provider. The contents of the buffer will remain valid until a subsequent read call against the CQ. Users may call fi_cq_strerror to convert provider specific error information into a printable string for debugging purposes.


The fi_cq_signal call will unblock any thread waiting in fi_cq_sread or fi_cq_sreadfrom. This may be used to wake-up a thread that is blocked waiting to read a completion operation. The fi_cq_signal operation is only available if the CQ was configured with a wait object.


The CQ entry data structures share many of the same fields. The meanings of these fields are the same for all CQ entry structure formats.

op_context : The operation context is the application specified context value that was provided with an asynchronous operation. The op_context field is valid for all completions.

flags : This specifies flags associated with the completed operation. The Completion Flags section below lists valid flag values. Flags are set for all relevant completions.

len : This len field only applies to completed receive operations. It indicates the size of received message data -- i.e. how many data bytes were actually placed into the associated receive buffer. If an endpoint has been configured with the FI_MSG_PREFIX mode, the len also reflects the size of the prefix buffer.

buf : The buf field is only valid for completed receive operations, and only applies when the receive buffer was posted with the FI_MULTI_RECV flag. In this case, buf points to the starting location where the receive data was placed.

data : The data field is only valid if the FI_REMOTE_CQ_DATA completion flag is set, and only applies to receive completions. If FI_REMOTE_CQ_DATA is set, this field will contain the completion data provided by the peer as part of their transmit request. The completion data will be given in host byte order.

tag : A tag applies only to received messages that occur using the tagged interfaces. This field contains the tag that was included with the received message. The tag will be in host byte order.

olen : The olen field applies to received messages. It is used to indicate that a received message has overrun the available buffer space and has been truncated. The olen specifies the amount of data that did not fit into the available receive buffer and was discarded.

err : This err code is a positive fabric errno associated with a completion. The err value indicates the general reason for an error, if one occurred. See fi_errno.3 for a list of possible error codes.

prov_errno : On an error, prov_errno may contain a provider specific error code. The use of this field and its meaning is provider specific. It is intended to be used as a debugging aid. See fi_cq_strerror for additional details on converting this error value into a human readable string.

err_data : On an error, err_data may reference a provider specific amount of data associated with an error. The use of this field and its meaning is provider specific. It is intended to be used as a debugging aid. See fi_cq_strerror for additional details on converting this error data into a human readable string. Providers are allowed to reuse a single internal buffer to store additional error information. As a result, error data is only guaranteed to be available until the next time the CQ is read.


Completion flags provide additional details regarding the completed operation. The following completion flags are defined.

FI_SEND : Indicates that the completion was for a send operation. This flag may be combined with an FI_MSG or FI_TAGGED flag.

FI_RECV : Indicates that the completion was for a receive operation. This flag may be combined with an FI_MSG or FI_TAGGED flag.

FI_RMA : Indicates that an RMA operation completed. This flag may be combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_REMOTE_WRITE flag.

FI_ATOMIC : Indicates that an atomic operation completed. This flag may be combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_REMOTE_WRITE flag.

FI_MSG : Indicates that a message-based operation completed. This flag may be combined with an FI_SEND or FI_RECV flag.

FI_TAGGED : Indicates that a tagged message operation completed. This flag may be combined with an FI_SEND or FI_RECV flag.

FI_READ : Indicates that a locally initiated RMA or atomic read operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.

FI_WRITE : Indicates that a locally initiated RMA or atomic write operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.

FI_REMOTE_READ : Indicates that a remotely initiated RMA or atomic read operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.

FI_REMOTE_WRITE : Indicates that a remotely initiated RMA or atomic read operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.

FI_REMOTE_CQ_DATA : This indicates that remote CQ data is available as part of the completion.

FI_MULTI_RECV : This flag applies to receive buffers that were posted with the FI_MULTI_RECV flag set. This completion flag indicates that the original receive buffer referenced by the completion has been consumed and was released by the provider. Providers may set this flag on the last message that is received into the multi- recv buffer, or may generate a separate completion that indicates that the buffer has been freed.

Applications can distinguish between these two cases by examining the completion entry flags field. If additional flags, such as FI_RECV, are set, the completion is associated with a received message. In this case, the buf field will reference the location where the received message was placed into the multi-recv buffer. Other fields in the completion entry will be determined based on the received message. If other flag bits are zero, the provider is reporting that the multi-recv buffer has been freed, and the completion entry is not associated with a received message.


fi_cq_open / fi_cq_signal : Returns 0 on success. On error, a negative value corresponding to fabric errno is returned.

fi_cq_read / fi_cq_readfrom / fi_cq_readerr fi_cq_sread / fi_cq_sreadfrom : On success, returns the number of completion events retrieved from the completion queue. On error, a negative value corresponding to fabric errno is returned. If no completions are available to return from the CQ, -FI_EAGAIN will be returned.

fi_cq_strerror : Returns a character string interpretation of the provider specific error returned with a completion.

Fabric errno values are defined in rdma/fi_errno.h.