Undefined Behavior · The Julia Language
Description: Documentation for The Julia Language.

# Undefined Behavior

In programming language design, it is prudent to separate the concepts of a language's semantics, and the behavior of a language's implementation. A language's semantics define the allowable set of observable behaviors (including defining what it means to be observable). A correct implementation will ensure that the actual behavior of executing a program has observable behavior that is allowable according to the language semantics. This is often referred to as the as-if rule in other languages.

To illustrate the distinction, consider a statement like `print(Ref(1).x)`. The language semantics may specify that the observable behavior of this statement is that the value `1` is printed to `stdout`. However, whether or not the object `Ref` is actually allocated may not be semantically observable (even though it may be implicitly observable by looking at memory use, number of allocations, generated code, etc.). Because of this, the implementation is allowed to replace this statement with `print(1)`, which preserves all semantically observable behaviors.

The julia compiler leverages this rule heavily to perform speculative execution. Where the compiler can prove that a function has no side effect (e.g. because it is a purely mathematical computation), the compiler may evaluate all or part of a function invocation during the compilation of that function's caller. The validity of this optimization is another way of thinking about which behaviors are observable.

Additionally, the allowable behaviors for a given program are not unique. For example, the `@fastmath` macro gives wide semantic latitude for floating point math rearrangements and two subsequent invocations of the same operation inside of that macro, even on the same values, are not semantically required to produce the same answer. The situation is similar for asynchronous operations, random number generation, etc.

Note

It is important to note that while multiple behaviors may be allowable, this does not mean that the optimizer is allowed to assume that all invocations of a particular function will exhibit a particular allowable behavior. To the contrary, unless the optimizer can prove that a particular behavior will happen at runtime (e.g. by private interface agreement with the code generator), it must generally assume that any allowable behavior may occur. In julia, we often refer to this concept as "IPO" safety.

Undefined Behavior (UB) occurs when a julia program semantically performs an operation that is assumed to never happen. In such a situation, the language semantics do not constrain the behavior of the implementation, so any behavior of the program is allowable, including crashes, memory corruption, incorrect behavior, etc. As such, it is very important to avoid writing programs that semantically execute undefined behavior.

The term `unsafe`, when it appears in julia function names, is typically intended to be interpreted in these terms: of causing UB if any of the arguments do not carefully follow the contract of that function.

Note that this explicitly applies to semantically executed undefined behavior. For example, undefined behavior present in a branch not taken at runtime does not cause problems. Because of this, undefined behavior is similar to (though a distinct concept from) a side effect as described above and in particular speculative execution is inhibited unless the code in question is proven to not execute undefined behavior.

The presence of undefined behavior is modeled as part of julia's effect system using the `:noub` effect bit. See the documentation for `Base.@assume_effects` for more information on querying the compiler's effect model or overriding it for specific situations (e.g. where a dynamic check precludes potential UB from ever actually being reached).

## List of sources of undefined behavior

The following is a list of sources of undefined behavior, though it should currently not be considered exhaustive:

- Various modifications of global state during precompile. Where possible, this is detected and yields an error, but detection is incomplete.

- Observation of mutable state inside a `@generated` function body (e.g. accessing a global `Dict`)

- Incorrect implementation of a `Core.OptimizedGenerics` interface [1]

- Any invocation of undefined behavior in FFI code (e.g. `ccall`, `llvmcall`) according to the semantics of the respective language

- Incorrect signature types in `ccall` or `cfunction`, even if those signatures happen to yield the correct ABI on a particular platform

- Incorrect use of annotations like `@inbounds`, `@assume_effects` in violation of their requirements [1]

- Observation or retention of pointers to GC-tracked objects outside of a `GC.@preserve` region (e.g. taking a pointer to a GC-allocated object and using it after the `@preserve` block ends)

- Memory modification of GC-tracked objects without appropriate write barriers from outside of julia (e.g. in native calls, debuggers, etc.)

- Violations of the memory model using `unsafe_` operations (e.g. `unsafe_load` of an invalid pointer, pointer provenance violations, etc)

- Violations of TBAA guarantees (e.g. by using `unsafe_wrap` to create a `Vector{Int}` from a pointer to a `Vector{UInt8}`)

- Mutation of data promised to be immutable (e.g. in `Base.StringVector`)

- Data races, although some limits are still placed upon the allowable behavior, per the memory model.

- Modification of julia-internal mutable state (e.g. task schedulers, data types, etc.)

- A value other than `false` (`reinterpret(UInt8, b) == 0x00`) or `true` (`reinterpret(UInt8, b) == 0x01`) for a `Bool` `b`.

- Invoking undefined behavior via compiler intrinsics.

## Implementation-defined behavior

Some behavior is technically forbidden by the semantics of the language, but required in certain parts of the implementation and thus allowed as long as implementation-defined constraints are obeyed. Nevertheless, these constructs should be avoided when possible, as the implementation-defined constraints may not be stable across julia versions.

- Construction of objects using `eval(Expr(:new))` rather than their constructors

## Special cases explicitly NOT undefined behavior

- Access to undefined bits types is not undefined behavior. It is still allowed to return an arbitrary value of the bits type, but the value returned must be the same for every access and use thereof is not undefined behavior. In LLVM terminology, the value is `freeze undef`, not `undef`.

  Julia 1.11

  Prior to Julia 1.11, access to undefined bits types was considered undefined behavior.

- Loops that do not make forward progress are not considered undefined behavior.

  Julia 1.12

  Prior to Julia 1.12, loops that did not make forward progress were considered undefined behavior.

- Replacement of `const` values.

  Julia 1.12

  Prior to Julia 1.12, replacement of `const` values was considered undefined behavior.

- Signed integer overflow is not undefined behavior. See also the manual section on Overflow Behavior.

- Revival of objects inside finalizers is permitted though discouraged.

- 1Incorrect use here may be UB, even if not semantically executed, please see the specific documentation of the feature.

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This document was generated with Documenter.jl version 1.17.0 on Saturday 29 August 2026. Using Julia version 1.14.0-DEV.3067.