fi_atomic - Man Page


fi_atomic — Remote atomic functions

fi_atomic / fi_atomicv / fi_atomicmsg / fi_inject_atomic

Initiates an atomic operation to remote memory

fi_fetch_atomic / fi_fetch_atomicv / fi_fetch_atomicmsg

Initiates an atomic operation to remote memory, retrieving the initial value.

fi_compare_atomic / fi_compare_atomicv / fi_compare_atomicmsg

Initiates an atomic compare-operation to remote memory, retrieving the initial value.

fi_atomicvalid / fi_fetch_atomicvalid / fi_compare_atomicvalid / fi_query_atomic : Indicates if a provider supports a specific atomic operation


#include <rdma/fi_atomic.h>

ssize_t fi_atomic(struct fid_ep *ep, const void *buf,
    size_t count, void *desc, fi_addr_t dest_addr,
    uint64_t addr, uint64_t key,
    enum fi_datatype datatype, enum fi_op op, void *context);

ssize_t fi_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
    void **desc, size_t count, fi_addr_t dest_addr,
    uint64_t addr, uint64_t key,
    enum fi_datatype datatype, enum fi_op op, void *context);

ssize_t fi_atomicmsg(struct fid_ep *ep, const struct fi_msg_atomic *msg,
    uint64_t flags);

ssize_t fi_inject_atomic(struct fid_ep *ep, const void *buf,
    size_t count, fi_addr_t dest_addr,
    uint64_t addr, uint64_t key,
    enum fi_datatype datatype, enum fi_op op);

ssize_t fi_fetch_atomic(struct fid_ep *ep, const void *buf,
    size_t count, void *desc, void *result, void *result_desc,
    fi_addr_t dest_addr, uint64_t addr, uint64_t key,
    enum fi_datatype datatype, enum fi_op op, void *context);

ssize_t fi_fetch_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
    void **desc, size_t count, struct fi_ioc *resultv,
    void **result_desc, size_t result_count, fi_addr_t dest_addr,
    uint64_t addr, uint64_t key, enum fi_datatype datatype,
    enum fi_op op, void *context);

ssize_t fi_fetch_atomicmsg(struct fid_ep *ep,
    const struct fi_msg_atomic *msg, struct fi_ioc *resultv,
    void **result_desc, size_t result_count, uint64_t flags);

ssize_t fi_compare_atomic(struct fid_ep *ep, const void *buf,
    size_t count, void *desc, const void *compare,
    void *compare_desc, void *result, void *result_desc,
    fi_addr_t dest_addr, uint64_t addr, uint64_t key,
    enum fi_datatype datatype, enum fi_op op, void *context);

size_t fi_compare_atomicv(struct fid_ep *ep, const struct fi_ioc *iov,
       void **desc, size_t count, const struct fi_ioc *comparev,
       void **compare_desc, size_t compare_count, struct fi_ioc *resultv,
       void **result_desc, size_t result_count, fi_addr_t dest_addr,
       uint64_t addr, uint64_t key, enum fi_datatype datatype,
       enum fi_op op, void *context);

ssize_t fi_compare_atomicmsg(struct fid_ep *ep,
    const struct fi_msg_atomic *msg, const struct fi_ioc *comparev,
    void **compare_desc, size_t compare_count,
    struct fi_ioc *resultv, void **result_desc, size_t result_count,
    uint64_t flags);

int fi_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
    enum fi_op op, size_t *count);

int fi_fetch_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
    enum fi_op op, size_t *count);

int fi_compare_atomicvalid(struct fid_ep *ep, enum fi_datatype datatype,
    enum fi_op op, size_t *count);

int fi_query_atomic(struct fid_domain *domain,
    enum fi_datatype datatype, enum fi_op op,
    struct fi_atomic_attr *attr, uint64_t flags);



Fabric endpoint on which to initiate atomic operation.


Local data buffer that specifies first operand of atomic operation

iov / comparev / resultv

Vectored data buffer(s).

count / compare_count / result_count

Count of vectored data entries. The number of elements referenced, where each element is the indicated datatype.


Address of remote memory to access.


Protection key associated with the remote memory.


Datatype associated with atomic operands


Atomic operation to perform


Local compare buffer, containing comparison data.


Local data buffer to store initial value of remote buffer

desc / compare_desc / result_desc

Data descriptor associated with the local data buffer, local compare buffer, and local result buffer, respectively. See fi_mr(3).


Destination address for connectionless atomic operations. Ignored for connected endpoints.


Message descriptor for atomic operations


Additional flags to apply for the atomic operation


User specified pointer to associate with the operation. This parameter is ignored if the operation will not generate a successful completion, unless an op flag specifies the context parameter be used for required input.


Atomic transfers are used to read and update data located in remote memory regions in an atomic fashion. Conceptually, they are similar to local atomic operations of a similar nature (e.g. atomic increment, compare and swap, etc.). Updates to remote data involve one of several operations on the data, and act on specific types of data, as listed below. As such, atomic transfers have knowledge of the format of the data being accessed. A single atomic function may operate across an array of data applying an atomic operation to each entry, but the atomicity of an operation is limited to a single datatype or entry.

Atomic Data Types

Atomic functions may operate on one of the following identified data types. A given atomic function may support any datatype, subject to provider implementation constraints.


Signed 8-bit integer.


Unsigned 8-bit integer.


Signed 16-bit integer.


Unsigned 16-bit integer.


Signed 32-bit integer.


Unsigned 32-bit integer.


Signed 64-bit integer.


Unsigned 64-bit integer.


A single-precision floating point value (IEEE 754).


A double-precision floating point value (IEEE 754).


An ordered pair of single-precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.


An ordered pair of double-precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.


A double-extended precision floating point value (IEEE 754). Note that the size of a long double and number of bits used for precision is compiler, platform, and/or provider specific. Developers that use long double should ensure that libfabric is built using a long double format that is compatible with their application, and that format is supported by the provider. The mechanism used for this validation is currently beyond the scope of the libfabric API.


An ordered pair of double-extended precision floating point values (IEEE 754), with the first value representing the real portion of a complex number and the second representing the imaginary portion.

Atomic Operations

The following atomic operations are defined. An atomic operation often acts against a target value in the remote memory buffer and source value provided with the atomic function. It may also carry source data to replace the target value in compare and swap operations. A conceptual description of each operation is provided.



if (buf[i] < addr[i])
    addr[i] = buf[i]


if (buf[i] > addr[i])
    addr[i] = buf[i]


addr[i] = addr[i] + buf[i]


addr[i] = addr[i] * buf[i]

Logical OR

addr[i] = (addr[i] || buf[i])

Logical AND

addr[i] = (addr[i] && buf[i])

Bitwise OR

addr[i] = addr[i] | buf[i]

Bitwise AND

addr[i] = addr[i] & buf[i]

Logical exclusive-OR (XOR)

addr[i] = ((addr[i] && !buf[i]) || (!addr[i] && buf[i]))

Bitwise exclusive-OR (XOR)

addr[i] = addr[i] ^ buf[i]

Read data atomically

result[i] = addr[i]

Write data atomically

addr[i] = buf[i]

Compare values and if equal swap with data

if (compare[i] == addr[i])
    addr[i] = buf[i]

Compare values and if not equal swap with data

if (compare[i] != addr[i])
    addr[i] = buf[i]

Compare values and if less than or equal swap with data

if (compare[i] <= addr[i])
    addr[i] = buf[i]

Compare values and if less than swap with data

if (compare[i] < addr[i])
    addr[i] = buf[i]

Compare values and if greater than or equal swap with data

if (compare[i] >= addr[i])
    addr[i] = buf[i]

Compare values and if greater than swap with data

if (compare[i] > addr[i])
    addr[i] = buf[i]

Swap masked bits with data

addr[i] = (buf[i] & compare[i]) | (addr[i] & ~compare[i])

Base Atomic Functions

The base atomic functions -- fi_atomic, fi_atomicv, fi_atomicmsg -- are used to transmit data to a remote node, where the specified atomic operation is performed against the target data. The result of a base atomic function is stored at the remote memory region. The main difference between atomic functions are the number and type of parameters that they accept as input. Otherwise, they perform the same general function.

The call fi_atomic transfers the data contained in the user-specified data buffer to a remote node. For connectionless endpoints, the destination endpoint is specified through the dest_addr parameter. Unless the endpoint has been configured differently, the data buffer passed into fi_atomic must not be touched by the application until the fi_atomic call completes asynchronously. The target buffer of a base atomic operation must allow for remote read an/or write access, as appropriate.

The fi_atomicv call adds support for a scatter-gather list to fi_atomic. The fi_atomicv transfers the set of data buffers referenced by the ioc parameter to the remote node for processing.

The fi_inject_atomic call is an optimized version of fi_atomic. The fi_inject_atomic function behaves as if the FI_INJECT transfer flag were set, and FI_COMPLETION were not. That is, the data buffer is available for reuse immediately on returning from from fi_inject_atomic, and no completion event will be generated for this atomic. The completion event will be suppressed even if the endpoint has not been configured with FI_SELECTIVE_COMPLETION. See the flags discussion below for more details. The requested message size that can be used with fi_inject_atomic is limited by inject_size.

The fi_atomicmsg call supports atomic functions over both connected and connectionless endpoints, with the ability to control the atomic operation per call through the use of flags. The fi_atomicmsg function takes a struct fi_msg_atomic as input.

struct fi_msg_atomic {
    const struct fi_ioc *msg_iov; /* local scatter-gather array */
    void                **desc;   /* local access descriptors */
    size_t              iov_count;/* # elements in ioc */
    const void          *addr;    /* optional endpoint address */
    const struct fi_rma_ioc *rma_iov; /* remote SGL */
    size_t              rma_iov_count;/* # elements in remote SGL */
    enum fi_datatype    datatype; /* operand datatype */
    enum fi_op          op;       /* atomic operation */
    void                *context; /* user-defined context */
    uint64_t            data;     /* optional data */

struct fi_ioc {
    void        *addr;    /* local address */
    size_t      count;    /* # target operands */

struct fi_rma_ioc {
    uint64_t    addr;     /* target address */
    size_t      count;    /* # target operands */
    uint64_t    key;      /* access key */

The following list of atomic operations are usable with base atomic operations: FI_MIN, FI_MAX, FI_SUM, FI_PROD, FI_LOR, FI_LAND, FI_BOR, FI_BAND, FI_LXOR, FI_BXOR, and FI_ATOMIC_WRITE.

Fetch-Atomic Functions

The fetch atomic functions -- fi_fetch_atomic, fi_fetch_atomicv, and fi_fetch atomicmsg -- behave similar to the equivalent base atomic function. The difference between the fetch and base atomic calls are the fetch atomic routines return the initial value that was stored at the target to the user. The initial value is read into the user provided result buffer. The target buffer of fetch-atomic operations must be enabled for remote read access.

The following list of atomic operations are usable with fetch atomic operations: FI_MIN, FI_MAX, FI_SUM, FI_PROD, FI_LOR, FI_LAND, FI_BOR, FI_BAND, FI_LXOR, FI_BXOR, FI_ATOMIC_READ, and FI_ATOMIC_WRITE.

For FI_ATOMIC_READ operations, the source buffer operand (e.g. fi_fetch_atomic buf parameter) is ignored and may be NULL. The results are written into the result buffer.

Compare-Atomic Functions

The compare atomic functions -- fi_compare_atomic, fi_compare_atomicv, and fi_compare atomicmsg -- are used for operations that require comparing the target data against a value before performing a swap operation. The compare atomic functions support: FI_CSWAP, FI_CSWAP_NE, FI_CSWAP_LE, FI_CSWAP_LT, FI_CSWAP_GE, FI_CSWAP_GT, and FI_MSWAP.

Atomic Valid Functions

The atomic valid functions -- fi_atomicvalid, fi_fetch_atomicvalid, and fi_compare_atomicvalid --indicate which operations the local provider supports. Needed operations not supported by the provider must be emulated by the application. Each valid call corresponds to a set of atomic functions. fi_atomicvalid checks whether a provider supports a specific base atomic operation for a given datatype and operation. fi_fetch_atomicvalid indicates if a provider supports a specific fetch-atomic operation for a given datatype and operation. And fi_compare_atomicvalid checks if a provider supports a specified compare-atomic operation for a given datatype and operation.

If an operation is supported, an atomic valid call will return 0, along with a count of atomic data units that a single function call will operate on.

Query Atomic Attributes

The fi_query_atomic call acts as an enhanced atomic valid operation (see the atomic valid function definitions above). It is provided, in part, for future extensibility. The query operation reports which atomic operations are supported by the domain, for suitably configured endpoints.

The behavior of fi_query_atomic is adjusted based on the flags parameter. If flags is 0, then the operation reports the supported atomic attributes for base atomic operations, similar to fi_atomicvalid for endpoints. If flags has the FI_FETCH_ATOMIC bit set, the operation behaves similar to fi_fetch_atomicvalid. Similarly, the flag bit FI_COMPARE_ATOMIC results in query acting as fi_compare_atomicvalid. The FI_FETCH_ATOMIC and FI_COMPARE_ATOMIC bits may not both be set.

If the FI_TAGGED bit is set, the provider will indicate if it supports atomic operations to tagged receive buffers. The FI_TAGGED bit may be used by itself, or in conjunction with the FI_FETCH_ATOMIC and FI_COMPARE_ATOMIC flags.

The output of fi_query_atomic is struct fi_atomic_attr:

struct fi_atomic_attr {
    size_t count;
    size_t size;

The count attribute field is as defined for the atomic valid calls. The size field indicates the size in bytes of the atomic datatype. The size field is useful for datatypes that may differ in sizes based on the platform or compiler, such FI_LONG_DOUBLE.


Completed atomic operations are reported to the initiator of the request through an associated completion queue or counter. Any user provided context specified with the request will be returned as part of any completion event written to a CQ. See fi_cq for completion event details.

Any results returned to the initiator as part of an atomic operation will be available prior to a completion event being generated. This will be true even if the requested completion semantic provides a weaker guarantee. That is, atomic fetch operations have FI_DELIVERY_COMPLETE semantics. Completions generated for other types of atomic operations indicate that it is safe to re-use the source data buffers.

Any updates to data at the target of an atomic operation will be visible to agents (CPU processes, NICs, and other devices) on the target node prior to one of the following occurring. If the atomic operation generates a completion event or updates a completion counter at the target endpoint, the results will be available prior to the completion notification. After processing a completion for the atomic, if the initiator submits a transfer between the same endpoints that generates a completion at the target, the results will be available prior to the subsequent transfer's event. Or, if a fenced data transfer from the initiator follows the atomic request, the results will be available prior to a completion at the target for the fenced transfer.

The correctness of atomic operations on a target memory region is guaranteed only when performed by a single actor for a given window of time. An actor is defined as a single libfabric domain (identified by the domain name, and not an open instance of that domain), a coherent CPU complex, or other device (e.g. GPU) capable of performing atomic operations on the target memory. The results of atomic operations performed by multiple actors simultaneously are undefined. For example, issuing CPU based atomic operations to a target region concurrently being updated by NIC based atomics may leave the region's data in an unknown state. The results of a first actor's atomic operations must be visible to a second actor prior to the second actor issuing its own atomics.


The fi_atomicmsg, fi_fetch_atomicmsg, and fi_compare_atomicmsg calls allow the user to specify flags which can change the default data transfer operation. Flags specified with atomic message operations override most flags previously configured with the endpoint, except where noted (see fi_control). The following list of flags are usable with atomic message calls.


Indicates that a completion entry should be generated for the specified operation. The endpoint must be bound to a completion queue with FI_SELECTIVE_COMPLETION that corresponds to the specified operation, or this flag is ignored.


Indicates that the user has additional requests that will immediately be posted after the current call returns. Use of this flag may improve performance by enabling the provider to optimize its access to the fabric hardware.


Indicates that the control of constant data buffers should be returned to the user immediately after the call returns, even if the operation is handled asynchronously. This may require that the underlying provider implementation copy the data into a local buffer and transfer out of that buffer. Constant data buffers refers to any data buffer or iovec used by the atomic APIs that are marked as 'const'. Non-constant or output buffers are unaffected by this flag and may be accessed by the provider at anytime until the operation has completed. This flag can only be used with messages smaller than inject_size.


Applies to transmits. Indicates that the requested operation, also known as the fenced operation, and any operation posted after the fenced operation will be deferred until all previous operations targeting the same peer endpoint have completed. Operations posted after the fencing will see and/or replace the results of any operations initiated prior to the fenced operation.

The ordering of operations starting at the posting of the fenced operation (inclusive) to the posting of a subsequent fenced operation (exclusive) is controlled by the endpoint's ordering semantics.


Specifies that the target of the atomic operation is a tagged receive buffer instead of an RMA buffer. When a tagged buffer is the target memory region, the addr parameter is used as a 0-based byte offset into the tagged buffer, with the key parameter specifying the tag.

Return Value

Returns 0 on success. On error, a negative value corresponding to fabric errno is returned. Fabric errno values are defined in rdma/fi_errno.h.



See fi_msg(3) for a detailed description of handling FI_EAGAIN.


The requested atomic operation is not supported on this endpoint.


The number of atomic operations in a single request exceeds that supported by the underlying provider.


Atomic operations operate on an array of values of a specific data type. Atomicity is only guaranteed for each data type operation, not across the entire array. The following pseudo-code demonstrates this operation for 64-bit unsigned atomic write. ATOMIC_WRITE_U64 is a platform dependent macro that atomically writes 8 bytes to an aligned memory location.

fi_atomic(ep, buf, count, NULL, dest_addr, addr, key,
      FI_UINT64, FI_ATOMIC_WRITE, context)
    for (i = 1; i < count; i ++)
        ATOMIC_WRITE_U64(((uint64_t *) addr)[i],
                 ((uint64_t *) buf)[i]);

The number of array elements to operate on is specified through a count parameter. This must be between 1 and the maximum returned through the relevant valid operation, inclusive. The requested operation and data type must also be valid for the given provider.

The ordering of atomic operations carried as part of different request messages is subject to the message and data ordering definitions assigned to the transmitting and receiving endpoints. Both message and data ordering are required if the results of two atomic operations to the same memory buffers are to reflect the second operation acting on the results of the first. See fi_endpoint(3) for further details and message size restrictions.

See Also

fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_cq(3), fi_rma(3)



Referenced By

fi_collective(3), fi_mr(3).

The man pages fi_atomicmsg(3), fi_atomicv(3), fi_atomic_valid(3), fi_compare_atomic(3), fi_compare_atomicmsg(3), fi_compare_atomicv(3), fi_compare_atomic_valid(3), fi_fetch_atomic(3), fi_fetch_atomicmsg(3), fi_fetch_atomicv(3), fi_fetch_atomic_valid(3), fi_inject_atomic(3) and fi_query_atomic(3) are aliases of fi_atomic(3).

2020-10-14 Libfabric Programmer's Manual Libfabric v1.12.1