5.9 Atomic Execution

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▼ | 5 | Derived Utilities

5.1 | Safe Homogenous Vectors
5.2 | Safe C Vectors
5.3 | Tagged C Pointer Types
5.4 | Serializable C Struct Types
5.5 | Static Callout and Callback Cores
5.6 | Defining Bindings
5.7 | Allocation and Finalization
5.8 | Custodian Shutdown Registration
5.9 | Atomic Execution
5.10 | Speculatively Atomic Execution
5.11 | Thread Scheduling
5.12 | Ports
5.13 | Process-Wide and Place-Wide Registration
5.14 | Operating System Threads
5.15 | Garbage Collection Callbacks
5.16 | Objective-C FFI
5.17 | Cocoa Foundation
5.18 | COM (Common Object Model)
5.19 | File Security-Guard Checks
5.20 | Windows API Helpers
5.21 | Virtual Machine Primitives

On this page:

start-atomic
end-atomic
start-breakable-atomic
end-breakable-atomic
call-as-atomic
call-as-nonatomic
in-atomic-mode?
start-uninterruptible
end-uninterruptible
call-as-uninterruptible

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### 5.9 Atomic Execution🔗ℹ

(require ffi/unsafe/atomic) | package: base

Atomic mode evaluates a Racket expression without switching among Racket threads and with limited support for events. An atomic computation in this sense is not atomic with respect to other places, but only to other threads within a place.

Atomic mode is unsafe, because the Racket scheduler is not able to operate while execution is in atomic mode; the scheduler cannot switch threads or poll certain kinds of events, which can lead to deadlock or starvation of other threads. Beware that many operations can involve such synchronization, such as writing to an output port. Even if an output target is known to be free of synchronization, beware that values can have arbitrary printing procedures attached through prop:custom-write. Successful use of atomic mode requires a detailed knowledge of any implementation that might be reached during atomic mode to ensure that it terminates and does not involve synchronization.

> procedure (start-atomic) → void?
> procedure (end-atomic) → void?

Disables/re-enables context switches at the level of Racket threads, and also suspends/resumes delivery of break exceptions (independent of the result of break-enabled). Calls to start-atomic and end-atomic can be nested.

Note that pairing start-atomic and end-atomic with dynamic-wind is useful only when

- the current exception handler is known to safely escape atomic mode, or else all possible escapes are through known continuation jumps or aborts (because breaks are disabled and no other exceptions are possible) that escape safely; and

- exception constructions, if any, avoid printing values in the exception message, or else the error value conversion handler is always used and known to be safe for atomic mode.

Using call-as-atomic is somewhat safer than using start-atomic and end-atomic, because call-as-atomic catches exceptions and re-raises them after exiting atomic mode, and it wraps any call to the error value conversion handler with call-as-nonatomic. The latter is safe for a particular atomic region, however, only if the region can be safely interrupted by a non-atomic exception construction.

Unlike call-as-atomic, start-atomic and end-atomic can be called from any OS thread as supported by ffi/unsafe/os-thread, although the calls have no effect in that case. Using start-atomic in a future that is not running in a Racket thread blocks the future until it is resumed by touch in a Racket thread.

See also the caveat that atomic mode is unsafe.

> procedure (start-breakable-atomic) → void?
> procedure (end-breakable-atomic) → void?

Like start-atomic and end-atomic, but the delivery of break exceptions is not suspended.

These functions are not significantly faster than start-atomic and end-atomic, so they provide no benefit in a context where breaks are disabled.

> procedure (call-as-atomic thunk) → any
> thunk : (-> any)

Calls thunk in atomic mode, where call-as-nonatomic can be used during the dynamic extent of the call to revert to non-atomic mode for a nested computation.

When call-as-atomic is used in the dynamic extent of call-as-atomic, then thunk is called directly as a non-tail call.

If thunk raises an exception, the exception is caught and re-raised after exiting atomic mode. Any call to the current error value conversion handler is effectively wrapped with call-as-nonatomic.

See also the caveat that atomic mode is unsafe.

> procedure (call-as-nonatomic thunk) → any
> thunk : (-> any)

Within the dynamic extent of a call to call-as-atomic, calls thunk in non-atomic mode. Beware that the current thread may be suspended or terminated by other threads during the execution of thunk.

When used not in the dynamic extent of a call to call-as-atomic, call-as-nonatomic raises exn:fail:contract.

> procedure (in-atomic-mode?) → boolean?

Returns #t when in atomic mode or uninterruptible mode (within the current place), #f otherwise.

> procedure (start-uninterruptible) → void?
> procedure (end-uninterruptible) → void?

Like start-atomic and end-atomic, but with the intent that the continuation after start-uninterruptible and before end-uninterruptible may run concurrently with other Racket threads, but in uninterruptible mode: it should reach end-uninterruptible without interruption from other threads. Uninterruptable mode is unsafe, just like atomic mode is unsafe.

Uninterruptable mode is currently implemented using start-atomic and end-atomic, except when used within a future. Unlike start-atomic, start-uninterruptible in the CS implementation does not block a future that is running concurrently with Racket threads. At the same time, it is the responsibility of such a future to not perform any action that blocks the future, otherwise the “uninterruptible” sequence is interrupted, and it will be completed only when a Racket thread touches the future (at which point it will run uninterrupted in the Racket thread until end-uninterruptible).

Calls to start-uninterruptible and end-uninterruptible can be nested, and they can be mutually nested with calls to start-atomic and end-atomic. A call to start-atomic nested in uninterruptible mode will still block a future that is running concurrently to Racket threads.

Added in version 8.17.0.7 of package base.

> procedure (call-as-uninterruptible thunk) → any
> thunk : (-> any)

Similar to call-as-atomic, but for uninterruptible mode.

Added in version 8.17.0.7 of package base.

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