GeezerVison is showing webassembly.github.io — Implementation Limitations — WebAssembly 3.0 (2026-11-04)

Implementation Limitations¶

Implementations typically impose additional restrictions on a number of aspects of a WebAssembly module or execution. These may stem from:

physical resource limits,
constraints imposed by the embedder or its environment,
limitations of selected implementation strategies.

This section lists allowed limitations. Where restrictions take the form of numeric limits, no minimum requirements are given, nor are the limits assumed to be concrete, fixed numbers. However, it is expected that all implementations have “reasonably” large limits to enable common applications.

Note

A conforming implementation is not allowed to leave out individual features. However, designated subsets of WebAssembly may be specified in the future.

Syntactic Limits¶

Structure¶

An implementation may impose restrictions on the following dimensions of a module:

the number of types in a module
the number of functions in a module, including imports
the number of tables in a module, including imports
the number of memories in a module, including imports
the number of globals in a module, including imports
the number of tags in a module, including imports
the number of element segments in a module
the number of data segments in a module
the number of imports to a module
the number of exports from a module
the number of sub types in a recursive type
the subtyping depth of a sub type
the number of fields in a structure type
the number of parameters in a function type
the number of results in a function type
the number of parameters in a block type
the number of results in a block type
the number of locals in a function
the number of instructions in a function body
the number of instructions in a structured control instruction
the number of structured control instructions in a function
the nesting depth of structured control instructions
the number of label indices in a \(\href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_table}}\) instruction
the number of instructions in a constant expression
the number of instructions between a branch instruction and its target
the nesting depth of blocks between a branch instruction and its target
the length of the array in a \(\href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new\_fixed}}\) instruction
the length of an element segment
the length of a data segment
the length of a name
the range of characters in a name

If the limits of an implementation are exceeded for a given module, then the implementation may reject the validation, compilation, or instantiation of that module with an implementation-specific error.

Note

The last item allows embedders that operate in limited environments without support for Unicode to limit the names of imports and exports to common subsets like ASCII.

Binary Format¶

For a module given in binary format, additional limitations may be imposed on the following dimensions:

the size of a module
the size of any section
the size of an individual function’s code
the size of a structured control instruction
the size of an individual constant expression’s instruction sequence
the number of sections

Text Format¶

For a module given in text format, additional limitations may be imposed on the following dimensions:

the size of the source text
the size of any syntactic element
the size of an individual token
the nesting depth of folded instructions
the length of symbolic identifiers
the range of literal characters allowed in the source text

Validation¶

An implementation may defer validation of individual functions until they are first invoked.

If a function turns out to be invalid, then the invocation, and every consecutive call to the same function, results in a trap.

Note

This is to allow implementations to use interpretation or just-in-time compilation for functions. The function must still be fully validated before execution of its body begins.

Execution¶

Restrictions on the following dimensions may be imposed during execution of a WebAssembly program:

the number of allocated module instances
the number of allocated function instances
the number of allocated table instances
the number of allocated memory instances
the number of allocated global instances
the number of allocated tag instances
the number of allocated structure instances
the number of allocated array instances
the number of allocated exception instances
the size of a table instance
the size of a memory instance
the size of an array instance
the number of frames on the stack
the number of labels on the stack
the number of values on the stack
the number of handlers on the stack
the number of labels dropped from the stack by a branch instruction
the number of values dropped from the stack by a branch instruction

If the runtime limits of an implementation are exceeded during execution of a computation, then it may terminate that computation and report an implementation-specific error to the invoking code.

Some of the above limits may already be verified during validation or instantiation, in which case an implementation may report exceedance in the same manner as for syntactic limits.

Note

Concrete limits are usually not fixed but may be dependent on specifics, interdependent, vary over time, or depend on other implementation- or embedder-specific situations or events.